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Oyu Tolgoi, Turquoise Hill, Hugo Dummett |
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Mongolia |
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Cu Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Oyu Tolgoi high sulphidation overprinted porphyry copper-gold-(molybdenum) deposit is located in the Gobi Desert of southern Mongolia, 550 km due south of the capital, Ulaanbaatar and 80 km north of the Chinese border (#Location: 43° 1' 17"N, 106° 51' 19"E).
Summary
The Oyu Tolgoi cluster of porphyry Cu-Au-Mo deposits in southern Mongolia, define a narrow, linear, 12 km long, almost continuously mineralised trend. Together, the seven main deposits of the cluster, which contain in excess of 42 Mt of Cu and 1850 t of Au, are among the largest high grade porphyry Cu-Au deposits currently known in the world, and are the biggest of Palaeozoic age. This cluster includes the fault offset Heruga deposits immediately to the south, which are the subject of the separate Heruga record.
These deposits occurs towards the southern margin of the Central Asian Orogenic Belt, a collage of magmatic arcs that were periodically active from the Late Neoproterozoic to Permo-Triassic, extending from the Urals Mountains to the Pacific Ocean. They lie within the Gurvansayhan island-arc terrane, a fault bounded segment of the broader Silurian to Carboniferous Kazakh-Mongol arc. This terrane is composed of dismembered ophiolite mélanges, Lower to Middle Palaeozoic volcaniclastics, sandstones, argillites, radiolarian cherts and basaltic rocks. In the vicinity of Oyu Tolgoi, this succession is dominated by Devonian (or older) juvenile, probably intra-oceanic arc-related basaltic lavas and lesser volcaniclastic rocks of the lower Oyu Tolgoi sequence.
This pile is unconformably overlain by the Late Devonian (~370 Ma) basaltic to dacitic pyroclastic and volcanosedimentary rocks of the upper Oyu Tolgoi sequence. Volcanism was terminated by a collisional event, and burial below an overthrust nappe of Devonian (or pre-Devonian) basalts and sedimentary rocks of the Heruga sequence, that are older or equivalent to the lower Oyu Tolgoi sequence. This was followed, after an erosional break, by a more mature calc-alkaline, basaltic to trachyandesitic volcanosedimentary succession of continental margin arc affinity, the Carboniferous Sainshandhudag Formation. These successions were progressively intruded by Late Devonian, and Carboniferous to Permian granitoids, ranging from dykes to stocks and batholiths.
Mineralisation at Oyu Tolgoi is associated with multiple, overlapping, intrusions of Late Devonian (~372 to 370 Ma) quartz-monzodiorite that are chronologically indistinguishable from the basaltic to dacitic pyroclastics of the upper Oyu Tolgoi sequence, into which they partially intrude. These quartz-monzodiorite intrusions range from early-mineral porphyritic dykes, characterised by A-type quartz veining, to larger, linear, syn-, late- and post-mineral dykes and stocks. Ore was deposited within the syn-mineral quartz-monzodiorites, but is dominantly hosted by augite basalts and to a lesser degree by upper Oyu Tolgoi sequence dacitic pyroclastic rocks. The ores were deposited prior to partial erosion of the upper Oyu Tolgoi sequence and over-thrusting by the Heruga sequence, which preceded intrusion of a post-ore biotite granodiorite at ~365 Ma.
Mineralisation is characterised by varying, telescoped stages of intrusion and alteration. The early A-type quartz veined dykes were followed by Cu-Au mineralisation associated with potassic alteration, mainly K-feldspar in quartzmonzodiorite and biotite-magnetite in basaltic hosts. Downward reflux of cooled, late-magmatic hydrothermal fluid, resulted in intense quartz-sericite retrograde alteration in the upper parts of the main syn-mineral intrusions, and an equivalent chlorite-muscovite/illite-hematite assemblage in basaltic host rocks. Uplift, facilitated by syn-mineral longitudinal faulting, brought sections of the porphyry deposit to shallower depths, to be overprinted and upgraded by late stage, shallower, advanced argillic alteration and high sulphidation mineralisation.
The key controls on the location, size and grade of the Oyu Tolgoi deposits include i). a long-lived, narrow faulted corridor, most likely part of an early regional, crustal scale, structure related to the oblique convergence of the magmatic arc; ii). multiple pulses of overlapping intrusion and injection of mineralising fluids and vapours, concentrated along a dilatant section of the same structure to repeatedly mineralise and upgrade the same wallrocks; and iii). enclosing reactive, mafic dominated wall rocks that buffered the Cu-Au-bearing fluids and restricted the lateral dispersion of mineralisation, focussing ore deposition over a narrow interval either side of the intrusions associated with the influx of those fluids.
Discovery and Development
Small historic workings, dating from the Bronze age, are found over the exposed South Oyu deposit, although no serious modern exploration was conducted over the area until the 1980's, when a joint Russian-Mongolian survey conducted a regional geochemical soil sampling survey and reported a single anomalous molybdenum value over the Central Oyu area.
One of the geologists involved in the survey, visited the site in 1983 and noted evidence of alteration and copper mineralisation at South Oyu, and in 1996 guided a Magma Copper team to the site. The outcrops with stockwork veining at Central Oyu were identified as a leached capping over a porphyry deposit. During 1997, geochemical and geophysical investigations were undertaken by BHP Limited which has acquired Magma Copper in late 1996. This was followed by drilling, principally to test the potential for supergene chalcocite mineralisation, with intersections of up to 70 m @ 1.65% Cu, 0.15 g/t Au from a depth of 56 m. By 1999, testing of the supergene and hypogene mineralisation at what was then described as the North, Central, and South mineralised zones, had outlined a preliminary resource of 438 Mt @ 0.52% Cu, 0.25 g/t Au (Perelló et al., 2001).
In 1999, BHP decided to not proceed further and offered the tenements for joint venture, resulting in an agreement with Ivanhoe Mines that allowed for acquisition of up to 100% of the project. Ivanhoe Mines Mongolia Inc. commenced exploration in May 2000, and achieved 100% of the project in 2002. Key moments in that program were a drill intersection in late 2001 of 508 m of 0.81% Cu and 1.17 g/t Au, from 70 m, which was the discovery hole at Southwest Oyu, and in September 2002, a hole that passed through 638 m at 1.6% Cu, from 230 m, and was the first intersection of what was to later be known as the Hugo Dummett South deposit. Drilling moved progressively to the north, until in January 2003, a 300 m step-out from known mineralisation intersected 164 m @ 4.0% Cu, 1.42 g/t Au, which was within the Hugo Dummett North deposit. Exploration on titles held in joint venture with Entrée Gold to test a deep, 3 km long, geophysical (IP) anomaly extending south from Southwest Oyu, discovered the Heruga deposit in 2007, with intersections such as 298 m @ 0.63% Cu, 0.29 g/t Au, 229 ppm Mo from a depth of 636 m.
In 2006, Rio Tinto entered into an agreement with Ivanhoe Mines and in July 2012 obtained majority control of the latter which was renamed Turquoise Hill Resources.
In early 2011, a joint venture between Ivanhoe and BHP Billiton Ltd. discovered a new zone of shallow Cu-Au-Mo mineralisation, Ulaan Khud North, ~10 km north of the Hugo North deposit.
The first copper concentrate was produced from the Oyu Tolgoi mine and exported in 2013. For more detail see Crane and Kavalieris, 2012.
Regional Setting
The Oyu Tolgoi deposits lie within the Middle Palaeozoic Kazakh-Mongol magmatic arc, part of the Transbaikal-Mongolian orogenic collage (Yakubchuk, 2002; 2005), which with the Altaid tectonic collage to the west (Sengör et al., 1993; Sengör and Natal'in, 1996; Yakubchuk, 2004), forms the 5000 km long, by up to 1000 km wide, Central Asian Orogenic Belt (Jahn et al., 2000) that extends from the Urals in the west to the Pacific coast in the east.
Within the Central Asian Orogenic Belt, a string of major porphyry copper deposits (e.g., Almalyk or Kal'makyr-Dalnee in Uzbekistan; Bozshakol, Kounrad, Koksai and Aktogai in Kazakhstan, Aksug in Russia, Tuwu and Duobaoshan in China, and Erdenet and Oyu Tolgoi in Mongolia) are hosted by subduction related magmatic arcs that developed from the late Neoproterozoic, through the early, mid and late Palaeozoic, up to the Jurassic intra-cratonic extension. Deposits are predominantly on the southern palaeo-Tethys Ocean margin of the proto-Asian continent, but are also associated with the closure of two rifted back-arc basins behind that ocean facing margin (e.g., Yakubchuk, 2004; Seltman and Porter, 2005; Seltman et al., 2014).
Although major deposits occur from the Ordovician (e.g., Bozshakol) through to the Triassic (e.g., Erdenet), the most prolific interval of ore formation, including the largest deposits, was during the Late Devonian and Early Carboniferous (Yakubchuk et al., 2002), prior to the amalgamation of Pangea during the late Carboniferous. Major orogenic gold deposits (e.g., the Permian Muruntau in Uzbekistan and Kumtor in Kyrgyzstan) also occur within the Central Asian Orogenic Belt.
Mongolia has been subdivided into two domains by the continental scale Irtysh fault (Yakubchuk et al., 2012), locally known as the Main Mongolian lineament in Mongolia (Badarch, 2005). This structure has a sinistral offset of several hundred kilometres and in western China, has been shown to comprise a crustal-scale, north vergent thrust that was active in the Permian (Briggs et al., 2007). It most likely telescoped and juxtaposed the two domains from different sections of the orogenic belt, rather than exerting an original temporal control on arc development.
The northern domain in Mongolia is characteristically a Caledonian orogen, containing Proterozoic and Lower Palaeozoic rocks, including the extensive late Neoproterozoic to Ordovician Tuva-Mongol magmatic arc. The domain includes a major, fault dislocated cratonic fragment of Precambrian rocks, and a series of back- and fore-arc sequences, Proterozoic to Lower Palaeozoic intrusions, and accretionary wedge sequences that include Late Neoproterozoic (Vendian) to Early Cambrian ophiolites.
The southern domain is largely a Mid- to Late Palaeozoic (Hercynian) orogen, dominated by the Kazakh-Mongol magmatic arc. This magmatic arc comprises a 100 to 275 km wide arcuate swathe of fault separated, arc-related terranes in southern Mongolia, following the southern border between Mongolia and China, with a trend that curves from NE in the east of Mongolia, to WNW where it passes into western China. The individual terranes are predominantly composed of Devonian to Carboniferous island arc volcanic rocks, but also include sporadic Ordovician and Silurian volcanism, as well as Ordovician to Carboniferous sedimentary rocks, and are extensively intruded by voluminous Permo-Carboniferous granitoids in the south.
The Kazakh-Mongol magmatic arc terranes are bounded by tectonically complex suites of underlying and laterally equivalent terranes, composed of Lower to Upper Palaeozoic back- and fore-arc sequences, slivers of Cambrian to Silurian sepentinite and gabbro, and basement Meso- to Neoproterozoic metamorphic rocks, and Proterozoic to Lower Palaeozoic intrusions. As in the northern domain, these elements are also in part separated by intra-arc, suture-related, accretionary wedge sequences.
In the both the north and south domains, the respective Caledonian and Hercynian terranes are succeeded by Permo-Triassic magmatic arcs.
To the south, the Manchurides (e.g., Mihalasky et al., 2015), or the Hingan Orocline of Yakubchuk (2005), comprise Proterozoic to Mesozoic rocks that were involved in orogenesis during the early Triassic collision between the North China craton (and Tarim microcontinent) to the south, and the previously accreted southern Central Asian Orogenic Belt terranes in the north. This collision is marked by the Solonker suture. To the south of Oyu Tolgoi, the Manchurides comprise i). a collage that included Ordovician to Silurian arc and accretionary wedge sequences that had been accreted to the North China Craton during the Lower Palaeozoic; ii). the southern margin of the previously accreted Central Asian Orogenic Belt; iii). variable Permo-Triassic arc-related volcanic sequences, and iv). voluminous Permo-Triassic granitoids that intruded all of these successions, as well as the adjacent Kazakh-Mongol arc successions and the margin of the North China Craton.
In the northern domain, the Mongol-Okhotsk suture marks the similar aged, progressive west to east, scissor like closure of the Mongol-Okhotsk sea that separated the main, previously accreted Central Asian Orogenic Belt from the Siberian craton, resulting in formation of the Mongolian Orocline. This closure was accompanied by formation of the Permo-Triassic Selenge-Gobi-Khanka magmatic arc, comprising extensive volcanism and intrusion.
The Oyu Tolgoi deposits are hosted within the Gurvansayhan Terrane of the southern domain, which is located in the central-southern section of the Kazakh-Mongol magmatic arc, where it trends roughly east-west and is at its widest. The 600 x 250 km triangular shaped terrane is bounded to the north and SE by the major, sinistral, Gobi-Hinggan and East Mongolian faults respectively. These faults merge to the east, but diverge to the SW, where the terrane is terminated by a WNW trending accretionary wedge complex (the Zoolen Terrane of Badarch, 2005, or the more extensive Atasbodg accretionary wedge of Yakubchuk et al., 2002). This wedge complex is composed of thrust sheets, tectonic slivers and mélanges containing Ordovician to Devonian tholeiitic pillow basalts, andesite, tuff and sedimentary rocks that range from olistostromes, through conglomerates, sandstones and argillites, to minor limestones, as well as minor peridotite, serpentinite and gabbro, and highly deformed, mylonitised and metamorphosed equivalents (Zonenshain et al., 1975; Badarch, 2005 and sources cited therein).
According to Yakubchuk (2005) and Yakubchuk et al., (2012), there is up to 400 km of displacement (including later reactivation), along the East Mongolian fault, mainly during the Mesozoic.
The Gurvansayhan terrane is predominantly composed of dismembered ophiolite mélanges, Ordovician to Silurian greenschist facies sandstones, argillite, chert and volcaniclastic rocks, Late Silurian to Mid-Devonian radiolarian chert, tholeiitic pillow basalt and andesitic tuff, overlain by Late-Devonian to Lower Carboniferous basaltic to dacitic volcanosedimentary rocks, chert, sedimentary rocks, and intermediate to felsic volcanic rocks. All of these are cut by extensive late Devonian granitoids and by Permo-Carboniferous diorite, monzodiorite, granite, granodiorite and syenite bodies, ranging in size from dykes, to batholiths that are tens of kilometres across (Lamb and Badarch, 1997; Badarch et al., 2002; Badarch, 2005). In the upper parts of the succession, mafic to intermediate volcanic units evolved from juvenile Devonian, to more mature Carboniferous calc-alkaline compositions, taken to be a reflection of progressive marine arc maturity and thickening, whilst felsic rock suites are dominantly high-K calc-alkaline, regardless of age (Crane and Kavalieris, 2012).
The original architecture of the magmatic arc and constituent sequence has been disrupted by intrusive masses, modified by both Mid- to Late-Palaeozoic accretion and Mesozoic thrust and sinistral strike-slip faulting, and masked by alluvium on a mature surface. This has resulted in a complex of imbricate thrust sheets, dismembered blocks, mélanges and high strain zones. In the type area, well to the north of Oyu Tolgoi, the arc also includes ~420 Ma serpentinite and oceanic basalts, which have a mid-ocean ridge basalt (MORB) geochemical signature (Ruzhentsev and Pospelov, 1992; Lamb and Badarch 1997; Badarch et al., 2002). Helo et al. (2006) suggest these basalts may belong to the fore-arc region of a juvenile island arc of the Gurvansayhan terrane. At Oyu Tolgoi, similar, but poorly dated basalts (Perelló et al., 2001), which have an island-arc affinity, are intruded by the Late Devonian quartz-monzodiorite of the Oyu Tolgoi porphyry system (Crane and Kavalieris, 2012). Dacitic extrusive rocks that are of a very similar age (Wainwright, 2011) to these quartz-monzodiorites, but most likely not comagmatic, are separated from the basalts by a significant hiatus, suggesting the Oyu Tolgoi porphyry system is not directly genetically related to older Gurvansayhan terrane island-arc basalts (Crane and Kavalieris, 2012). These sequences are unconformably overlain by Carboniferous successions of continental margin affinity (Yarmolyuk et al., 2008), in both the Oyu Tolgoi district and elsewhere.
During the late Palaeozoic, southern Mongolia underwent a period of basin and range style extension, accompanied by bimodal, basalt-peralkaline granite-comendite magmatism (Kovalenko and Yarmolyuk, 1995) in a mature continental setting (Perelló et al., 2001 and references cited therein). This included the abundant Carboniferous and Permian intrusive complexes of the Gurvansayhan terrane, (e.g., the mid Carboniferous Northwest Granitic Complex and Javkhlant granodiorite to the west and south of Oyu Tolgoi, and the Late Permian sodic alkalic Khanbogd granite to the east; Fig. 3), which appear to be closely related to the major northeast-trending structural zone, the East Mongolian or Zuunbayan fault zone, which cuts obliquely across the arc and forms the south-eastern boundary of the terrane (Crane and Kavalieris, 2012).
To the north, the Gurvansayhan terrane is juxtaposed against another magmatic arc, the Mandalovoo terrane, by the large sinistral Gobi-Hinggan fault zone. This latter terrane is long (>2000 km) and narrow (<100 km), composed of Ordovician to Carboniferous volcanic and sedimentary rocks, including some Lower Palaeozoic volcanic rocks, but mainly Ordovician to Silurian and Lower to Middle Devonian sedimentary rocks (conglomerate to limestone), Upper Devonian pillow basalt, andesite tuff and volcaniclastic sedimentary rocks, Lower Carboniferous marine sediments, as well as Devonian dioritic intrusions (Badarch, 2005 and references cited therein). This arc grades northwards into the equally extensive Gobi-Altai back-arc basin, containing basinal equivalents of the same rocks that are among the least deformed in Mongolia, and are bounded to the north by the Irtysh fault zone (Badarch, 2005).
The Baydrag and Edren terranes comprise Mid to Late Palaeozoic volcanic arc rocks. Lower Palaeozoic sequences appear to be absent from both of these terranes, which predominantly contain Devonian to Carboniferous sequences. The Baydrag terrane, further to the SW, has a much higher volcanic content, including Lower Devonian tholeiitic basalt and andesites, and Mid to Upper Devonian volcaniclastic sedimentary rocks and Lower Carboniferous limestones, which further west in China contains a 479±27 Ma ophiolite suite. The Edren terrane is intruded by Permian alkaline granitic plutons (Badarch, 2005).
To the south of Oyu Tolgoi, a further belt of magmatic arcs and cratonic terranes are found in fault contact with the Gurvansayhan terrane, paralleling the main Mandalovoo terrane arc to the north. These include the Hashaat and Enshoo volcanic terranes of Badarch et al. (2002) and Badarch (2005). The cratonic terrane immediately to the south of the Enshoo arc expands to become the more extensive Songlai basement block further to the NE in the Hegenshan (Hinggan Range). The Enshoo island arc terrane contains variably metamorphosed quartzo-feldspathic gneisses and schists, Devonian calc-alkaline basalt, andesite, dacite, tuff, volcaniclastic rocks and minor limestone, and Carboniferous shallow marine sedimentary and volcanic rocks. These are succeeded by Permian andesite, dacite, tuffaceous sandstone, siltstone, shale and limestones of the Manchuride overlap regime (Badarch, 2005). In the Hashaat magmatic arc terrane, which continues well to the west into China, Proterozoic basement marble and quartzite, is overlain by Ordovician greenschist facies andesite, tuff, volcaniclastic turbidites and minor limestone, followed by Devonian pillow basalt, andesite, gabbro, chert and volcaniclastic rocks, and then by Lower Carboniferous conglomerate, sandstone, siltstone and chert (Badarch, 2005).
Although faulting has shuffled the individual terranes laterally by up to several hundred kilometres, the broad Gurvansayhan terrane is wedged between two narrow parallel, semi-contemporaneous magmatic arcs represented by the Mandalovoo and Hashaat-Enshoo magmatic arc terranes. The latter has been interpreted as a separate arc and cratonic sliver, that converged northward towards the accreted Central Asian Orogenic belt, ahead of the approaching North China craton (Badarch et al., 2002). The broader, more heavily mineralised, Gurvansayhan terrane has been interpreted to have been transported eastward along the Gobi-Hinggan and East Mongolian fault complex (Yakubchuk, 2005), and may have originally been related to the Baydrag terrane, and represented a separate arc docked between the Mandalovoo and Hashaat-Enshoo terranes.
The amalgamation of continental blocks and magmatic arcs within the main Central Asian Orogenic Belt was largely completed by the end of the Palaeozoic (according to Enkin et al., 1992; Sengör et al., 1993; Hendrix et al., 1996), to be fully assembled during the early Mesozoic (Ruzhentsev et al., 1985; Zonenshain et al., 1990; Meng and Zhang, 1999), accompanied by widespread uplift and associated thrusting which unroofed the magmatic arcs. This corresponded to the approach and amalgamation of the North China craton from the SW, accompanied by development of the Manchurides, and rotation and formation of the Mongolian orocline (and Mongol Okhotsk suture) between latest Carboniferous and Early Triassic (Edel et al., 2014). During this period, early Mesozoic continental sediments were deposited in thrust-controlled foreland basins over the Gurvansayhan terrane (Hendrix et al., 1996). Major regional structures related to this period in western and central Mongolia include the SE-trending Gobi Altai Fault system, which forms a complex zone of sedimentary basins, over-thrust by basement blocks, to the NW of, and into, the Gurvansayhan terrane, and the overprinting, generally east-west trending Gobi-Tien Shan sinistral strike-slip fault system that splits eastward into a number of splays in the western half of the terrane.
Extensive intracontinental rifting and subsidence took place in southeastern Mongolia during the Late Jurassic to Early Cretaceous, with associated uplift of metamorphic core complexes (Webb et al., 1999), and formation of syn-rift basins with thick (up to 2 km) sedimentary accumulation of at least five distinct stratigraphic sequences. Deposition was controlled by strike-slip movements on NE-SW structures, including the reactivated Zuunbayan and Gobi-Hinggan fault zones (Johnson, 2004). These cover rocks preserved the porphyry deposits of the Gurvansayhan terrane. Deposition of alluvial plain and aeolian red beds continued into the Late Cretaceous.
This extension was followed by Cenozoic transpressional tectonic events related to the Himalayan collision (Cunningham, 2010), which dominate the present-day regional structure of southern Mongolia, with the onset from the Late Cretaceous of increasingly arid conditions (Jerzykiewicz et al., 1993; Jerzykiewicz, 1998; Loope et al., 1998).
Geology
The sequence surrounding Oyu Tolgoi is predominantly composed of Devonian (or possibly in part older) basaltic to intermediate volcanic and volcaniclastic rocks, overlain by layered dacitic pyroclastics and sedimentary rocks with basaltic doleritic sills. Layered Carboniferous basaltic to trachyandesitic pyroclastic and sedimentary rocks, also cut by doleritic sills, unconformably overlie and are in fault contact with the Devonian hosts. Several large masses of granitic rocks surround the deposit, the largest being the Permian 287±2 Ma (K-Ar) Khanbogd Mountain per-alkaline granite, 5 km to the east, while the Carboniferous 308±2 Ma (U-Pb zircon) Javhalant Mountain Batholith is a similar distance to the south and the 348±2 Ma (U-Pb zircon) North Granite is closer to the north and west (Perelló, et al., 2001).
The Oyu Tolgoi porphyry Cu-Au-Mo deposit complex comprises four discrete zones hosted at different stratigraphic levels within a sequence of Devonian (or older) juvenile calc-alkaline basaltic volcanic and lesser pyroclastic rocks, unconformably overlain by Upper Devonian dacitic clastic sedimentary and volcaniclastic rocks. The Cu-Au-Mo mineralisation is associated with Late Devonian intermediate- to high-K, porphyritic quartz-monzodiorite, emplaced as structurally controlled dykes and small plugs, followed by Late Devonian, post-mineral, biotite granodiorite intrusions.
The pre-Carboniferous stratigraphy of the Oyu Tolgoi district has been informally divided into two main units, the Heruga and Oyu Tolgoi sequences, separated by the Late Devonian (365.3±1.5 to 367±3 Ma) district-scale, reverse, Contact fault. This fault has been folded, and parallels the bedding at the top of the Oyu Tolgoi sequence, extending for >12 km, from the Heruga deposit in the south, to Hugo Dummett in the north, and dips at moderate to steep angles to the ESE. The upward facing, and relatively undeformed, Oyu Tolgoi sequence lies in the footwall of the Contact fault, where it hosts the Oyu Tolgoi deposits, and has only limited surface exposure, mainly around the Southwest and South Oyu deposits, and locally to the west of the Heruga deposit. The allochthonous Heruga sequence, in the hanging wall of the Contact fault, is most likely overturned, and outcrops extensively in the south. Although it's relationship to the Oyu Tolgoi sequence is uncertain, it is believed to be older (Crane and Kavalieris, 2012).
The stratigraphic sequence in the Oyu Tolgoi area is as follows, from the oldest units (after Crane and Kavalieris, 2012):
The Heruga sequence (unit DA4), has not been reliably dated, but is regarded as Late Devonian or older, and is interpreted to be older than the Oyu Tolgoi Sequence, but throughout the Oyu Tolgoi district is allochthonous, and structurally overlies the latter. It is extensively exposed to the south, around Heruga, and has been intersected in drill holes overlying deposits from Heruga in the south, to Hugo Dummett in the north. The sequence, which is ~1.5 km thick, is only weakly altered to unaltered, and is typically structurally disrupted and mostly overturned. It primarily comprises an upper succession of dark green basaltic flow breccias with vesicular, fine-grained to coarsely porphyritic basaltic clasts. These breccias are commonly interstratified with volcanogenic sandstones and conglomerates that appear to be directly derived from the basalts. They stratigraphically succeed a comparable thickness of green to red, finely laminated siltstone with interbedded thick, massive, green, volcanogenic sandstone. Subordinate, but characteristic, red to green, basalt-trachyandesitic hyaloclastite includes ash to lapilli-sized, tuff-like beds (partially densely welded) in addition to angular boulder beds interbedded with reworked polymictic conglomerate and fine-grained laminated siltstone. South of Heruga, sills and dykes of the same composition are intimately associated with the hyaloclastite. Rare crinoid fossil detritus in siltstones, and possible poorly developed pillow textures in basalts, suggest the sequence was deposited in a subaqueous marine environment (Crane and Kavalieris, 2012; Peters et al., 2012).
Basaltic breccias within the Heruga sequence are classified as normal to high-K, calc-alkaline island-arc basalts, although alkalic intraplate basalts are also recognised in the same succession (Crane and Kavalieris, 2012). As such it has different trace element composition to the structurally underlying Oyu Tolgoi sequence (Wainwright et al., 2011a) and may be exotic relative to that sequence.
Minjin et al. (2005) correlated the Heruga sequence with the upper Khalzan-Ovoo Formation of the Late Devonian Alagbayan Group, although Crane and Kavalieris (2012) regard this correlation as uncertain.
Crane and Kavalieris (2012) subdivide the sequence as follows, from the base (assuming it is overturned):
• Red and green laminated siltstones with greywacke sandstone interbeds;
• Red and green alkalic basalt breccia/conglomerate, characterised by fine green needles of hornblende;
• Basaltic volcanic breccia, with a thinner wedge of basaltic-trachyandesite hyaloclasite rocks at the top;
• Medium to fine basaltic volcaniclastic rocks, with minor breccia and siltstone;
• Basaltic volcanic breccia.
The Oyu Tolgoi sequence, which is mainly Late Devonian in age, has been subdivided, as follows from the base:
• Unit DA1a - An unknown thickness of grey to green, finely laminated, volcanogenic siltstone and interbedded fine sandstone, which contains fine- and coarse-graded units, possibly of turbiditic origin, is the lowest unit of the sequence below all of the deposits. It passes upwards, across a transitional contact, into augite basalt and associated volcaniclastic rocks of the overlying unit.
• Unit DA1b - Massive, dark green, coarsely porphyritic augite basalt, which characteristically contains ~35% augite phenocrysts that are 5 to 10 mm across in a fine-grained matrix of plagioclase and augite. The basalt forms an up to >1 km thick volcanic complex, extending over a known area of 12 x 3 km along the Oyu Tolgoi trend. Overall, it is internally massive, without evidence of individual flows, pillow structures, or vesicles, and lacks any flow alignment fabric in thin section. The massive augite basalt appears to be transitional with basaltic volcaniclastic rocks (or peperites) towards the base, and to the west of the Southwest deposit, where it comprises a coarse sandy matrix supporting irregular, up to 10 to 20 cm clasts. To the north of Hugo Dummett, the complex tapers to <200 m in thickness, with a basal siltstone clast breccia above underlying laminated siltstones (Crane and Kavalieris, 2012; Peters et al., 2012).
Wainwright (2008) and Wainwright et al. (2011a) classified the augite basalts as being of primitive tholeiitic basaltic magmatic arc origin. However, on the basis of their whole rock geochemistry, Crane and Kavalieris (2012), re-classified the same rocks as juvenile calc-alkaline island-arc basalts, with characteristically low Al2O3 (<16%), relatively high K2O, and flat REE patterns. In addition, they have high TiO2 contents, which suggests they are alkalic.
• Unit DA1c - Although obscured by patchy, porphyry related alteration, an interval of tens of metres thick in the upper section of the augite basalt, appears to be fragmental, possibly a volcaniclastic (Crane and Kavalieris, 2012). It is described as a monomictic to slightly polymictic basaltic lapilli tuff to volcaniclastic conglomerate/breccia, occurring above or partially interbedded with the massive augite basalt by Peters et al. (2012).
The Unit DA1 stratigraphy has not been reliably dated, and may be older than Late Devonian (Crane and Kavalieris, 2012).
• Erosional unconformity.
• Unit DA2a - Coarse, poorly sorted, polymictic volcanic conglomerate and minor sandstone, dominated by porphyritic andesitic volcanic clasts up to 2 m in diameter, accompanied by lesser augite basalt, trachyandesite and quartz-diorite fragments, although quartz-monzodiorite clasts are absent. Clasts at the base of the unit are commonly highly angular, and may represent an eruptive breccia that has been reworked and incorporated into conglomerate. It is characterised by the presence of quartz vein fragments, and less commonly, altered and mineralised feldspar porphyritic intrusive clasts (Savage, 2010; Peters et al., 2012). This unit overlies the unconformity at Heruga, where it is dominantly a polymictic volcanic conglomerate, and in the Wedge Zone and sections of Hugo South, although it is absent at Hugo North where the succeeding dacitic pyroclastic rocks directly overlie the unconformity (Peters et al., 2012).
• Unit DA2b - Dacitic pyroclastic deposits, occurring as two facies, corresponding to dacitic block and ash tuff (unit DA2b1), and dacitic ash flow and lapilli tuffs (unit DA2b2) respectively. The two interfinger locally, but the dacitic block and ash tuff generally occurs lower in the section. The dacitic block and ash tuff is dominated by dacitic volcanic clasts that are up to several tens of centimetres in size. The dacitic ash flow tuff shows evidence of semi-molten blocks, fiamme and altered igneous clasts and generally lacks lithic fragments, and is regarded to be ignimbritic (Peters et al., 2012). It has been dated at 367±3 Ma by SHRIMP-RG zircon and 369±2 Ma by TIMS U-Pb (Wainwright et al., 2011a). This date is bracketed by the ages of the main causative porphyritic quartz-monzodiorite intrusion. The dacitic ash-flow tuff has whole rock geochemical similarities to the quartz-monzodiorite, and could be comagmatic, although it differs in its high TiO2 content and less pronounced concave REE pattern (Crane and Kavalieris, 2012).
The combined unit DA2 is generally >200 m thick.
• Unit DA3 - Dark grey, carbonaceous, laminated siltstone-sandstone, >300 m thick, which overlies the dacitic tuff and conglomerate, and is characterised by silicified carbonate nodules and conglomerate lenses, with quartz, chert and siltstone clasts toward the base (Crane and Kavalieris, 2012). This clastic sequence is only weakly altered and comprises two main rock types, commencing with a coarser facies, which is characterised by polymictic conglomerate, sandstone and siltstone (DA3a), that is abundant in the Southern Oyu Tolgoi and Heruga deposits and parts of Hugo South. The conglomerate typically has a muddy matrix, and is transitional downward to boulder conglomerate and volcanic breccia, where it directly overlies the dacite sequence. A finer facies of rhythmically interbedded siltstone, mudstone carbonaceous mudstone and fine brown sandstone (DA3b) forms a thick lithologically distinctive unit, and is ubiquitous throughout the Oyu Tolgoi deposit area. It usually overlies a relatively thin layer of DA3a, or is directly on the top of the dacite where the conglomeratic facies is absent (Peters et al., 2012).
Wainwright (2008) and Wainwright et al. (2011a) divided the Oyu Tolgoi sequence above the unit DA1b augite basalt into a lower mineralised fragmental suite and an unconformably overlying upper unmineralised dacite tuff, and concluded that the mineralising quartz-monzodiorite was emplaced in the time gap represented by the unconformity. However, Crane and Kavalieris (2012) interpret the contact between unmineralised and mineralised suites (where not faulted or intruded by dykes), to be an alteration front, with rocks above and below being lithologically similar, except where the alteration front corresponds to the unconformity between augite basalt below, and conglomerate or ignimbrite above. They note the presence of quartz and altered igneous clasts in fresh ignimbrite and conglomerate and interpret that they came from an older system, but not necessarily from the altered rocks immediately below.
According to Crane and Kavalieris (2012), at some time following deposition of the 367±3 Ma dacitic ash-flow tuff in the upper section of the Oytu Tolgoi sequence, but prior to deposition of the Carboniferous Sainshandhudag Formation, in the Oyu Tolgoi district, the Heruga sequence was overthrust, as a recumbent fold, onto the Oyu Tolgoi sequence along the Contact fault, uplifted and partially eroded.
The Sainshandhudag Formation, which comprises volcano-sedimentary rocks of Early Carboniferous age (Minjin et al., 2005). It occurs in subcrop over more than half of the Oyu Tolgoi district, and it is separated by a major unconformity from the underlying Heruga sequence. In contrast to the Heruga and Oyu Tolgoi sequences, it is readily correlateable with sequences throughout the South Gobi region. The underlying Heruga sequence had been overturned prior to deposition of the Sainshandhudag Formation above an unconformity that is also recognised throughout southern Mongolia (Minjin et al., 2005).
The Sainshandhudag Formation was important in insulating the Oyu Tolgoi mineralisation from erosion by rapidly burying it during the early Carboniferous, under mass-wasted and pyroclastic material from the volcaniclastic apron of the post-mineral dacitic volcanoes.
At Oyu Tolgoi, the Sainshandhudag Formation is ~1 km thick and has been subdivided, from the base, into:
• Ulgii Member (unit CS1), which comprises 50 to 200 m of andesitic to dacitic ash flow tuffs, with local sandstone and siltstone, and boulder conglomerate at the base. It has been dated at 354±2 Ma age (early to mid Tournaisian; Wainwright et al., 2011).
• Tsagaan Suvarga Member (unit CS2), a 50 to 200 m thick sedimentary sequence comprising, from the base up,
- a distinctive 2 m thick 'coaly' carbonaceous bed;
- a sequence of fluvial conglomerate and sandstone with thin coal beds; and
- an upper shallow marine sequence of fossiliferous bioturbated siltstones with interbedded waterlain tuffs.
• Aman-Us Member (unit CS3), comprising up to 1000 m of basaltic trachyandesite flows and flow-related fragmental rocks, with an erosional base overlying the Tsagaan Suvarga Member, subdivided into:
- an ~100 m thick basal sequence, comprising pink volcanic sandstone, conglomerate and intercalated andesitic lava that has a characteristic crowded texture defined by aligned feldspar phenocrysts;
- a thin unit of basaltic andesite lava;
- a thick suite, >500 m, of basaltic volcanic rocks, mainly basaltic trachyandesite breccia and flows, which include peperites, and are commonly red, due to hematite alteration/oxidation, and locally contain native copper.
Basalts within this sequence have typical subduction-related geochemical signatures and moderately fractionated REE patterns, and may be assigned to the high-K calc-alkaline series (Crane and Kavalieris, 2012).
Throughout southern Mongolia, the Sainshandhudag Formation represents a period of slow deposition in a stable environment. Detrital quartz, K feldspar grains and granitic clasts suggest a continental origin for these sediments, while marine fossils across the region are all of similar Early Carboniferous (late Tournaisian-early Visean) age (Minjin et al., 2005).
Cretaceous clays, sandstones and conglomerates fill a 6 km diameter basin-shaped depression above the Hugo Dummett North and South, Central and Southwest deposits. It is interpreted to represent a fault scarp-bounded internal drainage basin in an arid environment (Hendrix et al., 1992; Perelló et al., 2001). The sequence is thickest (~100 m) adjacent to a fault contact with Early Carboniferous granodiorite, the Granodiorite Boundary fault. The dominant lithology is a red montmorillonitic clay, with lesser thin carbonate-cemented sandstone and poorly sorted conglomerate lenses, which are suggested to be of Cretaceous age, based on dinosaur eggs found near the base of the sequence near Hugo Dummett South.
Bedrock weathering at Oyu Tolgoi was related to the Cretaceous palaeosurface, as reflected by the Cretaceous K-Ar dates of supergene alunite at the Central deposit (Perelló et al., 2001), and extends to depths of 60 to 100 m below that surface.
Recent aeolian sand and gravel cover, up to 3 m thick, blankets much of the area, commonly including a gypsum layer immediately above bedrock.
Intrusive rocks in the Oyu Tolgoi district range from Late Devonian or older to Jurassic in age, and from gabbro to granite in composition, as follows in chronological order (after Crane and Kavalieris, 2012):
• Quartz-monzodiorite of Late Devonian age (374±3 to 364±4 Ma; Wainwright et al., 2011a). A porphyritic quartz-monzodiorite constitutes the main causative intrusion related to the Oyu Tolgoi porphyry copper deposits. It passes downward into equigranular varieties at deeper levels, where it represents the top of a larger intrusion, which below the Southern Oyu Tolgoi deposits, is 3 km wide. This lower, voluminous, massive quartz-monzodiorite may also contain weaker mineralisation, flanking and underlying the high-grade ore zones.
Quartz-monzodiorite intrusions that are apparently similar, accompany all of the deposits at Oyu Tolgoi, although deformation and alteration locally mask subtle distinctions. Multiple stages of intrusion are recognised at Southwest Oyu, and most likely also occur at the other deposits. The quartz-monzodiorite varies from early, strongly quartz veined and mineralised dyke-like bodies that are either porphyritic or fine grained, to moderately and poorly mineralised and altered equigranular intrusives, all of which may occur in close proximity to ore. This suggests multiple generations of syn-, late- and post-mineral intrusions. In the core of the Southwest Oyu deposit, younger, fine grained, xenolithic quartz-monzodiorite dykes entraining early quartz vein clasts, but lacking strong sulphide mineralisation, are also evident. Progressive intrusion is generally coaxial, with newer phases not breaching earlier outer shells.
The porphyritic quartz-monzodiorite has a characteristic crowded-crystal texture, composed of ~45 vol.% elongate plagioclase phenocrysts up to 8 mm long, with 15 to 20% K feldspar occurring as discrete euhedral crystals, or more commonly intergrown with quartz in the groundmass. The equigranular varieties are generally coarse grained and contain 20 to 30% of 2 to 4 mm plagioclase, a similar proportion of 2 to 4 mm hornblende and 10 to 20% of 1 to 3 mm K feldspar, with minor quartz and biotite, and accessory zircon, apatite and magnetite. In the deepest samples, below the Southwest Oyu deposit, relatively unaltered quartz-monzodiorite contains up to 20% each of hornblende and biotite, with augite, lesser magnetite (~5%) and titanite (~2%). Where unaltered, these rocks commonly have a distinct red colouration, although more typically, the quartz monzodiorites are grey-green to buff-white, cut by quartz veins, sericite-altered with abundant disseminated and vein-hosted pyrite and copper sulphides (Wainwright, 2008).
Age dating by Wainwright (2008), discussed in Wainwright et al. (2011), conducted on multiple zircons from 5 samples using SHRIMP-RG zircon 207Pb-corrected 206Pb/238U, range from 374±3 to 368±3 Ma, whilst more precise U-Pb TIMS ages are more closely clustered between 371±1 and 372±1 Ma. Another sample from the larger, poorly mineralised quartz-monzodiorite pluton to the west of Central Oyu, returned younger ages of 367.9±3.4 Ma (SHRIMP-RG zircon only).
Within the limits of accuracy, these dates for the porphyritic quartz-monzodiorite intrusion are close to and overlap the 367±3 Ma obtained for the dacitic ash-flow tuff of unit DA2b2 in the upper part of the Oyu Tolgoi sequence, and the two may have originated from the same deep pluton.
The quartz-monzodiorite is characterised by a typical subduction-related geochemical signature, and appears to belong to the high-K calc-alkaline series. Based on multi-element plots, they may be of island-arc provenance rather than continental margin affinity (Crane and Kavalieris, 2012).
To the east of the Central and Southwest Oyu deposits, the upper margin of the quartz-monzodiorite, appears to be sub-parallel to the enclosing volcanic and sedimentary units of the Oyu Tolgoi sequence, as well as to the Contact fault. However, while dacitic tuffs of Oyu Tolgoi sequence units DA2b1 and 2 are mineralised, the quartz-monzodiorite has not been seen to intrude any higher in the sequence than the underlying volcanic conglomerates and sandstones of unit DA2a at Heruga, where lithological distinctions are not masked by alteration (Crane and Kavalieris, 2012). However, geological interpretations in the more strongly altered Hugo South, Central and Southwest Oyu areas infer that the quartz-monzodiorite does intrude into the DA2b2 dacitic ash flow tuffs to as high as the base of unit DA3 sedimentary rocks.
• Hydrothermal breccias are not common or volumetrically significant at Oyu Tolgoi. At the Central Oyu deposit, local early hydrothermal breccias dominantly contain angular quartz-monzodiorite and quartz-vein clasts, while in the core of the Southwest Oyu deposit, similar fragmental rocks are found, comprising shard-like clasts of intensely biotite altered basaltic wall rock, quartz vein fragments, and <10 vol.% quartz-monzodiorite clasts (the latter in some cases milled). Later pulses of the quartz-monzodiorite intrude these breccias. Late, rare porphyry-related breccias are only represented by thin tuffisite or pebble dykes (several cm wide) in the upper parts of the alteration system.
• Post-mineral biotite granodiorite intrusions of Late Devonian age (U-Pb SHRIMP-RG zircon ages that range from 367±2 Ma to 363±2 Ma, and a single U-Pb TIMS age of 365.3±1.5 Ma; Wainwright et al., 2011a). This suite typically intruded the Oyu Tolgoi sequence as dykes and sills, and also occur as sills along the Contact fault. They do not cut the Heruga sequence, except in the Hugo Dummett deposits, where they flare upward through the Contact fault, breaching the Heruga sequence, above the zone of highest grade mineralisation. A similar upward flaring of the biotite granodiorite occurs at the Ulaan Khud prospect to the north of Hugo Dummett. These dykes and sills are compositionally and texturally varied and probably include several intrusive phases. The age date quoted above marginally post-dates that of the dacitic ash-flow tuff in the Oyu Tolgoi sequence unit DA2b2 (Crane and Kavalieris, 2012).
The granodiorite porphyry suite occurs as both a coarse, sparsely porphyritic, and a fine-grained, crowded porphyritic phase. The first is characterised by 40% of
4 to 6 mm euhedral to subhedral, locally zoned plagioclase phenocrysts, ~10% of 0.5 to 1 mm quartz phenocrysts, ~10% of 1 to 2 mm biotite phenocrysts, all set in a fine-grained, granular to aphanitic, grey-brown to beige-yellow feldspathic groundmass with abundant apatite micro-phenocrysts. The fine-grained, crowded porphyritic phase comprises ~40% of 0.5 to 3 mm euhedral to subhedral plagioclase phenocrysts, 10% of 0.5 to 1 mm quartz phenocrysts, 5 to 10% fine-grained biotite, in a brown aphanitic to microcrystalline groundmass with abundant apatite and zircon (Wainwright et al., 2011). It is relatively more felsic, and poorer in ferromagnesian and opaque minerals compared to the quartz-monzodiorite, which are hornblende-dominant, and has a different REE pattern (Crane and Kavalieris, 2012).
• Carboniferous dykes and small intrusions, which are common at Oyu Tolgoi. All postdate the Early Carboniferous Sainshandhudag Formation, although some dolerite dykes, sills and plugs of this group of intrusive may predate early Carboniferous granodiorite intrusions.
Porphyritic hornblende-biotite andesite and dacite dykes have a similar age (345±2 Ma; Wainwright et al., 2011a) to the Early Carboniferous granodiorite and are probably comagmatic, and are part of a trachyte suite, which may be genetically related to flows and pyroclastic units of the Sainshandhudag Formation. They are typically strongly porphyritic with feldspar, hornblende, and biotite. Quartz phenocrysts are common within the dacite dykes (Wainwright, 2008).
Rhyolite forms dykes and sills, commonly intruded as dilational arrays, but may also occur as volcanic domes. Rhyolite dykes and sills are abundant throughout the deposit area and are up to a few metres to tens of metres in thickness. Dykes typically have steep dips, with variable strike orientations. Intrusive breccias are common along dyke contacts, frequently incorporating both rhyolitic and wall rock fragments within a flow-banded groundmass. Texturally they are aphanitic and aphyric, and have been dated at 330 to 340 Ma (U/Pb zircon; Wainwright et al., 2011a; Peters and Sylvester, 2014).
Basalt and dolerite dykes and small plugs are widespread and are mostly the youngest intrusive rocks recognised at Oyu Tolgoi. Typically, they are aphanitic to fine-grained, and locally vesicular, variably containing plagioclase phenocrysts. In the main deposit area, they occur as dykes from metres to at most a few tens of metres wide, although to the
SW, they also occur as large, sill-like intrusive masses. One of these dykes is a strongly magnetic, north-striking subvertical dolerite that cross-cuts the Hugo Dummett deposit (Peters et al., 2012).
• Large granite and granodiorite intrusions at and surrounding the Oyu Tolgoi area, are grouped into five units (after Crane and Kavalieris, 2012):
- Unit 1 - medium-grained equigranular granite of inferred Carboniferous age, which occurs as a 2 km wide, NE-trending strip, ~5 km NW of the Oyu Tolgoi deposits. It is cut along its eastern margin by a major ductile fault zone, represented by an up to 100 m wide zone of foliated and mylonitic granite, juxtaposing it against mylonitised mafic rocks, greenschist, phyllite and granodiorite.
- Unit 2 - a small, fine-grained granite, to the east of, and separated from Unit 1 by the mylonite zone described above.
- Unit 3 - Early Carboniferous (350±4 Ma; Wainwright et al., 2011a) hornblende-biotite granodiorite which occurs as a lensoid, ~15 x 5 km, NNE oriented pluton, intruding metamorphic rocks and possibly the mylonite zone. The Early Carboniferous (mid-Tournaisian) age overlaps that of the Sainshandhudag Formation, and large granitic boulders common at its basal conglomerate may have been eroded from this granodiorite. The SE margin of the intrusion is juxtaposed against Carboniferous basaltic trachyandesite belonging to the Aman Us Member, and dacitic ring dykes cut the granodiorite.
- Unit 4 - hornblende-biotite granodiorite, which occurs in a number of bodies of possible mid-Carboniferous age (based on nearby rocks dated at 320 to 310 Ma; Amaramgalan, 2008; Wainwright et al., 2011a)). This unit intruded the western margin of the Oyu Tolgoi trend, resulting in moderate hornfelsing of the surrounding country rocks, with zones rich in epidote.
- Unit 5 - the late Permian alkalic Khanbogd Granite (288 to 284 Ma; Kovalenko et al., 2006; Amaramgalan, 2008), which occurs a large, ~35 km diameter, circular body, ~5 km east of the Hugo Dummett North deposit. The complex has a concentric structure, defined by abundant pegmatite dykes enriched in rare earth elements and Zr, and has a flat roof, as indicated by numerous basaltic wall rock roof pendants. It dips outward at a moderate angle away from its exposed margins and has an associated zone of strong hornfelsing about 1 km wide surrounding the contact. It is interpreted to have a lopolithic intrusive form. Contact metamorphic effects from the Khanbogd Granite do not affect the Hugo Dummett deposits, although the porphyry mineralisation at Ulaan Khud, 10 km to the north, is strongly hornfelsed.
Structure
Overview
The Oyu Tolgoi cluster of porphyry deposits postdate the early, arc-related, basaltic volcanic host rocks of the Oyu Tolgoi sequence, and are aligned along the >25 km long, NNE (20°) oriented Oyu Tolgoi trend. The deposits are developed within and over the elongate apex of a larger, buried quartz-monzodiorite intrusion that follows the trend (Crane and Kavalieris, 2012). This trend is part of a poorly defined, ~300 km long, NE striking, arc-oblique, alignment of coeval copper occurrences, that extended to the Tsagaan Suvraga deposit in the north, but has subsequently been dislocated by faulting, and may reflect a highly permeable, crustal scale structure (Yakubchuk et al., 2012).
Locally, within the Oyu Tolgoi deposit area, the expression of this structure is the mainly conceptual Axial Fault, for which direct evidence is largely overprinted by the intrusive phases. Apparent offsets of stratigraphic contacts across the main post-mineralisation biotite granodiorite dyke have been interpreted to support the existence of the structure. The basal contact of the upper Oyu Tolgoi sequence unit DA2 shows an apparent stratigraphic displacement of ~200 to 300 m across the biotite granodiorite dyke. There is no clear evidence for any displacement after emplacement of the dyke, which would restrict the timing of movement to the narrow time interval between deposition of the upper Oyu Tolgoi sequence and intrusion of the biotite granodiorite in the Late Devonian (Peters et al., 2012).
The Late Devonian thrust fault, the Contact fault (described above), parallels the bedding at the top of the Oyu Tolgoi sequence, dominantly following the uppermost carbonaceous, laminated siltstone-sandstone unit DA3, with subordinate parallel structures (e.g., the Lower and Intermediate faults described below) partitioning the underlying porphyry and Oyu Tolgoi sequence. The age of faulting is constrained by the 367±3 Ma dacitic ash-flow tuff (unit DA2) it cuts, and the 365.3±1.5 Ma biotite granodiorite that intrudes it (Wainwright et al., 2011a). The Contact fault and enclosing rocks drape over the deposits, and to the east, extends for 12 km from the Heruga to the Hugo Dummett deposits, and dips at moderate to steep angles to the ESE. The Oyu Tolgoi sequence in the footwall of the Contact fault is upright and relatively undeformed. In contrast, the older Heruga sequence, in the hanging wall of the same fault, is mostly overturned with thicknesses of up to 1 km, implying the presence of major recumbent folding (Crane and Kavalieris, 2012; Ayush, 2006).
In the Heruga deposit area, outcrops of Devonian rocks have a structural grain that differs from that of the typically little deformed Carboniferous Sainshandhudag Formation that unconformably overlie or are faulted against them. The Sainshandhudag Formation dips gently both to the east and west of the Oyu Tolgoi trend, while narrow fault-controlled slivers are found directly above the Heruga and Heruga North deposits, suggesting continued activity along a narrow zone in the core of the trend. Large-scale fold interference patterns in exposures of Devonian rocks below the unconformity, to the east of the Heruga deposits, indicate multiple generations of overprinted deformation, implying large changes in the principal compressive stress regime (Crane and Kavalieris, 2012; Wainwright et al., 2011).
According to the relationships described by Crane and Kavalieris (2012), at some stage during and following deposition of the 367±3 Ma dacitic ash-flow tuff in the upper section of the Oyu Tolgoi sequence, but before emplacement of the 365.3±1.5 Ma post-mineral biotite granodiorite, the Heruga sequence was overthrust, as a large scale recumbent fold, onto the Oyu Tolgoi sequence along the Contact fault, with the uneroded overturned lower limb preserved above the fault.
The subsequent Mid-Carboniferous faults are usually vertical, displace older rocks, and are consumed and truncated by Late Carboniferous granodiorites e.g., the West Bat fault, which has an apparent vertical displacement of ~1.5 km, and appears to define the western margin of, and most likely cut the northwestern side of the Hugo Dummett North deposit. Similar structures displace the Heruga deposit, and form part of a NNE trending fault system across the Oyu Tolgoi district (Crane and Kavalieris, 2012).
A set of three major ENE trending faults, Javkhlant, Solongo and Granodiorite Boundary, displace the Carboniferous granodiorites. The Solongo fault offsets the West Bat fault, with a vertical displacement of ~2 km, and is, in turn, intruded at South Oyu by Late Carboniferous rhyolite (Wainwright et al., 2011a). A subparallel fault system truncates the Solongo fault, and displaces the contact of the Permian Khanbogd Granite. The Javkhlant fault also displaces the Khanbogd Granite contact, and shows evidence of recent movement. The most recent activity on the Granodiorite Boundary fault is implied to be Cretaceous or younger, as it in part forms the faulted NW margin of the Cretaceous clay basin (Crane and Kavalieris, 2012).
A major NNE trending ductile fault zone, the Northwest Shear Zone, parallels the Oyu Tolgoi trend about 5 km to the west of the line of deposits. This structure is expressed as mylonitic to ultramylonitic rocks in the centre, grading outward over about 200 m to rocks lacking visible ductile strain. It merges with the Boundary Fault to the north, and together the two structures mark the break between the Devonian and Carboniferous volcanic and sedimentary units hosting and surrounding the Oyu Tolgoi deposits, and the Carboniferous granitic complex to the west (Peters et al., 2012). Along the eastern margin of the ductile zone there are exposures of metamorphic rocks, phyllite, schist, amphibolite, and metadiorite (Crane and Kavalieris, 2012). Shear fabric and displacement of contacts suggest the Northwest Shear Zone accommodated dominantly dextral strike-slip movement. Displacement of at least several kilometres is indicated. Sheared 335±1 Ma rhyolite dykes in the shear zone give a maximum age (Peters et al., 2012).
Major structures in the deposit area strongly influence the distribution of mineralisation, both controlling the original location and form of mineralised bodies, and by modifying, displacing and preserving them during post-mineral deformation events. The significant structures in each of the main deposit groupings are as follows:
Hugo Dummett
The major faults cutting the Hugo North and South deposits have been grouped into the following sets (after Peters et al., 2012):
• East-west striking, moderately north-dipping structures, e.g., the 110, Central Faults:
The 110 Fault defines the boundary between the Hugo North and South deposits, although mineralisation is continuous across it. It strikes east-west, dips at 45 to 55°N, and comprises zones of non-cohesive gouge and breccia up to several metres thick. The stratigraphic offset is of from 100 to 200 m, north-side-down. Thickness variations across the structure suggest it may have been a growth fault during deposition of the host upper Oyu Tolgoi sequence unit DA2 dacitic tuffs, and has little affect on the post-mineral biotite granodiorite dyke contact. Kinematic indicators suggest sinistral-reverse movement contrary to the stratigraphic offset, suggesting later movement of a lesser magnitude that does not significantly offset mineralisation. The 110 fault is cut and offset by the West and East Bat, and Rhyolite faults.
The Central Fault dips shallowly to moderately to the north, and separates the Hugo South and Central deposits. It strikes at ~90 to 100°, and at shallow levels, has an apparent normal displacement, whilst at deeper levels, has an apparent reverse sense of movement, implying multiple periods of displacement, similar to the 110 Fault. It consists of a zone of fault breccia and gouge that can be up to several metres thick.
• East-west striking, subvertical faults, e.g., the East-West Fault, Bogd, Dugant and Bumbat faults.
The East-West Fault, which cuts the Hugo South deposit, is subvertical with a strike of ~80 to 90°, and abruptly terminates alteration and mineralisation in several locations, cuts biotite granodiorite dyke contacts, and the Contact, East Hugo and East Bat faults, as well as the up-dip extension of the 110 fault. It occurs as a zone of clay-rich breccia and locally foliated gouge up to several metres in width. Narrow basaltic dykes commonly occur within the fault zone. Overall it has an oblique, dextral, north side down net slip of roughly 150 to 200 m, along a gently eastward-plunging slip vector.
The Bogd, Bumbat and Dugant faults, which are steeply dipping structures, in the Hugo North to North-east Extension transition area, with locally varying dips between north and south. They offset the mineralisation and lithologies, and have an oblique slip component associated with them, including significant dextral movement across the Bogd Fault.
• Steep NNE striking faults, e.g., the East and West Bat, East Hugo and 160 faults.
The West and East Bat faults are steep to vertical post-mineral structures that straddle the deposit, controlling the uplifted block, that hosts Hugo Dummett deposits and bring them to mining depths. Offset on the West Bat fault is as much as 1500 m, west side down, but has undergone multiple periods of movement, although the strike-slip displacement is indeterminate. The East Bat fault shows ~200 to 300 m of east-side down stratigraphic offset of the base of the Sainshandhudag Formation.
The East Hugo Fault occurs as a zone of strong to intense brecciation and clay gouge occurring along the east limb of the Hugo South and Hugo North deposits. It post-dates mineralisation, with east-side-down displacement of ~200 to as much as 400 m. It cuts the 110 Fault, but is dextrally displaced by the East-West Fault at Hugo South (Peters et al., 2012). Although described in a number of reports, none illustrate it on a plan or detail its location relative to the East Bat fault.
The 160 Fault strikes north to NNW, subparallel to the deposit trend, and dips steeply to the east. It can be traced through the southern part of the Hugo North deposit, although gradually loses expression northward, suggesting a corresponding decrease in displacement. It cuts across stratigraphic contacts at moderate angles and forms a sharp break in alteration intensity and copper grade, and displaces mineralisation (Peters et al., 2012).
• NNE striking faults that dip moderately east, subparallel to lithologic contacts, e.g., the Contact, Lower and Intermediate Faults. The nature of these faults suggests that they may have formed as gently-dipping thrusts during regional contractional deformation. They are localised along contacts between units with differing competencies, or along relatively weak layers within the stratigraphic sequence.
The Contact Fault has been described previously, and predates emplacement of the biotite granodiorite. At Hugo Dummett it is a bedding-parallel detachment zone that normally lies at the contact between tectonised uppermost Oyu Tolgoi sequence mudstones that stratigraphically overlie the deposits, and the overlying Heruga sequence basalt flows and volcaniclastic rocks. It occurs as a strongly foliated and deformed zone up to 10 m or more in thickness, and does not truncate mineralisation.
The Lower Fault occurs as an intensely brecciated, clay gouge-rich zone within the middle or lower portion of the mineralised Hugo South and North bodies, typically 200 to 400 m below the Contact Fault. It can be traced westward through at least part of the biotite granodiorite dyke as a narrow zone of breccia and gouge. In the Hugo North deposit, the Lower Fault appears to displace the Hugo North gold zone and biotite granodiorite dykes by up to 400 m (Peters et al., 2012).
The Intermediate Fault is sub-parallel to the contact between the unit DA1 augite basalt and unit DA2 ignimbritic dacite tuffs of the Oyu Tolgoi sequence, and lies between the Contact and Lower faults.
• ENE striking faults, e.g., the Granodiorite Boundary Fault System, Kharaa, Eroo and Rhyolite fault.
The Granodiorite Boundary Fault System is a corridor of subparallel, anastomosing faults, located to the NW of, and partially truncating Hugo North. It merges southward into the Northwest Shear Zone, as discussed above. Together these two fault zones juxtapose the Carboniferous granitic complex to the NW over the Devonian and Carboniferous sequences hosting and overlying the Oyu Tolgoi deposits to the SE (i.e., SE side down). The faults within this system dip steeply to the north or NW, and are strongly-developed, foliated gouge and breccia zones ranging from a few tens of centimetres to several tens of metres wide.
The Kharaa and Eroo faultscut the northern part of the Hugo North deposit, ~500 m SE of the Boundary Fault, and may be reactivated faults associated with the Boundary Fault System.
The Rhyolite fault is subvertical, and cuts across the southern part of the Hugo North deposit, where it coincides with a wide rhyolite dyke zone. It is usually manifested as strong to intense breccia and clay gouge zones along dyke contacts. Several tens of metres of apparent dextral or south-side-down displacement is indicated, although the absolute slip direction is unknown. The Rhyolite Fault cuts and displaces the Contact fault.
• NW striking faults, e.g., the 7100 Fault.
The 7100 Fault is one of a series of steep, parallel, oblique slip structures (including the Noyon, Gobi and Burged faults) that cut through the Hugo North deposit, displacing all rocks units with an offset of north side down.
There is an apparent fault hierarchy at Hugo North (Peters et al., 2012), namely: i). early stage, which are deposit parallel, e.g., the Contact, Lower, Intermediate and 160 faults. ii). mid stage, that includes the north-west trending faults, which are earlier than the East and West Bat faults. However, the 7100 cuts the West Bat Fault and consequently may have been reactivated. iii). mid to late stage, e.g., the ENE trending Boundary, Kharaa, Eroo, 110 and Rhyolite faults. iv). late stage, east-west trending oblique slip faults in the Hugo North to NE extension transition area, e.g., the Bogd Bumbat and Dugant faults.
Southern Oyu Tolgoi Deposits
The key structures influencing the Southwest, Central and South Oyu deposits are (after Peters et al., 2012) the:
• Central Fault, on the northern margin of the cluster of deposits, which dips shallowly to moderately to the north, and separates the Hugo Dummett South and Central deposits, as described above.
• West Bounding and East Bounding faults, two NE striking faults that bracket the entire high-grade, gold-rich core of the Southwest Oyu deposit. Both faults are clearly defined on ground magnetic images, and locally have subparallel splays. They consist of foliated cataclasite, gouge/breccia, and mylonitic bands in zones ranging from a few, to a few tens of metres wide, striking at ~40° and dipping at 80°NW. Kinematic indicators within the fault zones imply dominantly subhorizontal, sinistral movement on the bounding faults. The East Bounding Fault juxtaposes younger rocks to the SE against the augite basalt of Oyu Tolgoi sequence unit DA1 hosting the deposit, whilst the West Bounding Fault is mainly intraformational within the augite basalt to the south, but separates the same basalt from quartz monzodiorite in the north. The West Bounding Fault is commonly intruded by hornblende-biotite andesite dykes, whilst rhyolite dykes are more common within the East Bounding Fault. The cataclasite of the fault zones contains abundant quartz, quartz sulphide and sulphide (pyrite, chalcopyrite, sphalerite, and galena) clasts in a comminuted matrix that is locally overprinted by fine-grained pyrite and chalcopyrite. These relationships suggest at least some of the fault movement was coeval with mineralisation.
• The West Bat fault, as described above, continues south, with a dextral offset across the Central Fault, and passes ~800 m west of, and subparallel to, the West Bounding fault, to be terminated by the Solongo fault in the south.
• Solongo Fault, an east- to ENE-striking, subvertical structure cutting across the Oyu Tolgoi trend just south of the Southwest and South Oyu deposits. It forms a major structural break, reflected by a strong linear anomaly in ground magnetic data. The structure has a minimum of ~1600 m of south-side-down stratigraphic offset, juxtaposing the subcropping mineralised augite basalt of the South and Southwest Oyu deposits with sedimentary rocks of the Heruga sequence which structurally overlie the deeply buried, north plunging Heruga North mineralisation trend, in the south. The fault zone typically occurs as a strongly tectonised, foliated zone of up to several tens of metres in width. Rhyolite dykes (340±3 Ma; Wainwright, 2008) commonly intrude the fault zone, and in turn, also have tectonically brecciated margins, although some locally cross the fault with little or no apparent displacement.
• Rhyolite Fault (separate from the fault of the same name in the Hugo Dummett deposit area to the north), a curved, east-west structure, now occupied by a rhyolite dyke swarm, that juxtaposes the Central and Southwest Oyu deposits, via the Bridge zone. Between the Southwest and Central Oyu zones, displacement on the East Bounding Fault appears to have been largely transferred to the Rhyolite Fault, leaving the Central Oyu zone as a structurally intact block that has been displaced downward relative to the Southwest and South Oyu zones.
• South Fault, a curvilinear NE to east-west striking fault that forms the NW and northern boundary to the South Oyu deposit, separating it from the Wedge zone to the NW. It is interpreted to branch from the Solongo fault in the southwest, and is cut by an ENE-dipping Contact fault thrust to the east. These three structures encircle the South Oyu deposit. The South fault comprises a zone that encloses several strands over a width of up to 90 m, and juxtaposes progressively younger strata in the NW against older strata to the SE. Individual faults typically consist of gouge and breccia zones over widths of up to several metres. To the west, however, the zone strikes into a large body of quartz-monzodiorite. The individual fault strands are difficult to trace through the intrusion, and offset of the intrusive contact is minimal, implying that most movement pre-dated emplacement of the quartz-monzodiorite.
Heruga and Heruga North
The Heruga deposits are separated from the Oyu South cluster by the subvertical, east- to ENE-striking, Solongo Fault as described above. A series of other ENE striking faults are prominently reflected on both magnetic and satellite images. Geological mapping shows an ~500 m apparent dextral displacement of dykes and stratigraphic contacts across each of these faults. Two of these, the Heruga North and Javkhlant (or South Sparrow) faults, offset the Heruga trend. The key structures are:
• The Solongo Fault, an east- to ENE-striking, subvertical structure cutting across the Oyu Tolgoi trend just south of the Southwest and South Oyu deposits.
• The Heruga North fault, which separates the Heruga North and Heruga zones, and strikes ENE to NE. It dips moderately to the north, with an estimated dextral displacement of ~1000 m, as well as a north side downward displacement in the order of 600 m, assuming Heruga North is the continuation of the Heruga mineralisation. To the NE it coalesces with the Solongo Fault, and to the SW with the Javkhlant fault (Peters et al., 2012). In the northern part of the deposit area, facing directions and repetitions of stratigraphy define a large-scale recumbent anticline in the hanging wall of the Contact fault. Whilst the magnitude of displacement on the fault is poorly constrained, the scale of the overturned folds, vertical stacking of dissimilar stratigraphic sequences, and the fault continuity throughout the Oyu Tolgoi area, all imply that displacement of kilometres to tens of kilometres is probable (Peters et al., 2012).
• The South Sparrow, or Javkhlant fault crosses the Heruga trend <250 m south of the known Heruga mineralisation. It has an apparent south side down and dextral offset.
• The Contact fault (at Heruga North) is similar to elsewhere, occurring as a low angle thrust that is generally parallel to bedding, placing overturned Heruga sequence rocks over upright Devonian Oyu Togoi sequence sedimentary and volcanic rocks. At Heruga, it varies from tens of centimetres to 40 m in thickness, with an average orientation striking 110° dipping 45°ESE. Lewis (2008) reports that kinematic indicators, such as shear bands and drag folds, record up-dip (thrust) displacement (quoted by Peters et al., 2012).
• The Heruga block, bounded to the north and south respectively by the Heruga North and South Sparrow/Javkhlant faults, is also cut by a number of NE- to NNE-trending faults, including the Bor Tolgoi and Bor Tolgoi West faults which are ~400 to 600 m apart, converging gradually to the south, straddling the west block of the Heruga deposit to the east and west, whilst the east block of the deposit is located to the east of the Bor Tolgoi fault. Each displays 200 to 500 m of west side down apparent offset of stratigraphic contacts. The west and east structural blocks of the Heruga deposit, separated by the Bor Tolgoi fault, appear to have a sinistral offset across that structure (Peters et al., 2012). In the northern part of the deposit area, facing directions and repetitions of stratigraphy define a large-scale recumbent anticline in the hanging wall of the Contact fault. Whilst the magnitude of displacement on the fault is poorly constrained, the scale of the overturned folds, vertical stacking of dissimilar stratigraphic sequences, and the fault continuity throughout the Oyu Tolgoi area, all imply that displacement of kilometres to tens of kilometres is probable (Peters et al., 2012).
• The South Bor Tolgoi trends ESE and displaces a small block of the Heruga deposit on its southern margin to the west. It also appears to displace the Bor Tolgoi and Bor Tolgoi West faults (Peters et al., 2012).
The Heruga North area is similarly cut by NNE-trending, west side down, faults similar to the Bor Tolgoi fault structures at Heruga and the West Bat fault at Hugo Dummett (Peters et al., 2012).
The deposit-scale faults at Heruga and Heruga North displace mineralised zones as a whole, but do not directly limit mineralisation and alteration zones, implying they post-date mineralisation. Lewis (2008 quoted by Peters et al., 2012) concluded that it is likely the Heruga porphyry formed within a relatively intact structural block, with most faulting and disruption of contacts related to post-mineralisation deformation.
Folding History
Measurements taken from oriented drill core, mainly the bedded intervals within the upper Oyu Tolgoi and Heruga sequence rocks, reveal two orientations of folds in the Hugo Dummett deposit area, specifically a dominant set with NNE trends, and a subordinate set of NW trending folds. Both of these orientations also occur in Carboniferous post-mineral strata, indicating that both post-date mineralisation and may have modified the form of the deposit. Within the Heruga sequence, NNE trending folds have wavelengths and amplitudes on the order of metres to tens of metres, with dominant east dipping bedding measurements suggesting the fold geometry is strongly asymmetric, and west-vergent. In contrast, NW trending folds have wavelengths of hundreds of metres and open, symmetric forms. Together, the two fold sets define an elongate dome and basin interference pattern.
Reversals of sedimentary facing direction occur locally in the upper part of the Heruga sequence, suggesting tight to isoclinal folding. These same overturned folds are cut by the Late Devonian biotite granodiorite dyke at Hugo North, effectively bracketing the timing of folding to Late Devonian, roughly contemporaneous or close in age to mineralisation (Peters et al., 2012).
Mineralisation and Alteration
Overview
There are six principal mineralised deposits in the Oyu Tolgoi deposit complex, namely: i). Hugo Dummett (divided into South and North Hugo, previously Far North Oyu); ii). Central Oyu; iii). South-West Oyu, iv). South Oyu, v). Heruga North and vi). Heruga, which are distributed over ~12 km of a more than 25 km long, NNE trending structural and mineralised corridor, with gaps between the deposits of <1 to 2 km. Where completely preserved, economic mineralisation persists over a vertical interval of ~1 km.
These deposits appear to represent five porphyry centres, grouped into the Hugo Dummett, Southern Oyu Tolgoi cluster, Heruga North and Heruga deposits (Crane and Kavalieris, 2012).
The Ulaan Khud South, Ulaan Khud North and Javkhlant prospects are all within the Oyu Tolgoi trend, respectively located ~7.5 km and 11 km to the NNE of Hugo North, and ~4.5 km SW of Heruga.
There is a distinct variation in deposit characteristics along the trend of the mineralised corridor, with a high sulphidation phase partially telescoped onto the underlying porphyry systems in the northern half of the trend, from the Hugo Dummett North to Central and Wedge deposits. These variations may be summarised as follows (after Kirwin et al., 2005; Crane and Kavalieris, 2012):
i). At the northern end, at Hugo Dummett North, the causative quartz-monzodiorite, which may represent multiple intrusions, is a major host rock, and has been subjected to extensive sericitic and advanced argillic alteration overprinting earlier potassic alteration assemblages. This part of the deposit is characterised by the presence of bornite and hypogene chalcocite, with an upper zone of molybdenite, although chalcopyrite becomes the dominant sulphide at deeper levels. These are overprinted by a high sulphidation assemblage of enargite, tennantite-tetrahedrite and covellite. The Au (g/t) to Cu (%) ratio is lower compared to the other centres, generally from 0.1:1 to 1:1.
ii). On the eastern margin of Hugo Dummett North and at Hugo Dummett South, the highest-grade copper mineralisation is related to a zone of intensely stockworked to sheeted quartz veins, centred on thin, east-dipping, porphyritic quartz-monzodiorite intrusions, extending into the adjacent augite basalt of Oyu Tolgoi sequence unit DA1. This zone is distinct in its high Au (g/t) to Cu (%) ratios of 0.5:1. Bornite is dominant in the highest-grade core, with 3 to 5% Cu, progressively grading outward through chalcopyrite in augite basalt, passing into a lower grade fringe of pyrite-chalcopyrite ±enargite, tennantite, bornite, chalcocite in advanced argillically-altered dacitic tuff of unit DA2.
iii). At the Central deposit, an upward-flaring, conical advanced argillic and sericitic alteration cone overprints an early porphyry system within quartz-monzodiorite, and hosts a high sulphidation assemblage of covellite and pyrite, with lesser associated enargite, tennantite, bornite and chalcopyrite. Erosion during the Cretaceous appears to have removed the high sulphidation system at South and South-West Oyu, with only the roots and mid levels of the porphyry system remaining, although part of the high sulphidation system from South-West Oyu is preserved in the adjacent, down-faulted Wedge zone. A supergene chalcocite blanket was developed over the Central deposit and protected by the overlying Cretaceous sequence.
iv). In the southern deposits of Southwest Oyu, South Oyu, Heruga North and Heruga, remaining mineralisation is dominantly hosted by biotite-magnetite altered augite basalt of Oyu Tolgoi sequence unit DA1, overprinted by hematite-chlorite-sericite alteration. Whilst the upper, probable high sulphidation, sections of the Southwest and South Oyu deposits were eroded, the full vertical section remains at Heruga North and Heruga. Syn-mineral quartz-monzodiorite accounts for <20 vol.% of the ore host at Heruga and Heruga North, occurring as relatively small plugs and dykes, compared to the Hugo Dummett area, and are often not correlatable between drill sections. At the Southwest Oyu deposit, the syn-mineral quartz-monzodiorite is only found as an ~250 m diameter by 700 m high, pipe-like body in the core of the deposit. However, whilst syn-mineral intrusions are limited, late-mineral quartz-monzodiorite bodies are more extensive in the vicinity of all of these deposits (Peters et al., 2012). This group of deposits is characterised by a generally higher Au (g/t) to Cu (%) ratio of from 0.8 to 3:1 compared to the northern deposits. Chalcopyrite is the principal copper sulphide, with lesser pyrite, bornite and molybdenite. As at Hugo Dummett North, a similar, but higher grade, molybdenite zone occurs above the main copper-gold mineralisation at Heruga.
The Oyu Tolgoi mineralised corridor appears to have been tilted to the north, such that erosion has removed as much as 500 m of the mineralised system at South and South-West Oyu. In contrast, at Hugo Dummett North, the entire high sulphidation system and underlying and over printed porphyry mineralisation is preserved and plunges north at depth, below poorly- and un-mineralised hosts. A NE-SW striking, steeply south dipping normal fault (the post-Carboniferous Solongo Fault), immediately to the south of the Southern Oyu cluster, has down-thrown the mineralised corridor to the south, where it becomes the similarly north plunging Heruga North and Heruga deposits. High sulphidation bornite-rich mineralisation recurs at depth in some drill holes at Heruga North, where it occurs along section of the eastern margin of the quartz-monzodiorite intrusion, on the opposite side to the main Heruga North deposit (Peters et al., 2012).
Where the full porphyry system (not including the late stage high sulphidation overprint) is preserved, at Hugo Dummett and Heruga, it has a vertical extent of ~1 km.
Significant alteration includes early K silicate (quartz-K feldspar-biotite), overprinted by extensive retrograde quartz-sericite at Hugo Dummett and sericite-chlorite at South Oyu. Advanced argillic and quartz-sericite-illite associations are dominant at Central and North Oyu, progressively overprinting and obliterating the earlier K silicate and then quartz-sericite stages, particularly in association with hydrothermal breccias. Peripheral, magnetite stable propylitic alteration of calcite, chlorite, illite and epidote is weak, low in pyrite and fringes the advanced argillic alteration at Central Oyu and Hugo Dummett. The main styles of alteration are as summarised below (after Crane and Kavalieris, 2012), and subsequently described in more detail for each of the deposits.
• Sodic-calcic alteration, represented by an early assemblage of actinolite-magnetite-albite-apatite-titanite and green biotite, common in the southern deposits within augite basalt, and generally preceding biotite alteration. Augite phenocrysts are replaced by actinolite, while groundmass plagioclase is converted to to albite, intergrown with actinolite-magnetite-apatite-titanite.
• Biotite-magnetite, which is characteristic of gold-rich chalcopyrite mineralisation in the southern deposits. Strong, characteristically brown biotite, partially replaces actinolite-altered augite phenocrysts, whilst secondary magnetite occurs as pervasive alteration or in micro-veinlets.
• K feldspar is generally restricted to quartz-monzodiorite, but can occur in basaltic wall rocks obliterating the original rock texture. Pink K feldspar rims, or completely replaces, plagioclase phenocrysts, and occurs as selvages to some quartz veins, whilst in strongly altered rocks, recrystallisation of the groundmass to coarser K feldspar is apparent.
• Quartz-sericite (muscovite) is largely found in quartz monzodiorite, most extensively developed at the Central Oyu and Hugo Dummett deposits, where it persists to depths of up to 1000 m. Alteration almost completely obliterates original quartz-monzodiorite texture to form a pale amorphous rock, although it is generally much less intense in augite basalt where it produces an assemblage of chlorite-muscovite/illite-hematite, and secondary quartz is less abundant. The deep core of the Hugo Dummett North deposit has a thick carapace-like zone of intense sericite/muscovite alteration, overprinting the underlying red K feldspar-altered quartz-monzodiorite and hosting the molybdenite zone, but immediately overlying the main deep copper and gold zone.
• Tourmaline-sericite is locally found in the southern deposits, occurring as late fine-grained tourmaline in the sericite alteration zone, characterised by large rosettes of tourmaline, commonly nucleated on pyrite and large crystals of pink-white albite.
• Intermediate argillic alteration occurs as a yellow-brown to greenish assemblage of chlorite-muscovite-illite-siderite-specular hematite, with minor pyrophyllite-kaolinite (after magnetite-biotite) in augite basalt wall rocks. It is best developed at Hugo Dummett South.
• Advanced argillic alteration, is dominantly composed of residual quartz and pyrophyllite, with lesser corundum, diaspore, K-alunite, aluminium-phosphate-sulphate minerals, zunyite, topaz, dickite, kaolinite, anhydrite, gypsum and relict andalusite. It is developed over a strike length of ~5.9 km from Hugo Dummett North to the Central deposit, before being down-faulted between the Southwest and South Oyu deposits. It has a maximum width of ~1 km at the Hugo Dummett South deposit. Minor pyrophyllite-dominated alteration is also encountered in the outer part of the Heruga and Heruga North deposits. The sericite-muscovite of the previous 'quartz-sericite (muscovite)' stage alteration and the early advanced argillic pyrophyllite likely formed from cooling of a late magmatic-hydrothermal fluid (Hemley and Hunt, 1992), without a significant meteoric water component (Khashgerel et al., 2006, 2008, 2009). However, stable isotope studies (Khashgerel et al., 2006, 2009) indicate that later alunite formed at moderate temperatures (~270°C) from condensation of magmatic vapour that mixed with up to 25% meteoric water, whilst dickite formed at low temperatures (~150°C) during ingress of further meteoric water into the advanced argillic zone.
• Propylitic alteration is only observed in the western parts of the Southwest and Heruga deposits, and is characterised by pervasive alteration and veining of epidote, magnetite and hematite, veins of semi-massive pyrite, and albite alteration.
The mineralisation and alteration at each of the main deposits is as follows:
Hugo Dummett
Hugo Dummett represents a high sulphidation system that has been telescoped onto porphyry-style mineralisation formed at an earlier stage in the evolution of the hydrothermal centre. High grade copper mineralisation extends over a distance of more than 4.5 km in two, NNE trending connected segments, Hugo South and Hugo North. The dividing line between the two deposits is defined by the thinning and locally discontinuous nature of the high grade copper mineralisation (i.e., >2.0% Cu). This line broadly coincides with the east striking 110 fault, which separates the gold- and copper-rich zone hosted in augite basalt and quartz-monzodiorite of the Hugo North deposit, from the more southerly, gold-poor, dacitic tuffs and augite basalt-hosted mineralisation at Hugo South.
Mineralisation in the two deposits is dominantly bornite, chalcocite and chalcopyrite, with subordinate pyrite, enargite and tetrahedrite-tennantite. The sulphides are directly related to the associated alteration assemblage, which in turn is partially dependent upon the lithology of the host rock, but also the position in the outward zonation from the core of the high-grade shell ellipse. The outward zonation from high to low grade Cu ore corresponds to the progression from bornite + chalcocite, to chalcopyrite (±tetrahedrite-tennantite) to pyrite (±enargite). The Hugo Dummett deposits lie within a NNE trending, post-ore, structural high, bounded by the West Bat and East Bat Faults, described above. Although the latest movement on these bounding faults displaces the Carboniferous post-mineral strata, they may represent the shallow expression of a longer-lived, deposit-controlling structural zone.
Hugo Dummett North
Hugo North is dominantly hosted within quartz-monzodiorite, and an easterly dipping sequence of basaltic volcanic and volcaniclastic strata of the Oyu Tolgoi sequence unit DA1. It is developed over a vertical interval of >900 m, has a strike length in excess of 1.8 km by ~500 m wide, and is characterised by assemblages of bornite+chalcocite and chalcopyrite, with minor enargite and tetrahedrite-tennantite.
Several different mineralised zones are recognised in association with the syn-mineralisation quartz-monzodiorite, on the basis of alteration characteristics and position within the deposit, namely:
i). Eastern and upper zone - An outer shell of advanced argillic alteration and intermediate to high sulphidation assemblages occur in the eastern and upper sections of the deposit, straddling the outer margin of the intrusive system, and unit DA1 basaltic pyroclastics, into unit DA2 dacitic ash flow tuffs of the Oyu Tolgoi sequence wallrocks (Crane and Kavalieris, 2012; Peters et al., 2012). The associated alteration assemblages have unusual textures due to the close spatial and temporal relationship between the porphyry Cu-Au and high sulphidation mineralised phases (Khashgerel et al., 2006, 2009). Enargite-pyrite occurs as veins in the outer margins of the Hugo Dummett deposit, within advanced argillic altered rocks, whilst enargite is also intergrown with tennantite and closely associated with high-grade bornite that commonly overprints it. Advanced argillic minerals, such as topaz, zunyite, and alunite, are found intergrown with bornite and chalcocite. Rarely hessite (Ag2Te) and clausthalite (PbSe) occur as inclusions in bornite (Crane and Kavalieris, 2012).
ii). Quartz veined lens - The high sulphidation zone passes downward and westward into a high-grade (>2.5 wt.% Cu), intensely quartz veined lens. This lens is localised along the upper eastern margin of the main quartz-monzodiorite intrusive body, centred on a zone of thin, anastomosing, east dipping dyke-like quartz-monzodiorite intrusions, or within the apex of the adjacent large quartz-monzodiorite body, and the intruded Oyu Tolgoi sequence unit DA1 augite basalts wall rocks. Crane and Kavalieris (2012) suggest the quartz-monzodiorite dykes may predate the main quartz-monzodiorite intrusion.
This high grade quartz veined lens extends for at least 1.6 km to the NNE from Hugo South, and is the richest part of the Oyu Tolgoi group of deposits. Bornite is dominant in the highest grade (3 to 5% Cu) parts of the lens, passing outward to predominantly chalcopyrite (~2% Cu) and then pyrite-chalcopyrite (<1% Cu). Mineralisation occurs as a narrow zone of intense A-type quartz veining, with >90% of the rock containing >15% quartz veining. It has a vertical extent that varies from ~100 m in the neck connecting Hugo South and North, but expands to more than 700 m further to the north. The corresponding horizontal width of the high grade lens ranges from 150 to 180 metres in the south, up to ~200 m in the north, and is entirely enveloped by the>1% Cu grade shell, which reaches a maximum horizontal thickness of ~450 m at RL 0 m (1160 m below surface). Elevated gold grades are also found within the up-dip (western) portion of this zone of high grade veining (Peters et al., 2012; Crane and Kavalieris, 2012).
Bornite is relatively coarse, impregnating quartz, filling spaces and fractures within the veins, and occurring as disseminations in wall rocks, usually intergrown with subordinate chalcopyrite. High-grade bornite has minor amounts of associated tennantite, sphalerite, hessite, clausthalite and gold that occur as inclusions, or at grain boundaries. Crane and Kavalieris (2012) suggest the structural control of the intense quartz veining primarily provided a structural pathway for later mineralising fluids. Chalcocite is intergrown with the most intense bornite mineralisation, although it is less common than at Hugo South, and can also occur as veins cutting bornite-dominant mineralisation. On the margins of the high-grade bornite-dominant zone, large chalcopyrite veins (up to 5 cm wide) crosscut earlier sulphide assemblages. Pyrite is rare to absent, except locally, where the host rocks are altered to an advanced argillic assemblage. Within the upper levels, in advanced argillic altered basaltic tuff, the assemblage comprises pyrite-chalcopyrite ±enargite, tennantite, bornite, chalcocite, and more rarely covellite (Peters et al., 2012; Crane and Kavalieris, 2012).
This high grade lens is overprinted by a deeply developed quartz-sericite assemblage, and closely enveloped by advanced argillic alteration, related to alteration phases in the adjoining zones to the west and east respectively (Crane and Kavalieris, 2012).
The quartz veined lens is bounded to the west by the east-dipping Lower fault which was active soon after ore deposition, and has displaced the lens by as much as 400 m relative to the main quartz-monzodiorite intrusion to the west (Peters et al., 2012).
Crane and Kavalieris (2012) note that high grade mineralisation associated with intense quartz veining locally exhibits strong ductile deformation within quartz. This is evident at the microscopic scale, where chalcopyrite is deformed and folded, and mechanically injected into fractured pyrite. These textures are found in high to intermediate sulphidation assemblages and imply that ductile deformation post-dates formation of advanced argillic alteration. Molybdenite occurs as kinked flakes enclosed in bornite or chalcopyrite and thus appears to be early in the sulphide paragenesis. However, elsewhere, the distribution of molybdenite is not well understood, and it appears to occur at all depths within the copper-mineralised zone. An Re-Os determination from molybdenite in the orebody gave an age for the ore of 372±1.2 Ma (Crane and Kavalieris, 2012).
iii). Upper western zone - The outer and upper margin of the main intrusive body to the west of, and below, the Lower Fault, typically has a lower vein density and lower copper and gold grades. Chalcopyrite and bornite are estimated to occur in varying proportions and are typically intergrown or deposited sequentially, with bornite replacing chalcopyrite. The upper sections of this zone have a thick carapace-like development of intense sericite (muscovite) alteration where gold grades are very low (Peters et al., 2012; Crane and Kavalieris, 2012). Throughout this zone, fine-grained, disseminated bornite-chalcopyrite is intimately intergrown with muscovite. A bornite-rich shell straddles the lower margin of this sericitic carapace, characterised by finely disseminated bornite and chalcopyrite, although in hand specimen the chalcopyrite is usually not visible. The sulphides are disseminated throughout the rock in the matrix as well as in quartz veins. The fine-grained sulphide gives the rocks a black "sooty" appearance (Peters et al., 2012). This bornite zone also corresponds to a zone of andalusite, which has been overprinted by the intense muscovite/sericite alteration (Crane and Kavalieris, 2012).
Above the bornite zone, there is a broad sub-horizontal zone of chalcopyrite-pyrite-molybdenite with elevated (≥75 ppm Mo) molybdenum levels, grading upward into pyrite and pyrite-enargite, coinciding with overlapping advanced argillic alteration (Peters et al., 2012). Similar Mo mineralisation is found in the western margin of the adjacent high-grade quartz veined lens east of the Lower fault (as described above), suggesting introduction following movement on the fault (Peters et al., 2012; Crane and Kavalieris, 2012).
Late chalcopyrite with chlorite locally overprints the sericite zone at the base of the molybdenite zone, above the bornite shell (Peters et al., 2012).
iv). Deep western core - A deep core of the Hugo North deposit is located to the west, where it is truncated by the West Bat fault, and is hosted within the main quartz-monzodiorite intrusion. It is characterised by gold-bearing, bornite-chalcopyrite mineralisation with subordinate tennantite, accompanied by red to pink and buff coloured potassic altered quartz-monzodiorite. This red colouration is attributed to fine hematite dusting, mainly associated with albite. This core underlies the outer fringe of the intrusive and the sericite alteration carapace (Peters et al., 2012).
The deep core is capped by a moderate- to high-grade gold and bornite zone (the Hugo Western Gold orebody), which is up to more than 100 m thick, with a horizontal extent of >300 m and a strike length in excess of 1 km. This zone is distinct and has a high Au (g/t) to Cu (%) ratio of 0.5:1. It is restricted to the western part of the main quartz-monzodiorite intrusion, truncated by the post-mineralisation West Bat fault in the west, and immediately underlies the carapace of gold-poor, intense muscovite alteration, and overlaps the upper bornite shell that straddles the boundary (Peters et al., 2012).
Below the Hugo Western Gold orebody, the proportion of bornite decreases and chalcopyrite becomes more abundant in the deep core (Peters et al., 2012).
Locally, Au (g/t) to Cu (%) ratios are >1, corresponding to areas where native gold is observed. Gold and silver assay values correlate well with those of copper, and native gold is closely associated with copper sulphides, particularly bornite and chalcopyrite, and to a lesser extent, pyrite and tennantite-tetrahedrite. Most native gold grains are <10 µm across, and commonly occur at sulphide grain boundaries (Crane and Kavalieris, 2012). An electron microprobe study (Oyunchimeg, 2008), combined with polished section petrography, indicates that gold is generally >95% pure in the porphyry Cu-Au mineralisation. Gold is also present in base metal veins that either cut or occur on the margins of the porphyry style mineralisation in the Hugo North deposit. Silver accompanies Cu and Au in the order of 3 to 5 g/t Ag to 1 g/t Au, although the mineralogy of silver is uncertain, and it may substitute for copper in chalcopyrite, bornite and tennantite (Crane and Kavalieris, 2012).
Cross-cutting relationships between the mineralised zones detailed above are ambiguous, and it is uncertain whether they represent temporally distinct intrusive and/or hydrothermal events, or simply variations in alteration intensity related to position within the hydrothermal system.
The main Hugo North deposit is cut by a zone of late- to post-ore biotite granodiorite, that varies from a swarm of narrow dykes in the north and south, to one thick intrusion up to ~400 m thick in the centre of the deposits strike extent. These dykes are located to the west of the high grade, quartz veined lens, and where most intensely developed, result in a steep internal barren zone, splitting Hugo North into a western and eastern orebody.
The West Bat fault cuts across the main quartz-monzodiorite intrusion to converge with the Lower fault northward, where the Hugo Western Gold orebody is juxtaposed with the high grade quartz veined lens, before the ore zone in the main quartz-monzodiorite intrusion is eventually truncated.
Alteration styles - The distribution of alteration zones is similar to that in the Hugo South deposit, as described below, except that the advanced and intermediate argillic zones are more restricted and mainly lie along the outer and upper margins of the intrusive system. The alteration styles, from youngest to oldest, at Hugo North includes the following (after Crane and Kavalieris, 2012; Peters et al., 2012):
• Chlorite-illite, which occurs as a weak outer boundary of the advanced argillic zone, mainly in the coarse, upper part of the basaltic tuff/breccia and not usually associated with significant mineralisation.
• Advanced argillic alteration, which is characterised by residual quartz and pyrophyllite, with lesser corundum, diaspore, K-alunite, aluminium-phosphate-sulphate minerals, zunyite, topaz, dickite, kaolinite, anhydrite, gypsum and relict andalusite. This style of alteration is typically buff or grey in colour, with ubiquitous late dickite on fractures. Alunite constitutes <10% of the entire advanced argillic zone, closely associated with high-grade (>2.5% Cu) bornite-dominated mineralisation, and occurring as discrete zones or layers up to ~50 m thick, including massive pink-brown quartz-alunite bedding-parallel lenses. At Hugo North, advanced argillic alteration is hosted mainly in the lower part of the dacitic tuff in the outer, eastern section of the mineralised system, although on some sections, it extends into strongly veined quartz-monzodiorite. Topaz is widespread as a late alteration phase, controlled by structures, cutting both the advanced and intermediate argillic zone. In certain areas, topaz appears to replace parts of the quartz-alunite zone. In addition, topaz may also occur disseminated with quartz, pyrophyllite and kaolinite.
• Intermediate argillic alteration, characterised by chlorite, muscovite, illite, siderite and specular hematite, with minor pyrophyllite, kaolinite and dickite. At Hugo North it forms a relatively narrow zone (>0.1 to <1 m) that defines the transition from advanced argillic to chlorite-sericite altered augite basalt. It is yellow-brown to greenish, and commonly hosted by augite basalt, but may also occur in dacitic ash-flow tuff. Hematite usually occurs as fine specularite and may be derived from early magnetite or Fe rich minerals such as biotite or chlorite.
• Quartz-sericite/illite-(muscovite), which generally occurs in the main quartz-monzodiorite intrusions, but is also a feature of the strongly mineralised zone. Alteration decreases with depth in the quartz-monzodiorite. This zone is strongly developed above the deeper K-silicate alteration of the deep core, forming a carapace-like zone of intense sericite (muscovite) alteration in the upper sections of the main quartz-monzodiorite intrusion. In quartz-monzodiorite, this alteration is particularly destructive, with complete replacement to a pale rock where the original porphyritic textures are barely discernible. By contrast, sericitic alteration is generally less intense in augite basalt, producing the chlorite-muscovite/illite assemblage described below. A layer of elongate prisms of andalusite, up to 1 mm in size, occurs at the base of this sericitic carapace-like zone, usually partly or completely replaced by muscovite. In rare occurrences without strong sericitic alteration, andalusite is intergrown with greenish biotite (or phlogopite), which is possibly an equilibrium assemblage.
• Chlorite-muscovite/illite-hematite/siderite-(biotite-magnetite), which is transitional with the intermediate argillic zone. This zone is the equivalent in augite basalt host rocks of the quartz-sericite/illite-(muscovite) zone that predominantly occurs in quartz-monzodiorite. Earlier mineral phases are only partially replaced by sericite, and secondary quartz is less abundant, while the rock retains a characteristic greenish colour and much of its original texture. Relict hydrothermal magnetite is present, either as disseminations or in veins.
• K-silicate alteration, mainly biotite-K feldspar - is generally restricted to quartz-monzodiorite, but less commonly occurs in basaltic wall rocks. Pink secondary K feldspar rims or completely replaces plagioclase phenocrysts and occurs as selvages to some quartz veins. Recrystallisation of the groundmass to coarser K feldspar is apparent in strongly K-silicate altered rocks. It represents early alteration, overprinted by the zones described above, and decreases in intensity with depth.
Hugo Dummett South
The Hugo South deposit is separated from the Southern Oyu Tolgoi deposit cluster by the north dipping Central fault and from the Hugo North deposit by the 110 fault, as described previously in the Structure section. The deposit tapers downward, with the >1% Cu contour defining a body that is ~750 m down-dip, up to 200 m thick and ~850 to 1300 m long. It is hosted by an easterly dipping sequence of volcanic strata of the Oyu Tolgoi sequence and by quartz-monzodiorite intrusive rocks. The Oyu Tolgoi sequence at Hugo South consists of porphyritic (augite) basalt flows and minor volcaniclastic strata of unit DA1, unconformably overlain by 100 to 200 m of dacitic to andesitic ash flow tuffs of the upper Oyu Tolgoi sequence unit DA2. The latter tuff suite hosts a larger proportion of the Hugo South mineralisation than at Hugo North, which is predominantly within the unconformably underlying augite basalt. Weakly altered to unaltered sedimentary and volcanic rocks of the Heruga sequence and Sainshandhudag Formation structurally overlie the mineralised sequence along the eastern flank of the Hugo South deposit. The thickness of the Heruga sequence commonly exceeds 600 m, although structural repetition may in part be responsible for this thickening. The Sainshandhudag Formation strata unconformably overlie, and are locally faulted against, the Heruga sequence (Peters et al., 2012; Peters and Sylvester, 2014).
Several intrusive phases are recognised in at Hugo South. The most extensive are weakly copper mineralised quartz-monzodiorite bodies, underlying the entire deposit area. Contacts are irregular, but overall have a preferred easterly dip, subparallel to lithological banding in the enclosing Oyu Tolgoi sequence. The quartz-monzodiorite is broadly coeval with alteration and mineralisation, although two varieties are differentiated on the basis of alteration characteristics and position within the deposit:
i). An intensely quartz veined phase occurring along the upper margin of the main intrusive body, or as a separate east-dipping tabular body in the overlying strata; and
ii). An underlying, lower-grade, more weakly veined phase, that constitutes the bulk of the large intrusive body (Peters et al., 2012).
Late- to post-mineral biotite granodiorite intrusions form a NNE trending, moderate to steeply west-dipping, dyke complex cutting across the western edge of the deposit.
Younger, mainly Carboniferous, intrusions include rhyolite, hornblende-biotite andesite, dacite and basalt/dolerite occurring as subvertical dykes, or less commonly as easterly-dipping sills emplaced along stratigraphic contacts. They are unmineralised and are only locally volumetrically significant in the deposit (Peters et al., 2012).
The Hugo South deposit lies within a NNE elongate block that comprises a homoclinal sequence, dipping moderately to the ENE, bounded to the north and south by moderately north-dipping faults, and on the east and west by steep, NNE-striking faults. Deformation is dominantly by brittle faulting.
Mineralisation at Hugo South is centred on a high-grade (typically >2% Cu) zone of intense quartz stockwork veining, which in much of the deposit is confined, both vertically and laterally, to a series of narrow, dyke-like porphyritic quartz-monzodiorite intrusions and the enclosing basalt and dacite tuff. The stockwork zone forms an elongate tabular body, with a long axis plunging shallowly to the NNW, and an intermediate axis plunging moderately to the east. Copper grades gradually decrease upwards from the stockwork zone through the upper section of the massive augite basalt and the tuffs, and a broader zone of lower grades occurs below and to the west in volcanic rocks and quartz-monzodiorite.
The dominant sulphide minerals are chalcopyrite, bornite, chalcocite and pyrite, with minor molybdenite, enargite, tennantite and covellite, with rare sphalerite and galena. The sulphides are zoned, with bornite ±chalcopyrite, chalcocite and tennantite corresponding to the highest grades (>2.5% Cu), grading outwards to chalcopyrite (1 to 2% Cu). Pyrite-chalcopyrite ±enargite, tennantite, bornite, chalcocite and rarely covellite form low-grade (<1% Cu) zones, mainly in advanced argillic-altered tuff (Peters and Sylvester, 2014).
Alteration includes a minor preserved potassic suite, which has been swamped by the more prominent, overprinting muscovite/sericite, intermediate and advanced argillic assemblages. The minerals of the individual assemblages are not necessarily coeval, but may represent several overlapping paragenetic stages. The character of the host lithology strongly influences both the alteration and sulphide species. The dacitic ash flow tuffs are characteristically subjected to strong advanced argillic alteration comprising alunite, pyrophyllite, diaspore, dickite, topaz, zunyite, minor fluorite and rare dumortierite, accompanied by enargite, bornite+pyrite and locally covellite. In contrast, the augite basalt tends to be chlorite-muscovite-hematite altered with magnetite and chalcopyrite veining at depth, and exhibit pyrophyllitic advanced argillic alteration in the uppermost parts. Pockets of advanced argillic alteration are present locally in the high-grade zone in the quartz-monzodiorites. In more detail, the main alteration styles are as follows (after Peters et al., 2012; Peters and Sylvester, 2014):
• Chlorite-illite, which occurs as a weak outermost zone, in the uppermost part of the dacitic tuffs. It is not texturally destructive, and has no associated mineralisation.
• Advanced argillic alteration - which is characterised by quartz, pyrophyllite, kaolinite, topaz, diaspore, zunyite, alunite and dickite. It is dominant in the basaltic to dacitic tuffs of the upper Oyu Tolgoi sequence, subdivided into three suites.
- Pyrophyllite-kaolinite, with ubiquitous late white to pink dickite on fractures. This is the most widespread assemblage, and is predominantly found in the dacitic tuffs, in the upper augite basalt and in the quartz-monzodiorite.
- Quartz-alunite, which is whitish-pink to brown in colour, and typically occurs as bedding-parallel lenses in the dacitic tuffs, just above and east of, the high-grade, vein-rich deposit core.
- Topaz, which is vuggy and whitish-brown in colour, occurring as an areally limited, but intense, completely texture destructive zone in the dacitic tuffs and locally in augite basalts.
• Intermediate argillic alteration, characterised by pyrophyllite, kaolinite, dickite and muscovite/illite, the dominant alteration type in the high-grade deposit core and is much more extensively developed than at Hugo North. Hematite is common, occurring as fine specularite. This suite occurs in the upper parts of the massive augite basalt and straddling the contact with the basaltic and dacitic tuff, and is brownish-yellow in colour but turns pinkish where pyrophyllite rich.
• Quartz-sericite/illite-(muscovite), generally occurs in the quartz-monzodiorite intrusions, and is a feature of the strongly mineralised zone. It has local muscovite-topaz zones, is pale green to grey, mostly texturally destructive, and decreases in intensity with depth in the quartz-monzodiorite. It is extensively developed at the Hugo South deposit, where the quartz-monzodiorite is altered to depths of up to 1000 m.
• Chlorite-muscovite/illite-hematite, which is characteristically green, and is the equivalent in augite basalt country rocks of the quartz-sericite/illite zone found in the adjacent quartz-monzodiorite. Locally biotite is evident. The original textures are partially preserved, e.g. relict pseudo-hexagonal augite crystals. Relict magnetite occurs either as disseminations or in veins.
Southern Oyu Tolgoi Cluster
The Southern Oyu Tolgoi cluster of deposits includes the main Southwest, South and Central Oyu deposits and the Wedge and Bridge zones, as well as a number of smaller, fault-bounded zones. Together these zones form contiguous sectors of mineralisation representing multiple mineralising centres, each with distinct styles of mineralisation, alteration and host rock lithology. The boundaries between the individual deposits and zones is defined by the major faults described in the Structure section. These faults resulted in differential movement of the deposits and variations in the erosional histories of adjacent zones, depending on the depth to which a zone has been down-faulted or uplifted relative to its neighbour.
Central Oyu
Central Oyu comprises an upward-flaring, high sulphidation systems developed above, and partly telescoped onto, an underlying centre of porphyry style chalcopyrite-gold mineralisation. It is about 2300 m long and tapers from more than 600 m thick in the west, to ~200 m to the east. A supergene-enriched chalcocite blanket tens of metres in thickness overlies the high-sulphidation hypogene covellite-pyrite zone (Peters and Sylvester, 2014).
The deposit is separated from Hugo Dummett South, to the north, by the shallowly to moderately north-dipping Central Fault, and from the Central and Southwest Oyu deposits (and Bridge zone) to the south by the east-west trending Rhyolite fault (see the Structure section above), but is otherwise free of major bounding faults.
Mineralisation is associated with a feldspar-phyric quartz-monzodiorite, emplaced as a swarm of coalescing dyke-like intrusions into porphyritic augite basalt flows and overlying basaltic to dacitic tuffs of the Oyu Tolgoi sequence units DA1 and DA2 respectively. The quartz-monzodiorite has been dated at 371±1 Ma by U/Pb [zircon] TIMS (Wainwright, 2011). The Oyu Tolgoi sequence rocks are preserved as a series of relatively small, isolated, irregular, moderately north to NE dipping relict windows, within the quartz-monzodiorite intrusions. These volcanic windows are up to 200 m wide and extend for several hundred metres down-dip to the limit of drilling. The dominant host rocks are dacite tuff and quartz-monzodiorite. The Oyu Tolgoi sequence tuffs and sedimentary rocks are structurally overlain, above the Contact fault, by unmineralised conglomerate, mudstone, siltstone and mafic volcanic rocks of the Heruga sequence, which dip moderately to the east (Peters and Sylvester, 2014).
Several pulses of intra- and late-mineral quartz-monzodiorite intrusions have been distinguished in the Central Oyu deposit area, based on textural variations and intensity of mineralisation and alteration. Most occur as dykes, emanating from a larger intrusive mass to the north and west, and are terminated within the upper Oyu Tolgoi sequence sedimentary units (Peters and Sylvester, 2014).
The upper limit to mineralisation is the contact between the Oyu Tolgoi sequence tuffs and the overlying carbonaceous siltstones of unit DA3, which are, in turn, structurally overlain by the sedimentary Heruga sequence rocks above the Contact fault, represented here by a wide zone of breccia and foliated breccia (Peters and Sylvester, 2014).
Post-mineral dykes are common, comprising biotite-granodiorite, rhyolite, hornblende-biotite andesite and dacite. Rhyolite dykes are the most abundant, with most striking to the west and WNW in the southern half and on the periphery of the deposit area. Biotite-granodiorite dykes on the eastern margin tend to strike north to NNE, whilst hornblende-biotite andesite dykes trend ENE, mainly along the north-eastern margin (Peters and Sylvester, 2014).
The Central Oyu deposit accounts for ~20% of the Southern Oyu Tolgoi open pit reserve, and is the only deposit at Oyu Tolgoi where high sulphidation mineralisation is significant, with about 80 vol.% of the copper occurring as disseminated covellite, accompanied by pyrite (~10 vol.%), but lacking significant gold (Crane and Kavalieris, 2012).
Relics of early gold-bearing porphyry-style mineralisation remain at depth, and on the southern and western margins of the deposit. Chalcopyrite-gold mineralisation is dominant within both basalt and quartz-monzodiorite, straddling the intrusive contacts between the two lithologies. Higher grades are associated with zones of intensely contorted quartz stockwork veins, in which Au (g/t) to Cu (%) ratios reach 2:1. Peripheral, lower-grade mineralisation has gold to copper ratios of <1:1. Hematite, pyrite, chalcopyrite, bornite, magnetite and gold occur as disseminations and as fracture fillings. Early, intense A-type veins are preserved in the core of the porphyry system, unaffected by the high sulphidation event. These veins are also recognisable where subsequently overprinted in the high sulphidation zone, in the supergene chalcocite blanket, and crop out at the surface in the oxidised zone (Crane and Kavalieris, 2012).
The porphyry style chalcopyrite-gold mineralisation in the Oyu Tolgoi sequence basalts is accompanied by early biotite-magnetite alteration, which is characteristic of gold-rich chalcopyrite mineralisation in the southern deposits. Hematite subsequently overprinted magnetite, which was both pervasive and concentrated in micro-veinlets, and only remains in minor amounts, interspersed with the hematite. This alteration suite was subsequently overprinted by a quartz-sericite (muscovite) phase. Quartz-sericite alteration is particularly destructive in quartz monzodiorite, with complete replacement producing a pale rock with original porphyritic textures barely discernible. In contrast, sericitic alteration is generally less intense in augite basalt, where earlier mineral phases are only partly replaced by sericite, secondary quartz is less abundant, and the rock retains a greenish colour. The resultant altered rock comprises an assemblage of biotite, chlorite, epidote, sericite, albite, carbonate and anhydrite. Anhydrite and carbonates occur as late veins, whilst remnant K feldspar, probably representing the earliest alteration, increases with depth, occurring as vein selvages within biotite-altered basalt (Crane and Kavalieris, 2012; Peters and Sylvester, 2014).
The high-sulphidation part of the Central Oyu deposit, which lacks significant gold, contains a mineral assemblage of pyrite, covellite, chalcocite/digenite, enargite, tennantite, cubanite, chalcopyrite and molybdenite. This mineralisation is accompanied by local hypogene chalcocite/digenite in millimetre wide veins. The dominant host rocks are dacite tuff and quartz-monzodiorite. Higher-grade mineralisation occurs as disseminated and coarse grained fracture-filling sulphides in the zones of intense, contorted, quartz stockwork veining and anastomosing zones of hydrothermal breccias. These breccias are composed of quartz vein and quartz-monzodiorite fragments within an intensely sericitised matrix. Sulphide-filled fractures cut both the quartz veins and enclosing wall rock, and the high-grade mineralisation passes outward into a broad, weakly veined, low-grade halo of dominantly disseminated sulphides. Within the breccias, pyrite, chalcopyrite, bornite and enargite occur as relic grains, replaced by chalcocite and covellite, whilst pyrite also hosts small inclusions of covellite. Covellite, chalcocite and enargite occur as intimate intergrowths or as free disseminations. Cubanite and tennantite occur intergrown with or replacing enargite, and molybdenite occurs locally in quartz (Peters et al., 2012). Molybdenite has been dated at 373±1.2 Ma (Re/Os [Mo] (Kirwin et al., 2005).
Advanced argillic and sericite alteration accompanies the high-sulphidation mineralisation within quartz-monzodiorite and dacitic tuff/breccia. This comprises an advanced argillic assemblage of topaz, quartz, zunyite, diaspore, alunite, illite, andalusite, late kaolinite and dickite, which is associated with higher grade hydrothermal breccia-hosted mineralisation, and grades into a muscovite, sericite-dominant peripheral zone, accompanying lower grade disseminations (Peters and Sylvester, 2014).
A narrow zone of intermediate argillic alteration separates the advanced argillic and sericite alteration of the high-sulphidation stage, from the biotite chlorite alteration of the porphyry system. Intermediate argillic alteration is characterised by a creamy yellow to pale green coloured assemblage of kaolinite, chlorite, pyrophyllite, and illite (Peters and Sylvester, 2014).
An ~500 x 700 m supergene enrichment blanket, which varies from 0 to 55 m in thickness, overlies the high sulphidation pyrite-covellite assemblage, below a partially preserved ~20 to 60 m thick, hematitic limonite, goethite-rich leached cap, mainly after quartz-monzodiorite. The supergene blanket comprises pyrite, hematite and chalcocite/digenite, with lesser colusite, enargite, tenorite, covellite, bornite, chalcopyrite, cuprite and molybdenite. Pyrite is the dominant sulphide and is found as disseminated crystals. Sooty chalcocite occurs as rims or micro-veinlets in pyrite and covellite, and as independent disseminations. Colusite occurs as single grains, or is intergrown with chalcocite/digenite and/or pyrite. Tenorite occurs interstitial to silicate-iron oxide grain boundaries. Micro-grains of chalcopyrite, replaced by bornite and covellite, occur as small inclusions within pyrite. The leached cap is generally devoid of mineralisation, except on the edges of the eastern and southern flanks of the deposit, where patchy malachite and neotocite are found (Peters et al., 2012). The chalcocite blanket is of Cretaceous age, based on K-Ar dates of 93 and 117±1 Ma for supergene alunite (Perelló et al., 2001).
Minor exotic copper oxide mineralisation occurs in a Cretaceous palaeo-channel on the north-eastern flank of the deposit, where chrysocolla, malachite and neotocite mineralisation is found over a 400 x 300 m area as a thin 2 to 4 m thick layer at the base of the gravels (Crane and Kavalieris, 2012).
Southwest Oyu
The Southwest Oyu deposit comprises a large part of the Southern Oyu Tolgoi ore reserve. It is a gold-rich porphyry system, characterised by a south-west plunging, pipe-like geometry that has a vertical extent of up to 700 m. The high-grade core (>1 g/t Au) of the zone has a diameter of ~250 m, while the low-grade shell (> 0.3% Cu and >0.3% Cu) surrounding the core may extend over an area of as much as 600 x 2000 m (Peters et al., 2012).
The deposit is largely hosted within augite basalt of Oyu Tolgoi sequence unit DA1, and is truncated to the NW and SE by the steep, NE striking, West and East Bounding faults respectively. The West Bounding fault is intraformational within the augite basalt in the north, and separates it from quartz-monzodiorite in the south. To the north the deposit, and its lower grade NE periphery, the Bridge zone (see below) is separated from Central Oyu by the east-west trending Rhyolite fault, while it is cut to the south by the ENE trending Solongo fault (south side down). For detail on these faults, Structure section above.
More than 80% of the deposit is hosted by massive to fragmental porphyritic augite basalt of the Oyu Tolgoi sequence unit DA1, with the remainder within intra-mineral, Late Devonian (372±1 Ma; U/Pb [zircon] TIMS, Wainwright, 2011), quartz-monzodiorite intrusions. The deposit is centred on a cluster of small 10 to 30 m wide syn- to late-mineral porphyritic quartz-monzodiorite dykes and irregular plugs, with mineralisation extending for more than 100 m from each dyke or plug into the basaltic volcanic wallrocks (Kirwin et al., 2005). Several distinct phases of quartz-monzodiorite intrusion are recognised (after Peters and Sylvester, 2014):
• Early, strongly altered quartz-veined dykes, intimately associated with a high-grade central core of the deposit.
• Superimposed younger fragmental dykes that entrain early quartz vein clasts, but lack strong sulphide mineralisation.
• A large, massive, quartz-monzodiorite intrusion, that flanks and underlies the high-grade core, and contains weaker mineralisation.
The deposit is cut by several phases of usually steeply dipping post-mineral dykes, including rhyolite, hornblende-biotite andesite and biotite-granodiorite. The rhyolite dykes generally strike west to WNW in the deposit core, and NE when within major faults. The hornblende-biotite andesite dykes strike ENE, except where they intrude major NE trending faults.
The faults that demarcate the deposit, particularly the West and East Bounding faults, are composed of foliated cataclasite, gouge/breccia and mylonite, from a few metres to a few tens of metres thick, containing abundant quartz, quartz sulphide and sulphide (pyrite, chalcopyrite, sphalerite and galena) clasts in a comminuted matrix that is locally overprinted by fine-grained pyrite and chalcopyrite. This suggests that at least some of the fault movement was contemporaneous with mineralisation (Peters and Sylvester, 2014).
The geometry and kinematics of these faults, and the vein orientations and overall geometry of the deposit, suggest the deposit is developed within a dilational fault transfer zone between the West and East Bounding faults, where the local stress regime determines the preferred vein orientation (Peters and Sylvester, 2014). The same authors suggest the deposit probably formed as a sub-vertical cylindrical body, but was rotated to its present WSW plunge during post-mineral regional deformation.
Relatively high temperature, contorted and sinuous networks of milky white quartz veins are developed in both the mineralised quartz-monzodiorite and augite basalt, with strong quartz veining (>20 vol.%) and secondary biotite alteration defining the core of the porphyry system. The system is low sulphide (<5%), and the Cu-Au is related to chalcopyrite with only minor bornite. Chalcopyrite with subordinate pyrite and bornite occurs disseminated and as late fracture fillings within the quartz veins and host rocks. The veins also contain variable amounts of K feldspar, chlorite and carbonate, and are ubiquitous throughout the deposit. There is a general correlation between vein density and copper-gold grades. Most are several millimetres to several centimetres thick, although within the core of the ore zone, they can be up to a metre or more in thickness. Vein margins may be either planar or variably deformed, and are commonly folded and/or faulted. Within the high-grade core, the veins may be sub-parallel, to sheeted, with a preferred SW dip, grading to more irregularly oriented vein stockworks in the peripheries of the mineralised zones, where sub-vertical north to NW striking orientations are most common (Kirwin et al., 2005; Peters and Sylvester, 2014).
Gold is very fine, ranging from I to 120 µm, and is intergrown with chalcopyrite as veinlet infill, healing hydro-fractured pyrite crystals, and occurring as inclusions within, or on grain boundaries of chalcopyrite and bornite or gangue minerals. Metal ratios increase with depth, from an Au (g/t) to Cu (%) ratio of 2:1 near surface, to 3:1 at depth, while low-grade propylitic basalts surrounding the main high grade core maintain a ratio of 1:1 over an area of 600 x 2000 m around the high-grade core (Kirwin et al., 2005).
The high grade core of the deposit is truncated by the West and East Bounding faults to the NW and SE respectively. However, to the NE and SW, it passes into lower grade mineralisation, represented by the Bridge and Far South zones respectively, before being terminated by the Rhyolite and Solongo faults. The Bridge zone to the NE, described below, lies between the high grade core and the Rhyolite fault that marks the southern margin of Central Oyu in the NE. The decrease in grade in these two marginal zones corresponds to a lowering of vein densities, in chlorite and epidote altered basalt to the SW, and sericite and albite-altered quartz monzodiorite to the NE. Where present, early quartz veining predates magnetite veinlets, which are, in turn, post-dated by the main sulphide event. Chalcopyrite, bornite and pyrite are mainly disseminated within these low grade zones, with fracture- or vein-controlled sulphides being less prominent. The Far South zone, encompasses mineralised basalt with an Au (g/t) to Cu (%) ratio of 1:1 on the south-west margin of the deposit, separating the main high grade core from the Solongo Fault to the SE (Kirwin et al., 2005; Peters et al., 2012).
In the main deposit, alteration within the quartz-monzodiorite is predominantly early pervasive albite, overprinted by a subsequent quartz-sericite phase, with minor fluorite and rare tourmaline (Kirwin et al., 2005). This is typically expressed as pervasive quartz, sericite and pyrite, with remnant albite in vein selvages, small radiating clusters of tourmaline, and fluorite in quartz veins (Peters and Sylvester, 2014). The altered basaltic volcanic rocks are composed of chlorite, biotite, hematite-magnetite, weak sericite, and pink albite fracture and vein selvages, representing an early sodic-calcic alteration, followed by a biotite-magnetite (with hematite after magnetite) phase, overprinted by a chlorite-sericite assemblage (Peters and Sylvester, 2014).
The sodic-calcic phase, which is common in the Southern Oyu Tolgoi deposits, comprises an assemblage of actinolite-magnetite-albite-apatite-titanite and green biotite (Crane and Kavalieris, 2012) and is largely the result of interaction between the contrasting quartz-monzodiorite and augite basalt.
Pervasive biotite is evident in the core of the deposit, persisting outwards as vein selvages (Kirwin et al., 2005; Peters and Sylvester, 2014).
Biotite-magnetite alteration is characteristic of gold-rich chalcopyrite mineralisation in the Southern Oyu Tolgoi deposits. Strong brown biotite alteration partly replaces
actinolite-altered augite phenocrysts, whilst secondary magnetite is commonly present as pervasive alteration or in micro-veinlets (Crane and Kavalieris, 2012).
Advanced argillic alteration, occurring as quartz, sericite and kaolinite with late dickite veins, is associated with the high-sulphidation mineralisation in the quartz-monzodiorite breccia, whilst a quartz-monzodiorite dyke in the East Bounding fault, is altered to sericite in its upper levels with weak disseminated pyrite and chalcopyrite and is believed to represent the root zone of an eroded high sulphidation system (Kirwin et al., 2005; Peters and Sylvester, 2014).
Propylitic alteration developed in the western parts of the deposit area, is characterised by epidote veining and alteration, magnetite and hematite veining and alteration, veins of semi-massive pyrite, and albite alteration (Kirwin et al., 2005).
South Oyu
The South Oyu deposit is mainly hosted by basaltic volcanic rocks of the Oyu Tolgoi sequence, and is related to small, strongly-sericite altered, quartz-monzodiorite dykes (dated at 374±3 Ma; U/Pb [zircon] SHRIMP RG; Wainwright, 2011). It has 'sub-circular' areal dimensions of ~600 x 400 m, and mineralisation extends to depths of more than 500 m. It is separated from the Heruga 'block' to the south by the steep, ENE trending, Solongo fault (>1600 m, south block down). To the NW and north, it is separated from both the Southwest and Central Oyu deposits, by an ~200 to 600 m wide wedge-shaped block of Oyu Tolgoi sequence volcanic and sedimentary rocks, across the curvilinear NE to east-west striking South fault that truncates South Oyu to the north and NW. The displacement across this fault is NW and north side down. This intervening block, which is dislocated by splays within the broad South fault zone to the north and NW of South Oyu, hosts the discontinuous Wedge zone mineralisation (see below). Structural relationships suggest that movement on at least some of these faults, pre-dated or accompanied intrusion of the quartz-monzodiorite (see the Structure section above for details of the character of these faults). Displacement across all of the faults bounding South Oyu, indicate it was uplifted relative to the neighbouring deposits of the cluster (Peters and Sylvester, 2014).
The South Oyu deposit is hosted within an east to NE dipping suite of Oyu Tolgoi sequence augite basalt and dacitic tuffs of units DA1 and DA2 respectively, intruded in the SW by an irregular quartz-monzodiorite body. Much of the basalt sequence contains fragmental textures with juvenile pyroclasts, texturally similar to the overlying dacitic tuff sequence. To the NE, the altered and mineralised rocks are structurally overlain, across the Contact fault, by mudstones and conglomerates of the allochthonous Heruga sequence, which passes up-section into a basalt and sedimentary rock sequence, and is ultimately unconformably overlain by the Carboniferous Sainshandhudag Formation (Peters and Sylvester, 2014).
These volcano-sedimentary rocks are cut by numerous barren dykes, most of which are post-mineral rhyolite and basalt intrusive suites, and are typically only a few metres thick. The exception is a major, east-west rhyolite dyke that is up to a few tens of metres thick, and cuts through the middle of the South Oyu deposit, locally ballooning into larger intrusive masses where it intersects the South and Solongo faults. Although irregular in form, the rhyolite dykes strike approximate west to WNW and dip steeply. In contrast, the basalt dykes have moderate NE dips, that are sub-parallel to contacts within the stratified host rocks.
Mineralisation in the deposit is dominantly hosted in quartz-monzodiorite in the SW, in basalt throughout the centre, and in a minor zone of dacitic tuff and breccia on the northern margin. Contorted quartz veins are present, but there is no clearly defined zone of high quartz vein density such as at the Central and Southwest Oyu deposits. At the surface, copper mineralisation is present as steep, NW striking, strongly sheeted veins sets, but at depth veining becomes stockworks of thin (usually <10 cm) quartz–sulphide veins. However, both of these vein styles are only of minor significance, and sulphides dominantly occur as disseminated chalcopyrite, bornite and molybdenite. The principal copper sulphide is chalcopyrite, although in higher-grade areas bornite is locally more abundant. Magnetite is present as disseminations and veins, whilst small zones of elevated gold values occur locally. A small zone of high-sulphidation mineralisation occurs within a quartz-monzodiorite breccia in the western part of the deposit, adjacent to the South Fault, with an assemblage of pyrite, chalcopyrite, bornite, covellite and primary chalcocite (Peters and Sylvester, 2014).
Alteration of the basaltic rocks at South Oyu produced moderate chlorite, biotite, hematite-magnetite, weak sericite, and pink albite fracture and vein selvages. Hematite overprints magnetite. quartz-monzodiorite is typically pervasively altered to quartz, sericite and pyrite, as well as albite within vein selvages, small radiating clusters of tourmaline and fluorite in quartz veins. Advanced argillic alteration, comprising quartz, sericite and kaolinite with late dickite veins, is associated with the high-sulphidation mineralisation in the quartz-monzodiorite breccia (Peters et al., 2012).
An ~60 m thick weathered oxide zone overlies the South deposit hosted within basalt and quartz-monzodiorite. Due to the low pyrite content of the ore, and the reactive mafic composition of the basaltic hosts, leaching has been minimal, with copper carbonate (mainly malachite, with associated azurite, cuprite, chrysocolla, neotocite and tenorite) coating fractures in the weathered basalt. The blue-green copper minerals at South Oyu were known and partly exploited since prehistoric time. Native copper occurs sporadically in a thin zone at the oxide-fresh rock transition (Crane and Kavalieris, 2012).
Wedge
The Wedge zone is the northwestern part of the same mineralised zone that includes the South Oyu deposit as shown on the bedrock geology map of Oyu Tolgoi above. It is ~1400 m long, ~500 to 600 m in width in the NE, tapering to ~200 m in the SW, and is mineralised to depths of >500 m. It has an irregular, crescent-like wedge shape, and lies between the curvilinear South fault on the NW and northern margin of South Oyu, and the East Bounding fault that defines the SE limit of Southwest Oyu. The East Bounding fault merges into the Rhyolite fault, to the NE. Within the Wedge zone fault block, stratigraphic contacts are continuous and relatively planar, showing little evidence of substantial structural disruption. In addition, stratigraphic contacts are also relatively continuous between the Wedge and the Central deposit zones, suggesting that displacement on the East Bounding Fault was largely transferred to the Rhyolite Fault (which separates the Southwest/Bridge and Central zones) and that the Wedge and Central zones are, overall, a structurally intact block. Movement on the East Bounding and South faults has juxtaposed younger strata within the Wedge zone against older strata on the adjacent blocks. Consequently the Wedge and Central zones have been displaced downward relative to the Southwest and South deposit zones (Peters and Sylvester, 2014).
Mineralisation is hosted within a steep NE dipping sequence of Oyu Tolgoi sequence rocks, similar to that hosting the adjacent South zone. However, in the Wedge zone, the preserved dacitic tuff unit DA2, which constitutes the dominant host to copper mineralisation in this zone, is up to 180 m thick, significantly greater than in the South Oyu deposit zone. To the NE, the immediate hanging wall to mineralisation is the structurally overlying unmineralised rocks of the Heruga sequence and the lower section of the Carboniferous Sainshandhudag Formation. A large part of the western half of the Wedge zone is occupied by quartz-monzodiorite (Peters and Sylvester, 2014).
Abundant barren dykes cut the mineralised rocks of the Wedge zone, including biotite granodiorite, hornblende-biotite andesite and rhyolite. Biotite granodiorite and hornblende-biotite andesite intrusions are more commonly found along the NW margin of the zone, typically striking NE, parallel to the nearby East Bounding Fault, and are interpreted as sills, frequently intruding along the stratigraphic contact between the dacitic tuff and the overlying sedimentary strata. Rhyolite dykes are common throughout the zone, with variable strike orientations, and typically steep but varied dips (Peters and Sylvester, 2014).
The Wedge zone contains a core of high-sulphidation mineralisation, composed of pyrite, chalcopyrite, bornite, enargite, covellite and primary chalcocite in advanced argillic-altered host rocks. This high-sulphidation core is hosted within dacitic tuff and breccia of the Oyu Tolgoi sequence, and grades downward and to the SW into chalcopyrite, with lesser bornite, within underlying massive augite basalt host rocks, and pyrite and chalcopyrite mineralisation in quartz-monzodiorite. Higher grades of copper (>0.8% Cu) occur in a shallowly east dipping zone in the upper hundred metres of the dacitic tuff/breccia unit. Gold is absent, except locally, adjacent to the South Fault (Peters and Sylvester, 2014).
The dacitic tuffs and breccias have been subjected to advanced argillic alteration, comprising an assemblage of kaolinite, zunyite, pyrophyllite, muscovite, illite, topaz, diaspore, alunite, montmorillonite, late dickite and fluorite. This alteration zone has increased Cu grades towards its centre, and progressively overprints an adjacent marginal barren, specular hematite-rich zone. The advanced argillic alteration grades downward into and overprints a biotite-magnetite-chlorite assemblage, mainly within underlying massive augite basalt host rocks, with hematite overprinting and replacing the magnetite. In the south-western part of the Wedge zone, sericite and pyrite alteration is present within the quartz-monzodiorite (Peters and Sylvester, 2014).
Crane and Kavalieris (2012) suggest that, while high sulphidation mineralisation is largely absent at the Southwest and South Oyu deposits due to erosion, it has been preserved in the relatively down-dropped Wedge zone and contiguous Central Oyu deposit, where it overlies porphyry style mineralisation similar to that at Southwest and South Oyu.
Bridge
The Bridge zone is a low grade interval between the Central and Southwest Oyu deposits, occurring as a 250 m long triangular zone, that tapers from 500 m wide in the north to 30 m in the south. It is separated from Central Oyu to the north by the Rhyolite fault and from the Wedge zone to the SE by the East Bounding fault. Mineralisation is hosted by chlorite and epidote-altered basalt of Oyu Tolgoi sequence unit DA1 and lesser sericite- and albite-altered quartz-monzodiorite. The principal sulphide minerals, chalcopyrite, bornite and pyrite are mainly disseminated, with fracture- or vein-controlled sulphides being less prominent than in surrounding ore zones. It appears to represent the low grade NE margin of the Southwest Oyu deposit (Peters and Sylvester, 2014).
Heruga and Heruga North
See the separate Heruga record for a detailed description.
Ulaan Khud South and North
Two prospect have been partially tested on the northern extremity of the Oyu Tolgoi deposit trend, Ulaan Khud South and Ulaan Khud North, which are ~7.5 and ~11 km to the north to NNE of Hugo North respectively.
Ulaan Khud South
Ulaan Khud South was discovered during a 35 diamond drill hole condemnation program in 2006 to early 2007 over a proposed airport site, and was originally known as the Airport North Zone. This program was undertaken by Ivanhoe Mines on behalf of the joint venture with Entrée Gold Inc. One additional hole was drilled in 2008 (Vann et al., 2008).
The oldest rocks in the prospect area are the Devonian (or older), structureless, porphyritic, augite basalts of unit DA1, ignimbrites and tuffs of unit DA2, laminated carbonaceous siltstone and fine sandstone of unit DA3, and basalt flows and breccias, and volcanic-derived sandstones of unit DA4. Units DA2 to DA4 were hornfelsed during intrusion of the nearby Permian Khanbogd Granite Complex, which intrudes DA1 <1 km to the east. The contact between unit DA4 and the structurally underlying unit DA3 is mapped as a zone of strong ductile deformation marking the Contact Fault (Vann et al., 2008).
Carboniferous rocks of the Sainshandhudag Formation occur to the west of the interpreted extension of the West Bat fault. Textures have been largely obliterated by hornfelsing, although basaltic tuffs and lavas of unit CS3 and coarse conglomerates and siltstones most likely of unit CS2 have been recognised (Vann et al., 2008).
Intrusive rocks include strongly hornfelsed quartz monzodiorite that is otherwise texturally similar to the causative intrusion at Hugo North. This intrusive has been dated at 361.4±3.71 (U/Pb zircon, SHRIMP; Crane and Kavalieris, 2012 and sources quoted therein), which is regarded as a minimum age. An upward flaring body of biotite granodiorite, associated with a large north-south-trending dyke swarm, is the most abundant intrusive phase at Ulaan Khud South, intruding all Devonian-age rocks. Where unaltered, it is pinkish-brown, but where hornfelsed, is black to green. It consists of 3 to 10 mm feldspar phenocrysts in a fine-grained aphanitic groundmass (Vann et al., 2008).
The Late Permian sodic-alkalic Khanbogd Granite Complex intrudes the Devonian and Carboniferous sequence. Thin pegmatitic or aplitic sills, and dykes and/or greisen zones, parallel the contact, and can be found up to several hundred metres into the country rock, whilst hornfelsing extends for ~1 km or more from the shallow outward dipping contact between of the granite and surrounding sedimentary and volcanic rocks. The hornfels zone at Ulaan Khud is a garnet-hornblende-diopside-epidote assemblage, indicating metamorphic grades of at least amphibolite-facies. Disseminated and massive pyrrhotite replacement within basic rocks is considered to be related to the hornfelsing (Vann et al., 2008).
Two generations of basalt dykes have been recognised, respectively pre- and post-dating the Khanbogd Granite (Vann et al., 2008).
The mineralised zone is overlain by 40 to more than 80 m of Cretaceous soil and gravel, that comprises (after Vann et al., 2008):
• a basal sandy unit, about 10 m thick, developed in the deeper parts of a northerly trending palaeochannel,
• a massive, 50 m thick, montmorillonite-bearing red clay,
• a poorly sorted clayey gravel, commonly with rounded to angular boulder size clasts of basement lithologies in a red clay matrix, postulated to be a talus slope or alluvial fan deposit possibly derived from a fault scarp that a fault may flank the palaeochannel.
A broad, shallowly southward-plunging, synclinal structure has been interpreted on the eastern side of the prospect, with DA4 basalts in its core. The north-south trending biotite granodiorite dyke swarm mainly intrudes the western side of the syncline. A north-south oriented fault is inferred to exist between the Carboniferous sequence to the west, and Devonian units to the east, in a similar orientation to the West Bat Fault (Vann et al., 2008).
A 30 to 50 m wide, steeply-dipping mineralised zone has been defined at Ulaan Khud South, over a north-south strike length of ~900 m, and vertical extent of up to 600 m. Narrow, patchy, high-grade copper and gold intervals have been encountered within this mineralised zone, although after >12 000 m of drilling the average grade of the mineralised zone has been <0.3% Cu (Vann et al., 2008).
Mineralisation is largely hosted within DA1 augite basalts and includes chalcopyrite infilling late brittle fractures in quartz veins, or as disseminations, and, more rarely, in association with pyrite in "railway line"-style quartz veins. Molybdenite is developed on late-stage fractures and in quartz veins. One drill hole intersected narrow, irregular veinlets of sphalerite, galena and chalcopyrite accompanied by pyrite and/or pyrrhotite plus epidote, chlorite and calcite. Gold is association with epidote in the veins. Pyrrhotite is common, usually occurring as veinlets, disseminations or massive replacement in basic rocks, especially the basaltic dykes. It can be associated with chalcopyrite and pyrite in porphyry style mineralisation, but increases downward towards the Khanbogd Granite and appears to be a later stage event that has been linked to the hornfelsing. Sericite, and lesser secondary biotite and magnetite alteration have been observed (Vann et al., 2008; Crane and Kavalieris, 2012).
Ulaan Khud North
The Ulaan Khud North prospect is approximately 3 km to the north of Ulaan Khud South, concealed below 60 to 80 m of Cretaceous clay and gravel (Ivanhoe Mines 2011).
A program of 25, mostly vertical, drill holes on a 100 m square grid, each of 182 to 377 m depth has outlined a zone of shallow mineralisation over an area of 600 x 300 m (Ivanhoe Mines 2011). No further significant testing has been reported since.
Mineralisation is hosted within quartz monzodiorite, similar to the hosts at Oyu Tolgoi, and occurs as porphyry-style stockwork, disseminations and massive veins of chalcopyrite, with molybdenite disseminations and veinlets, and trace bornite (Ivanhoe Mines 2011).
Drilling has encountered intervals of from 5 to 132 m @ 0.25 to 0.35% Cu, 0.01 to 0.045 g/t Au and 20 to 80 ppm Mo. Within these broader intersections, there are numerous intervals of 3.5 to 20 m @ 0.5 to 1% Cu, 0.05 to 9.2 g/t Au and 100 to 400 ppm Mo. The best intersections were 9.6 m @ 2.43% Cu, 0.121 g/t Au and 1490 ppm Mo; 19.0 m @ 0.94% Cu, 0.098 g/t Au and 290 ppm Mo; and 7.65 m @ 2.24% Cu, 0.203 g/t Au and 810 ppm Mo (Ivanhoe Mines 2011).
Ivanhoe Mines (2011) suggested the presence of intensively mineralised and altered quartz monzodiorite and basalt xenoliths within moderately mineralised quartz monzodiorite may indicate an earlier phase of stronger mineralisation exists at depth and along margins of the tested area. No mention is made of hornfels alteration similar to that at Ulaan Khud South. Otherwise the geology is assumed to be similar to that described above at Ulaan Khud South.
Javkhlant
The Javkhlant prospect is located ~4.5 km SW of Heruga, and is considered to be the southernmost known mineralisation on the Oyu Tolgoi trend. It lies to the south of the east-west to ENE-WSW trending, dextral, Javkhlant Fault, and on the basis of geological mapping, may represent a faulted offset of the Heruga mineralisation, displaced 5 km to the west (Peters et al., 2012).
The Javkhlant target principally consists of two induced polarisation anomalies, Javkhlant I and II to the NE and SW respectively, most likely representing a continuous zone of mineralisation that has been cut and displaced by a NW-SE trending cross-fault (Peters et al., 2012; 13).
The geology at surface is complicated by a series of steep NNE-SSW, SW-NE and NW-SE trending faults. It is dominated by rocks of the Heruga sequence unit DA4, with lesser Oyu Tolgoi sequence DA1 augite basalts, Carboniferous basaltic and rhyolitic dykes, and in-faulted slivers of Carboniferous volcanic and sedimentary rocks (Peters et al., 2012).
The Heruga sequence is composed of a lower unit DAba, comprising a basaltic to dacitic hyaloclastite with intrusive basaltic trachyandesite sills, extrusive hyaloclastite breccias and hyaloclastite tuffs. A pelagic or turbiditic facies is Interbedded with the extrusive rocks. The other main component of the sequence is unit DA4a, a dark green basaltic volcanic breccia with vesicular, fine-grained to coarsely porphyritic basaltic clasts, interlayered with volcanogenic sandstones and conglomerates. A third, subordinate unit, DA4b is interbedded with both of these units, and is composed of thinly interbedded red and green siltstone, with lesser basalt layers near its base. At Javkhlant I, unit DAba predominates to the east, and DA4a to the west. The sequence is near vertical, but becomes steeply east dipping at depth. A downward thickening, fault bounded wedge of Oyu Tolgoi sequence rocks occurs in the core of the Javkhlant I target area in the west, sandwiched between unit DA4 rocks. It is composed of unit DA1b augite basalts, DA2b dacitic pyroclastic rocks and DA3b fine carbonaceous siltstone and fine brown sandstone (Peters et al., 2012; 2013).
The Javkhlant area is cut obliquely at depth to both the NW and south by the Carboniferous (324±3 Ma; Wainwright, 2008) Javkhlant Mountain pluton, a late hornblende-biotite granodiorite (Gd2). The contact zone is shrouded by hornfels alteration. The Heruga and Oyu Togoi sequences appear to represent a deep roof pendent in this area, between the two main batholith exposures (Peters et al., 2012; Wainwright, 2008).
Drilling at Javkhlant I intersected mineralisation within unit DA2b, containing >10% pyrite at down-hole depths of from 1384 to 1679 m, before passing into unmineralised granitoids. Grades were low, although the intersection included 30 m @ 0.96% Cu, 0.01 g/t Au from 1422 m. The chalcopyrite-pyrite mineralisation is associated with high temperature advanced argillic alteration, typical of the shallower parts of mineralised systems at Oyu Tolgoi (Peters et al., 2012). However, it is also characterised by a higher temperature assemblage of corundum, andalusite, muscovite, pyrophyllite, kaolinite and distinctive blue lazulite (Crane and Kavalieris, 2012).
A second intersection at Javkhlant I, 250 m lower than the first, passed through a ~700 m thick interval of abundant fine disseminated pyrite with minor zones of semi-massive pyrite ±chalcopyrite within units DA2b, DA1b and in-faulted DA4A rocks, before passing into barren granitoids at 2352 m. Better intersections included 76 m @ 0.31% Cu, 0.04 g/t Au from 2000 m, and 36 m @ 0.39% Cu, 0.05 g/t Au from 2110 m, both in unit DA1b augite basalts. In addition, two intervals within unit DA4a were 16 m @ 0.02% Cu, 0.41 g/t Au from 2208 m, and 18 m @ 0.05% Cu, 0.63 g/t Au from 2296 m (Peters et al., 2012).
Peters et al. (2013) suggest the presence of mineralisation in both the DA4 and DA1 units, hornfelsing, and a lack of porphyry-style quartz veining, imply the mineralisation and alteration at Javkhlant I may be related in part to the Carboniferous granodiorite. This interpretation is supported by the identification of the phosphate mineral lazulite. The only other known occurrence of lazulite in the district is a Carboniferous-age mineralising system related to nearby granites.
Drilling at Javkhlant II encountered strongly hornfelsed rocks throughout, due to the close proximity to the Javkhlant granodiorite batholith. The hornfelsing produced abundant secondary biotite and sparse garnet, and has destroyed much of the primary textures, although the first half of the hole to around 500 m, is largely DA1b augite basalt, intruded by andesitic dykes. A coarse conglomerate, assigned to unit DA2a, was also intersected, containing some DA1b clasts. The DA1b basalt contains disseminated and vein pyrite/pyrrhotite with pyrite up to 8% and trace chalcopyrite. There are rare porphyry style quartz veinlets. Core from 500 m to the end of the hole at 877.4 m, was mainly unmineralised trachyandesite volcaniclastics and fine siltstones, most likely correlatable with the Carboniferous Aman-Us Member unit CS3 (Peters et al., 2012).
Reserves and Resources
The main Oyu Tolgoi deposits for which resources have been estimated, are distributed over a 12 km interval of a 25 km corridor of mineralisation that defines the NNE aligned Oyu Tolgoi trend. From north to south, the deposits comprise Hugo Dummett, divided into a north and a south deposit, the Southern Oyu Tolgoi cluster, including the Central, South and Southwest Oyu deposits and smaller intervening satellite zones, and Heruga and Heruga North.
Total reported JORC compliant measured + indicated + inferred resources + reserves (which are reported separately) for all of the deposits, as at 31 December, 2014 (Rio Tinto Annual Report, 2015) amounted to 6.382 Gt @ 0.67% Cu, 0.29 g/t Au, of which 1.494 Gt @ 0.85% Cu, 0.31 g/t Au, 1.23 g/t Ag were proven + probable reserves. This resource amounts to 42.76 Mt of contained copper and 1850 tonnes of gold. The richest part of the deposit is North Hugo, with proven + probable reserves of 499 Mt @ 1.65% Cu, 0.35 g/t Au, 3.39 g/t Ag at the northern end of the string of deposits, whilst Heruga at the opposite end of the trend, which includes a molybdenum zone, has total resources (reserves not yet announced) of 1.817 Gt @ 0.39% Cu, 0.36 g/t Au, 1.40 g/t Au, 113 ppm Mo (=0.64% Cu equivalent).
In February 2003, at a 0.3% Cu equivalent cut-off, the four deposits of the Hugo Dummett and Southern Oyu cluster had:
- an indicated resource of 508.9 Mt @ 0.4% Cu, 0.59 g/t Au + an inferred resource of 1.602 Gt @ 0.63% Cu, 0.17 g/t Au.
In February 2003, at a 0.6% Cu equivalent cut-off, the same deposits yielded:
- an indicated resource of 266.9 Mt @ 0.53% Cu, 0.86 g/t Au + an inferred resource of 811.7 Mt @ 0.90% Cu, 0.21 g/t Au.
The indicated resource was all at SW Oyu.
In March 2007, at a 0.6% Cu equivalent cut-off, the same deposits yielded a:
- measured + indicated resource of 1.387 Gt @ 1.33% Cu, 0.47 g/t Au + an inferred resource of 1.397 Gt @ 0.98% Cu, 0.24 g/t Au.
- Total resource = 2.785 Gt @ 1.15% Cu, 0.35 g/t Au.
In March 2010, resource figures include (at a 0.6% Cu cut-off - source Ivanhoe Mines, 2010):
• All Hugo North deposits:
- an indicated resource of 820.2 Mt @ 1.82% Cu, 0.42 g/t Au + an inferred resource of 818.3 Mt @ 1.00% Cu, 0.30 g/t Au.
• Hugo South deposits:
- an inferred resource of 490.3 Mt @ 1.05% Cu, 0.09 g/t Au.
• Southern Oyu deposits (Central, Southern and SW):
- a measured + indicated resource of 567.23 Mt @ 0.62% Cu, 0.55 g/t Au + an inferred resource of 88.5 Mt @ 0.47% Cu, 0.41 g/t Au.
• Heruga deposits (Ivanhoe and Javkhlant JV Heruga):
- an inferred resource of 970 Mt @ 0.48% Cu, 0.48 g/t Au.
Total resource figures at March 2012, at a 0.6% Cu eq. cut-off for the full Oyu Tolgoi project (Ivanhoe Mines, 2012), including the Hugo North and South deposits, the Southern Oyu deposits (Central, Southern and SW), Heruga and Hugo North Extensions:
- Total measured + indicated resources of 1.38743 Gt @ 1.33% Cu, 0.47 g/t Au; plus
- Total inferred resources of 2.36713 Gt @ 0.78% Cu, 0.33 g/t Au
Total measured + indicated + inferred resources of 3.754 Gt @ 0.98% Cu, 0.38 g/t Au.
Total reserve figures at March 2012, which are included within resource figures, (Ivanhoe Mines, 2012), including the Hugo North and Southern Oyu deposits were:
• Hugo Dummett deposits: - a probable reserve of 438 Mt @ 1.93% Cu, 0.42 g/t Au.
• Southern Oyu deposits: - a proven + probable reserves of 957 Mt @ 0.49% Cu, 0.32 g/t Au.
Total Oyu Tolgoi proven + probable reserves of 1.395 Gt @ 0.94% Cu, 0.35 g/t Au.
Total resource figures at December 31, 2014, at cut-offs of 0.0.37% Cu eq. (underground) and 0.22% CuEq. (open pit) for the full Oyu Tolgoi project (Rio Tinto Annual Report, 2015; Turquiose Hill Resources, Technical Report, October 2014), including the Hugo North and South deposits, the Southern Oyu Tolgoi deposits (Central, Southern and SW), Heruga and Hugo North:
- Heruga
measured + indicated + inferred underground resources of 1817 Mt @ 0.39% Cu, 0.36 g/t Au, 1.40 g/t Ag, 113 ppm Mo, 0.64% Cu eq.
- Hugo Dummett North and North Extension
measured + indicated + inferred underground resources of 1476 Mt @ 0.96% Cu, 0.30 g/t Au, 2.65 g/t Ag, <50 ppm Mo.
- Hugo Dummett South inferred underground resources of 839 Mt @ 0.77% Cu, 0.07 g/t Au, 1.78 g/t Ag, 66 ppm Mo, 0.83% Cu eq.
- Southern Oyu Tolgoi (Oyut) measured + indicated + inferred open pit resources of 491 Mt @ 0.30% Cu, 0.18 g/t Au, 0.91 g/t Ag.
- Southern Oyu Tolgoi (Oyut) measured + indicated + inferred underground resources of 265 Mt @ 0.38% Cu, 0.44 g/t Au, 0.99 g/t Ag.
Total measured + indicated + inferred resources
Total reserve figures at December 31, 2014, which are NOT included within resource figures, (Rio Tinto Annual Report, 2015), were:
- Hugo Dummett North underground: - a proven + probable reserve of 499 Mt @ 1.65% Cu, 0.35 g/t Au, 3.39 g/t Ag.
- Southern Oyu Tolgoi open pit: - a proven + probable reserves of 995 Mt @ 0.45% Cu, 0.29 g/t Au, 1.23 g/t Ag.
Total Oyu Tolgoi proven + probable reserves of 1.494 Gt @ 0.85% Cu, 0.31 g/t Au, 1.95 g/t Ag.
TOTAL Oyu Tolgoi reserves + resources of 6.382 Gt @ 0.67% Cu, 0.29 g/t Au.
The most recent source geological information used to prepare this decription was dated: 2007.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Oyu Tolgoi
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Crane, D. and Kavalieris, I., 2012 - Geologic Overview of the Oyu Tolgoi Porphyry Cu-Au-Mo Deposits, Mongolia: in Hedenquist J W, Harris M and Camus F, 2012 Geology and Genesis of Major Copper Deposits and Districts of the World - A tribute to Richard H Sillitoe, Society of Economic Geologists Special Publication 16, pp. 187-213.
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Dolgopolova, A., Seltmann, R., Armstrong, R., Belousova, E., Pankhurst, R.J. and Kavalieris, I. 2013 - Sr-Nd-Pb-Hf isotope systematics of the Hugo Dummett Cu-Au porphyry deposit (Oyu Tolgoi, Mongolia): in Lithos v.164 pp. 47-64
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Gao, J., Klemd, R., Zhu, M., Wang, X., Li, J., Wan, B., Xiao, W., Zeng, Q., Shen, PO., Sun J., Qin, K. and Campos, E., 2017 - Large-scale porphyry-type mineralization in the Central Asian metallogenic domain: A review: in J. of Asian Earth Sciences Available on-line from October 18, 2017, 30p.
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Gao, J., Qin, K., Zhou, M.-F. and Zaw, K., 2018 - Large-scale porphyry-type mineralization in the Central Asian Metallogenic Domain: Geodynamic background, magmatism, fluid activity and metallogenesis: in J. of Asian Earth Sciences Online, https://doi.org/10.1016/j.jseaes.2018.08.023.
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Hart-Madigan, L., Wilkinson, J.J., Lasalle, S. and Armstrong, R.N., 2020 - U-Pb dating of hydrothermal titanite resolves multiple phases of propylitic alteration in the Oyu Tolgoi Porphyry District, Mongolia: in Econ. Geol. v.115, pp. 1605-1618.
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Khashgerel B E, Rye R O, Hedenquist J W and Kavalieris I, 2006 - Geology and Reconnaissance Stable Isotope Study of the Oyu Tolgoi Porphyry Cu-Au System, South Gobi, Mongolia: in Econ. Geol. v101 pp 503-522
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Khashgerel B E, Rye R O, Kavalieris I and Hayashi K, 2009 - The Sericitic to Advanced Argillic Transition: Stable Isotope and Mineralogical Characteristics from the Hugo Dummett Porphyry Cu-Au Deposit, Oyu Tolgoi District, Mongolia : in Econ. Geol. v104 pp 1087-1110
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Khashgerel, B.-E., Kavalieris, I. and Hayashi, K.-I., 2008 - Mineralogy, textures, and whole-rock geochemistry of advanced argillic alteration: Hugo Dummett porphyry Cu-Au deposit, Oyu Tolgoi mineral district, Mongolia: in Mineralium Deposita v.43, pp. 913-932.
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Kirwin D J, Forster C N, Garamjav D 2003 - The discovery history of the Oyu Tolgoi porphyry copper-gold deposit, South Gobi, Mongolia: in NewGenGold 2003, Conference Proceedings, Perth WA, Louthean Media, Perth pp 130-146
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Kirwin D J, Forster C N, Kavalieris I, Crane D, Orssich C, Panther C, Garamjav D, Munkhbat T O and Niislelkhuu G, 2005 - The Oyu Tolgoi Copper-Gold Porphyry Deposits, South Gobi, Mongolia: in Seltmann R, Gerel O and Kirwin D J, (Eds.), 2005 Geodynamics and Metallogeny of Mongolia with a Special Emphasis on Copper and Gold Deposits: SEG-IAGOD Field Trip, 14-16 August 2005, 8th Biennial SGA Meeting; CERCAMS/NHM, London, IAGOD Guidebook Series 11, pp 155-168
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Perello J, Cox D, Garamjav D, Sanjdorj S, Diakov S, Schissel D, Munkhbat T-O, Oyun G 2001 - Oyu Tolgoi, Mongolia: Siluro-Devonian Porphyry Cu-Au-(Mo) and high-sulfidation Cu mineralization with a Cretaceous Chalcocite blanket: in Econ. Geol. v96 pp 1407-1428
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Porter, T.M., 2015 - The geology, structure and mineralisation of the Oyu Tolgoi porphyry copper-gold-molybdenum deposits, Mongolia: A review : in Geoscience Frontiers v.7, pp. 375-407.
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Seltmann R and Porter T M, 2005 - The Porphyry Cu-Au/Mo Deposits of Central Eurasia: 1. Tectonic, Geologic & Metallogenic Setting and Significant Deposits: in Porter, T.M. (Ed), 2005 Super Porphyry Copper & Gold Deposits - A Global Perspective, PGC Publishing, Adelaide, v.2 pp. 467-512
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Seltmann, R., Dolgopolova, A. and CERCAMS team, 2012 - Porphyry Cu-Au/Mo Deposits of Central Eurasia: Geodynamics and Metallogeny: in Existing Resources, New Horizons, KazGeo 2012, Almaty, Kazakhstan, 29-31 October 2012, Conference Proceedings, 4p.
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Seltmann, R., Porter, T.M. and Pirajno, F., 2014 - Geodynamics and metallogeny of the central Eurasian porphyry and related epithermal mineral systems: A review: in J. of Asian Earth Sciences, v.79, pp. 810-841.
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Shen, P., Pan, H., Hattori, K., Cooke, D.R. and Seitmuratova, E., 2018 - Large Paleozoic and Mesozoic porphyry deposits in the Central Asian Orogenic Belt: Geodynamic settings, magmatic sources, and genetic models: in Gondwana Research v.58, pp. 161-194.
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Wainwright, A.J., Tosdal, R. M., Wooden, J.L., Mazdab, F.K., and Friedman, R.M., 2011 - U-Pb (zircon) and geochemical constraints on the age, origin, and evolution of Paleozoic arc magmas in the Oyu Tolgoi porphyry Cu-Au district, southern Mongolia: in Precambrian Research v.19 pp. 764-787
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Wainwright, A.J., Tosdal, R.M., Lewis, P.D. and Friedman, R.M., 2017 - Exhumation and Preservation of Porphyry Cu-Au Deposits at Oyu Tolgoi, South Gobi Region, Mongolia: in Econ. Geol. v.112, pp. 591-601.
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Wan, B., Xiao, W., Windley, B.F., Gao, J., Zhang, L. and Cai, K., 2017 - Contrasting ore styles and their role in understanding the evolution of the Altaids: in Ore Geology Reviews v.80, pp. 910-922.
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Yakubchuk, A., Degtyarev, K., Maslennikov, V., Wurst, A., Stekhin, A. and Lobanov, K., 2012 - Tectonomagmatic Settings, Architecture, and Metallogeny of the Central Asian Copper Province: in Hedenquist J W, Harris M and Camus F, 2012 Geology and Genesis of Major Copper Deposits and Districts of the World - A tribute to Richard H Sillitoe, Society of Economic Geologists Special Publication 16, pp. 403-432
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