Heruga, Heruga North
Cu Au Mo
Super Porphyry Cu and Au|
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The Heruga and Heruga North Mo rich Cu-Au porphyry deposits are located in the Gobi Desert of southern Mongolia, 550 km due south of the capital, Ulaanbaatar and 80 km north of the Chinese border, and immediately to the south of the Oyu Tolgoi deposits (#Location: 42° 58' 48"N, 106° 49' 35"E).
The two contiguous deposits lie within the SW trending, >20 km long, mineralised Oyu Tolgoi structural corridor, immediately to the SSW of the southernmost of the Oyu Tolgoi deposits, Southwest and South Oyu. It falls within a >3 km long, north-south oriented zone of high chargeability that is up to 1000 m wide and wedges out to the NE towards the Southwest Oyu deposit. Drilling to early 2014 has delineated a >5 km long coherent shell of Mo rich Cu-Au mineralisation at Heruga and Heruga North, underlain by a zone of Au-rich Cu mineralisation. The Mo-rich mineralisation, which is largely restricted to the Heruga deposit, is similar, but of higher grade, to a zone in the upper parts of the Hugo Dummett North deposit on the northern extremety of the Oyu Tolgoi deposit trend.
Regional Setting, Geology and Stratigraphy
The regional setting, local geology and stratigraphy, of the Oyu Tolgo trend and Heruga deposits are described in the separate Oyu Tolgoi record. The district geology is illustrated in the map below.
The Heruga deposits are separated from the Southern Oyu Tolgoi cluster by the subvertical, east- to ENE-striking, Solongo Fault. 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. 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; Wainright, 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.
• 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).
• The Javkhlant, or South Sparrow fault crosses the Heruga trend <250 m south of the known Heruga mineralisation. It has an apparent south side down 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). 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 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).
• The South Bor Tolgoi fault (Fig. 6) 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.
The Heruga and Heruga North porphyry copper-gold-molybdenum deposits are the most southerly of the currently known deposits within the >25 km long, NNE elongated (20°) Oyu Tolgoi trend for which resource estimates have been calculated.
The Heruga deposit is hosted by both late-Devonian basaltic volcanic rocks of the Oyu Tolgoi sequence unit DA1b, and quartz-monzodiorite intrusives that are nearly identical to the host rocks at the Oyu Tolgoi deposits, whilst Heruga North mineralisation is restricted to the former suite of basaltic rocks and the adjacent quartz-monzodiorite is largely barren. The Oyu Tolgoi structural corridor that bounds the Heruga deposits is flanked by Devonian and Carboniferous volcanic and sedimentary rocks.
The NNE trending, north plunging Heruga deposit is ~2.5 km long, with a width of 200 to 450 m. The shallowest mineralisation is obscured at a depth of 500 to 600 m below the current surface. The major ENE trending Javkhlant (or South Sparrow) fault passes just to the south of Heruga, while the subparallel Heruga North fault, ~3 km to the north, with a 1000 m dextral and ~600 m north side down displacement, separates the Heruga and Heruga North deposits. The Heruga North mineralisation continues to plunge to the north from its shallowest depth of ~1200 m below surface, and is assumed to be limited by the steep, NNE trending, north side up Solongo fault, 2.5 km further to the north. The same fault marks the southern boundary of the Southern Oyu Tolgoi deposits. Heruga North is assumed to represent the southward, down-dropped, continuation of the Southern Oyu Tolgoi deposits (Peters et al., 2013; Crane and Kavalieris, 2012).
The Heruga deposit is cut by a series of NNE trending vertical faults that each step down 200 to 500 m to the west, and have divided the deposit into at least two structural blocks of mineralisation. These structures include the Bor Tolgoi and Bor Tolgoi West faults, which are ~400 to 600 m apart, and converge to the south. In the eastern block, to the east of the Bor Tolgoi fault, mineralisation continues down-dip to the east with apparently declining grades, whilst ore in the western block, west of the Bor Tolgoi fault is terminated against the Bor Tolgoi West fault. Further weak mineralisation is developed within the augite basalts to the west of the latter fault (Peters et al., 2013).
The Heruga deposit is largely hosted by Devonian augite basalts of the Oyu Tolgoi sequence unit DA1b, and to a lesser extent within intrusive quartz-monzodiorite. The augite basalts are unconformably overlain by Devonian polymictic volcanic conglomerate, breccia and minor sandstone of unit DA2a and dacitic block and ash tuffs of unit DA2b. Units DA2a and DA2b are variably developed, and both are not always present at any one point. Either may unconformably overlie the basalts and dominate in a specific area. Unit DA2 has been logged as predominantly dacitic tuffs in the southern sections of the Heruga deposit (Crane and Kavalieris, 2012), whilst polymictic volcanic conglomerate of unit DA2a is the principal lithology further to the north (Peters et al., 2013). Unit DA2 is followed by thinly bedded siltstone, mudstones and carbonaceous mudstone of unit DA3b over much of the Heruga and Heruga North deposits.
The fine sedimentary rocks of unit DA3 are structurally overlain, above the Contact fault, by the moderately east dipping, but isoclinally folded and overturned, allochthonous Heruga sequence, comprising red-green, bedded siltstone with subordinate volcanogenic sandstone and basalt, passing structurally upwards, via a green massive sandstone into a thick (>800 m) succession of basaltic breccia, flows with interbedded siltstones to mudstone, and tuff. In places, thick lenses of green massive sandstone also occur within the upper basaltic breccia and flows (Peters et al., 2013).
An irregular dyke-sill-plug complex of Late Devonian porphyritic quartz-monzodiorite intrudes the Devonian augite basalts, along and to the east of the Bor Tolgoi Fault, and is considered to be the causative intrusions related to mineralisation and alteration. West of the Bor Tolgoi Fault, only a small quartz-monzodiorite dyke is known. In the south of the defined deposit, the quartz monzodiorite takes the form of an upward tapering wedge, the eastern margin of which dips east and is generally concordant with the east dipping intruded sequence below the Devonian Oyu Tolgoi sequence unit DA2b dacitic block and ash tuffs and breccias. The western contact dips steeply to vertically, following or cut by the steep Bor Tolgoi fault where it is juxtaposed against the massive augite basalt found below the down-thrown dacitic tuffs and breccias. Towards the central to northern parts of the known deposit, this intrusive wedge becomes one or more east dipping sills, truncated to the west by the Bor Tolgoi fault. Further to the north, the quartz monzodiorite occurs as a steep, upward tapering dyke controlled by the same fault, but appears to flare to the east and form a large stock within the Heruga North area. Within the Heruga deposit block, these intrusions are small compared to the stocks found in the Hugo Dummett and Southern Oyu Tolgoi areas (Peters et al., 2013).
Largely unmineralised dykes, mainly hornblende-andesite and biotite-granodiorite, make up about 15% of the volume of the deposit area, and cut all other rock types (Peters et al., 2013).
The Heruga deposit is split into an eastern and a western fault block, juxtaposed across the Bor Tolgoi fault. Towards the south of the Heruga deposit, virtually all of the orebody is within the eastern block (see section N4758400), with an apparent displaced extension across the fault progressively increasing to the north, until the ore is almost all within the west block (see section N4759300). The two blocks appear to have a sinistral offset across the Bor Tolgoi fault, although the eastern block hosts a significant body of quartz-monzodiorite, which is largely absent from the west block (Peters et al., 2012). See also the Structure section above.
The Heruga deposit, in both fault blocks, is composed of vertically overlapping zones of mineralisation, consisting of a molybdenum-rich carapace at higher elevations, with increasing gold-rich mineralisation at depth. A copper-rich interval overlaps both the molybdenum- and gold-rich zones. At a 100 ppm Mo cut-off, the Mo zone has a vertical extent of 300 to 400 m. The top of the copper zone, defined by a 0.3% Cu cut-off, approximately corresponds with the top of the molybdenum zone, but extends to greater depths, for a total vertical extent of ~600 m. The top of the gold zone, at a 0.3 g/t Au cut-off, is generally ~280 m below the top off the copper and molybdenum zones, locally resulting in a small overlap with the molybdenum zone, and a more significant overlap with the copper zone. The bottom of the gold zone generally extends below the base of the copper zone by about 200 m, giving the gold zone a total vertical extent of ~450 m. The three zones together, combine to define a continuous zone of mineralisation with a vertical extent of close to 800 m.
As at Southwest Oyu, chalcopyrite dominates in the Heruga and Heruga North deposits, although in their deeper levels, bornite contents increase (Crane and Kavalieris, 2012).
Mineralisation is associated with quartz chalcopyrite stockwork veins which are frequently deformed (Peters and Sylvester, 2014). The density of mineralised veining is much lower at Heruga than in the Southern Oyu Tolgoi and Hugo Dummett deposits (Peters and Sylvester, 2014), where higher grades closely correspond to increased vein density, possibly explaining the lower overall grade of the Heruga deposit.
There is a general spatial correlation between mineral species and alteration. At deeper levels, mineralisation consists of chalcopyrite and pyrite in veins and disseminations within biotite-chlorite-albite-actinolite altered basalt or sericite-albite altered quartz monzodiorite. Bornite appears within the lower copper zone, before the Cu grade begins to decline into the gold zone. The bornite commonly impregnates quartz stockwork veins and may precede the precipitation of chalcopyrite, which is also generally disseminated throughout the host rocks. Salmon pink albite altered selvages accompany chalcopyrite veining associated with higher grade mineralisation in both basalt and quartz monzodiorite. At higher elevations, the orebody is overprinted by strong quartz-sericite-tourmaline-pyrite alteration, where mineralisation consists of disseminated and vein controlled pyrite, chalcopyrite and molybdenite, representing an increase in the pyrite content, decrease in gold and stronger quartz-sericite alteration (Crane and Kavalieris, 2012).
Gold appears to correlate with strong biotite alteration in the deeper levels of the deposit, where the Au (g/t) to Cu (%) ratio may be >10:1. This ratio decreases rapidly upwards to ~1:1 in the copper-rich zone, and is <1:1 in the molybdenum zone (i.e. within the >100 ppm Mo shell). Below the molybdenum zone, there is an inverse relationship between gold and copper, reflecting increasing gold and decreasing copper grades with depth. In the deepest parts of the deposit, chalcopyrite diminishes to only trace amounts, and is replaced by pyrite as the dominant sulphide. This suggest that gold is not related to chalcopyrite, but is possibly associated with pyrite at the deepest levels (Peters et al., 2012).
High-grade copper and gold have a strong spatial association with contacts between quartz-monzodiorite porphyry intrusion in the southern part of the deposit and east of the Bor Tolgoi Fault, occurring both within the outer portion of the intrusion and in adjacent enclosing augite basalt country rock. However, west of the Bor Tolgoi Fault, where only a small quartz monzodiorite dyke is known, the possible association is less apparent. No significant high-sulphidation style mineralisation has been identified to date at Heruga (Peters et al., 2012).
Gold-base metal veins, composed of chalcedony, calcite, sphalerite, galena, electrum, native gold and sulphosalts, are found on the periphery of the Heruga deposits and in thin (millimetric) veins in a late quartz-monzodiorite intrusion at Heruga (Crane and Kavalieris, 2012).
The alteration paragenesis which produced the composite assemblages described above, is as follows, after Crane and Kavalieris (2012):
• Sodic-calcic alteration is the earliest phase at Heruga and Heruga North, comprising an assemblage of actinolite-magnetite-albite-apatite-titanite and green biotite (Crane and Kavalieris, 2012), which is largely the result of interaction between the contrasting silica rich quartz-monzodiorite and mafic augite basalt. It is most evident from replacement of augite phenocrysts in the basalts by actinolite.
• Propylitic alteration, peripheral to the sodic-calcic core, is clearly developed in the western parts of the Heruga deposit, where it is characterised by pervasive epidote, magnetite/hematite and albite alteration, and associated epidote, magnetite-hematite and massive pyrite veining. A younger, post-Carboniferous epidote alteration event is common in the region, and cannot be readily distinguished from the older porphyry-related propylitic phase.
• Biotite-magnetite alteration, which is characteristic of the gold-rich chalcopyrite mineralisation at Heruga and Heruga North, particularly within the augite basalt host rocks. Strong, secondary, brown biotite partly replaces actinolite-altered augite phenocrysts, whilst secondary magnetite is commonly present as pervasive alteration or in micro-veinlets. Titanite and possibly apatite are invariably associated with secondary biotite alteration as by-products of the modification of primary augite or hornblende.
• Hematite-chlorite-sericite assemblages overprint the biotite-magnetite alteration as a retrograde stage.
• Quartz-sericite-pyrite alteration, which is well developed in quartz-monzodiorite, but only partially replaces earlier assemblages within augite basalt, where earlier mineral phases are not completely replaced by sericite, secondary quartz is less abundant, and the rock retains a greenish colour. This zone hosts the disseminated and vein controlled pyrite, chalcopyrite and molybdenite of the molybdenum zone in the upper parts of the deposit.
• Tourmaline-sericite alteration is present at the Heruga deposit, where fine-grained tourmaline occurs in the uppermost parts of the sericite alteration that overprinted quartz-monzodiorite. This late alteration is characterised by large rosettes of tourmaline, commonly nucleated on pyrite, and large crystals of pink-white albite up to several centimetres in size.
• Post-mineral quartz-calcite veining.
The Heruga North mineralisation is similar to that at Heruga, and consists of strong chalcopyrite-pyrite mineralisation, but with only trace molybdenite occurring in veinlets and as disseminations. A similar lower gold-chalcopyrite zone is evident. The host sequence comprises intensely quartz veined massive augite basalt with strong pervasive biotite-magnetite alteration and minor pink albite on vein and fracture selvages, and represents the northward extension of the mineralisation at Heruga. Quartz-monzonite at Heruga North would appear to largely represent a barren post-mineral phase.
A deep drill hole (OTD1495A) in the northern third of the Heruga North deposit passed into a zone of bornite-rich, high sulphidation associated mineralisation on the eastern side of the quartz monzodiorite. This hole intersected a 343 m interval to the end of the hole at a depth of 2377 m, averaging 0.87% Cu, 0.06 g/t Au, 12 ppm Mo, including 47.2 m @ 1.32% Cu, 0.1 g/t Au, 8 ppm Mo. This mineralisation is on the opposite side of the intrusion to the main Heruga North deposit outlined to date, and has been regarded as similar to the ore found on the eastern margin of the intrusion at the Hugo Dummett deposits (Peters et al., 2012).
In March 2008, the deposit was estimated to contain an inferred resource of:
760 Mt @ 0.48% Cu, 0.55 g/t Au, 0.0142 % Mo, using a 0.60% Cu equiv. cut-off grade.:
Based on this initial estimate, the Heruga Deposit contains at least 415 t of gold.
At a 1% Cu equiv. cut-off grade, the inferred resources is estimated at:
210 Mt @ 0.57% Cu, 0.97 g/t Au and 0.0145% Mo (containing over 200 t Au).
In May 2010, the deposit was estimated to contain an inferred resource of:
910 Mt @ 0.48% Cu, 0.49 g/t Au, 0.0141 % Mo. Based on this initial estimate, the Heruga Deposit contains at least 445 t of gold.
In September 2014, the estimated inferred resource at a 0.37% Cu Equiv. cut-off, was (Peters and Sylvester, 2014):
1.816 Gt @ 0.39% Cu, 0.37 g/t Au, 1.40 g/t Ag, 0.0113% Mo, 0.64% Cu Equiv.
The most recent source geological information used to prepare this summary was dated: 2014.
Record last updated: 14/6/2015
This description is a summary from published sources, the chief of which are listed below.
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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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