Yanacocha, Kupfertal, La Quinua
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Yanacocha comprises a cluster of major high sulphidation Au-Cu sulphide deposits that have undergone strong supergene oxidation in their upper 50 to 300 m, hosted by mid to late Miocene volcanic rocks of the Cajamarca Mineral Belt in the high Andes of northern Peru. They are ~18 km north of the city of Cajamarca and 625 km north of Lima, at elevations of 3400 to 4250 m (#Location: 6° 58' 48"S, 78° 30' 44"W).
The epithermal ores are underlain by sub-economic porphyry Au-Cu mineralisation, including the Kupfertal deposit. They are also underlain by supergene enriched Cu-Au blankets, such as Yanacocha Verde, which are superimposed on the shallowly dipping high sulphidation Cu-Au sulphides of the lithocap mineralisation above the porphyry system. The mineralised system is overlain in part by the La Quinua transported deposits, which lie within a fault controlled basin, hosted by unconsolidated Quaternary glacial gravels.
The Yanacocha cluster of Au(-Cu) deposits are hosted by Miocene volcanic rocks that occur at the southern termination of the Northern Peruvian volcanic belt, a continuous NNW trending suite of Cainozoic volcanic rocks that extend into Ecuador.
These deposits define an ~18 x 6 km corridor aligned along the 50 to 60° trending Chicama-Yanacocha trans-orogen structure, focussed at the intersection with a major, long-lived NW-SE trending, broad structural zone that had controlled Mesozoic basin architecture.
The oldest Cainozoic rocks in the Yanacocha district are volcanic suites of the Calipuy Group, which include 19.5 to 15.9 Ma andesites and andesitic lahars, and 15.5 to 15.1 Ma dacitic pyroclastics. These thin and pinch-out towards the main Yanacocha deposits, where they are overlain by the 14.5 to 8.4 Ma Yanacocha Volcanic Complex, which is composed of three alternating cycles of lavas and pyroclastic rocks, that locally rest unconformably on Cretaceous sedimentary rocks.
The Yanacocha Volcanic Complex has been subdivided into the:
i). Lower Andesite Sequence, comprising the ~14.5 to 13.3 Ma Atazaico Andesite, an intercalated succession of pyroxene→hornblende andesitic flows and tuffs with flow-breccia facies and rare associated flow domes, interspersed with lesser block and ash flow tuffs, and lahars, with rare, associated flow domes. These were erupted from small stratovolcanoes progressively younging from SW to NE. They pass up into a transitional unit dominated by ignimbrites and fine-grained, laminated epiclastic sequences that grade into the overlying unit. These extrusives are intruded by plugs of the Quilish dacite, which predate the 12.6 Ma Cori Coshpa ignimbrite.
ii). Yanacocha Pyroclastic Sequence, which erupted in the centre of the district, is a variable suite of andesitic to dacitic ignimbritic to lithic crystal tuffs of the 12.6 Ma Cori Coshpa and 12.5 to 12.4 Ma Maqui Maqui ignimbrites. These porous rocks predate most of the mineralisation, but are extensively altered in the central portion of the district and are the primary host to the majority of gold deposits;
iii). Upper Andesite Sequence, which commences with hornblende→pyroxene bearing andesitic to dacitic lavas, flow-domes and minor pyroclastic rocks of the 12.1 to 11.6 Ma Azufre Andesite. The andesite porphyry dykes and plugs of the Yanacocha porphyries temporally overlap the Maqui Maqui ignimbrites and the Azufre Andesite. The Azufre Andesite is overlain by the 11.5 to 11.2 Ma San Jose Ignimbrite, which includes three members of hornblende-pyroxene (biotite) dacite and andesite that erupted in the centre of the district and flowed southward. Mineralogically similar flow domes were emplaced into the inferred vents. These ignimbrites are also altered and mineralised, and are succeeded by the late, low volume and more siliceous ~11.1 to 8.2 Ma Coriwachay dacite-rhyolite magmatism, dominated by the ~10.8 Ma Corimayo dacitic multiple flow dome complexes, and the intrusive ~9.9 Ma Cerro Yanacocha dacite porphyry and ~8.4 Ma Yanacocha Lake rhyolitic dykes and plugs.
The entire volcanic pile has been crosscut by multiple phases of phreatic, phreatomagmatic and hydrothermal breccias, mostly spatially associated with the late Cerro Yanacocha dacite porphyry.
Pervasive acid-sulphate alteration produced advanced argillic assemblages in multiple coalescing centres across the Yanacocha district. In individual deposits/centres, mineral assemblages are zoned outward from a pervasive, massive siliceous, generally gold-bearing core, which grades abruptly, both laterally and downward, into an intense, acid-leached vuggy quartz envelope, and where preserved, cappings of granular quartz, which are interpreted to represent near-surface, vapour-dominated horizons. These siliceous cores are surrounded by broader envelopes of advanced argillic alteration (alunite-pyrophyllite-quartz) to still broader zones of intermediate argillic and propylitic alteration halos. These alteration zones are broadly stratabound, and are zoned over hundreds of metres laterally and metres to tens of metres vertically.
Ar-Ar dating of hydrothermal alunite, has defined 6 separate stages (1 to 4, 5A and 5B) of quartz-alunite alteration, ranging from 13.5 to 8.2 Ma, each temporally and spatially associated with intrusions, flow dome cluster and brecciation pulses i.e., the Quilish dacitic intrusions; the Azufre Andesite flow domes; the Corimayo dacitic flow domes; the Cerro Yanacocha dacite porphyry; the main phreatomagmatic/hydrothermal brecciation; and the Yanacocha Lake rhyolitic dykes respectively. On a district scale, there is a general temporal progression of multiple alteration centres from the SW to NE ends of the mineralised corridor, and back to the centre.
The broadly tabular, gently dipping, advanced argillic quartz-alunite-pyrophyllite alteration zones are locally overprinted by cross-cutting, steep intermediate sulphidation zones of characterised by 'creamy' coloured cryptocrystalline chalcedony plus barite with higher gold grades.
The deeper Kupfertal Cu-Au porphyry system has secondary hydrothermal biotite dated at 10.7 Ma, associated with gold and chalcopyrite in porphyry style 'A' and 'D' veins and as disseminations. The deep potassic zone was overprinted upward by a chlorite-sericite zone and then by an intermediate argillic suite straddling the boundary between the two preceding phases, and finally at shallower depths by an advanced argillic assemblage. The deep potassic alteration is temporally equivalent to the stage 3 quartz-alunite alteration, the shallow Corimayo dacitic flow domes and the main stage high sulphidation gold-copper mineralisation event. The latter is characterised by fine-grained gold bearing pyrite with minor enargite and covellite at most of the Yanacocha epithermal gold deposits. The potassic porphyry and stage 3 quartz-alunite alteration are, in turn, overprinted by stage 4 and 5 quartz-alunite alteration that are 0.8 and 1.5 m.y. younger, respectively. The stage 4 alteration is accompanied by a high grade gold event, apparently associated with intermediate sulphidation, recognised by the occurrence of coarse gold and late barite and/or creamy chalcedony. The stage 5A alteration is related to a late copper-gold pulse, closely associated with dacitic intrusions and phreatomagmatic breccias, and is characterised by an enargite-covellite-gold bearing pyrite with quartz-alunite alteration at shallow levels, and pyrophyllite-diaspore alteration at depth. The final stage 5B alteration appears to accompany sparsely distributed veinlets of rhodochrosite-dolomite and base metal sulphides, and is coeval with the Yanacocha Lake rhyolitic dykes.
During the Pleistocene, glaciation scoured the upper sections of the hypogene Cerro Yanacocha mineralised system and deposited the detritus en masse as moraines and winnowed periglacial gravels in the La Quinua and La Pajuela basins to produce the large, unconsolidated, La Quinua deposits.
Finally, supergene oxidation and leaching to depths of between 50 to 300 m was responsible for the oxidation of sulphides, principally covellite-enargite±trace chalcopyrite, leaching of copper from the weathering profile to leave a copper depleted capping of iron oxides and remnant liberated gold, and the formation of copper enrichment sulphide blankets of covellite-chalcocite intermixed with hypogene enargite below the base of oxidation.
Prior to the era of modern exploration, pre-Columbian workings for mercury, hematite, native sulphur and gold were known over a number of the currently delineated deposits of the Yanacocha cluster, and the historical name for one of the stream draining to the NW of the deposits was Corimayo, which in the Quechua language translates to 'gold river'. The first documented report of these workings at Yanacocha was by Ramondi (1913), reporting on his expedition
of 1859 in northern Peru. Following the discovery of the Michiquillay porphyry Cu deposit, ~20 km to the ESE of Yanacocha by Asarco in 1957, that company undertook exploration over silicified breccia pipes in the area, including 670 m of drilling in 5 holes, but did not pursue further work. From 1968 to 1970, the Nippon Mining completed 13 exploration diamond drill holes, each of ~200 m depth, testing for porphyry copper mineralisation in the centre of the Yanacocha cluster. This drilling only returned marginal copper grades, although no core was assayed for gold.
In 1970-71, the British Geological Survey (BGS), on behalf of the Peruvian Government, completed a regional stream sediment sampling program, covering a large part in northern Peru, including what is now the Yanacocha district, assaying for Cu, Pb, Zn and Ag but not Au. The Yanacocha district was highlighted as being anomalous in Ag with ~10 ppm Ag values, and >20 ppm where streams penetrated the massive quartz replacement cores. In 1973, St. Joe Minerals Co. completed an induced polarisation survey and three diamond drill holes in the western part of the district. As a follow-up to the BGS results, a joint-venture alliance between the BRGM of France and Compañia de Minera Buenaventura acquired mineral concessions within the Yanacocha district and commenced trenching at Cerro Yanacocha in 1982 that yielded significant >20 ppm Ag and >300 ppm Pb anomalies.
In 1983, Newmont was invited to visit the site and were shown exposures of the epithermal system with bulk tonnage potential. A letter of intent was signed with Newmont in 1984 and a program of 25 percussion dill holes completed near Cerro Yanacocha, to suggest a bulk tonnage silver deposit with 3.125 Mt @ ~90 g/t Ag. The best gold result from this program was 7 m @ 9.6 g/t Au. An exploration joint veture between Newmont (40%) and BRGM-Compañia de Minera Buenaventura was signed in September 1985, with Newmont as operator.
Exploration between 1985 and 1993 led to the recognition of the Yanacocha cluster as an ~18 x 6 km, NE aligned corridor, defined by gold-bearing, high-sulphidation systems, and the delineation of an oxide reserve of ~125 t of contained gold in the Cerro Carachugo, San Jose and Yanacocha Norte deposits. Production commenced in September 1993.
A second generation of discoveries between 1994 and 2001 included a series of buried and blind high sulphidation oxide gold deposits and the large, gold-bearing, glacial gravel-hosted fan deposit at La Quinua. Following that period, the recognition of the low grade Kupfertal Au-Cu porphyry deposit, led to a phase of deeper exploration beneath the gold bearing high sulphidation oxidised caps, into underlying higher grade, sulphide-dominant zones and the transitional environment into shallow Au-Cu porphyry mineralisation. In 2006 a system of high-level, nested Yanacocha Verde Au-Cu porphyry deposits and associated supergene sulphide blankets were discovered beneath the Yanacocha Sur oxide deposit, extending for ~1 km southwest beneath the high sulphidation cap.
This discovery history summary is drawn from Teal and Benavides, 2010, which should be consulted for more detail.
The Yanacocha (or Cajamarca) district lies within the Peruvian section of the Andean Cordillera, south of the Huancabamba Deflection, where cordillera has a NNW structural trend. The Peruvian Andes are part of the Andean orogenic belt, and consists of folded and thrusted Proterozoic and Ordovician to Cretaceous sedimentary basement rocks, overlain by early Tertiary to Holocene volcanic sequences and early to late Tertiary intermediate to felsic intrusive rocks. These Tertiary rocks include the northern Peruvian volcanic belt, a continuous, NW to NNW trending Oligocene to Eocene and Miocene to Pliocene suites of bimodal andesite to rhyolite volcanic rocks that extend into southern Ecuador. The Yanacocha epithermal gold deposits are hosted by volcanic rocks that occur at the southern terminus of this belt.
For more details of the broader regional setting and geology of the Peruvian Andes, see the separate Peruvian Andes record.
In the Cajamarca-Yanacocha district, there is a major bend in the Cordillera, referred to as the Chimu Andes trend or Cajamarca Curvature (Benavides-Caceres, 1999), which deflects the Mesozoic strata into a series of west and WNW trending fold axes and faults that project beneath the Tertiary Yanacocha Volcanics, before resuming the regional NW-SE trend further to the north.
Within this section of the Andes, there are a series of prominent parallel, NE striking faults and fractures, oriented transverse to the trend of the Cordillera. These structures define the Cutervo-Cajamarca structural zone, and include four major lineaments the Cajamarca (south of Yanacocha), Sipan-Hualgayoc (north of Yanacocha, and 40 km NW of the Cajamarca lineament), La Zanja-Tantahuatay and Cutervo (south of La Granja, and ~120 km NW of the Cajamarca lineament). The Yanacocha cluster of deposits lies along the 50 to 60° trending Chicama-Yanacocha structural corridor, which parallels and is located between the Cajamarca and Sipan-Hualgayoc lineaments (Quiroz, 1997; Turner, 1999).
The principal tectonic events of the Andean Cycle in Peru (Cobbing et al., 1981; Megard, 1984), which commenced with subduction of the Farallon plate in the late Triassic to early Jurassic, have persisted for ~200 m.y. (Petford and Atherton, 1995), punctuated by the split of the Farallon plate into the Nazca and Cocos plates in the late Oligocene to early Miocene. However, crustal thickening in the overriding plate only started in the Upper Cretaceous, whereas during Jurassic and Lower Cretaceous times have essentially been marked by arc and back-arc extension (e.g., Mpodozis and Ramos, 1989; Jaillard and Soler, 1996).
Northern Peru lies within the interval of current flat slab subduction between the east-west Carnegie Ridge at the Equator (adjacent to the southern coast of Ecuador), and the NE-SW Nazca Ridge intersecting the Peru-Chile Trench at ~15°S, offshore south central Peru, south of Lima. This interval is marked by an absence of any current volcanic activity. Flat slab subduction, and the absence of magmatism, is interpreted to be the result of thicker, more buoyant oceanic plateau in this section of the Nazca Plate. As a consequence, the subducted oceanic plate directly underlies the crust, without an intervening asthenosphere wedge, which has been pushed hundreds of kilometres inland below thick crust. The same phenomena is regarded as being responsible for breaks in magmatism, both spatially and temporally, in the past (Martinod et al., 2010). Even thicker oceanic plateaux to the north of the Carnegie Ridge were responsible for the suppression of subduction and accretion of oceanic terranes to the continental margin in Colombia (e.g., Cediel et al., 2003).
Tectonic shortening occurred during three major, discrete, but multistage pulses in the Upper Cretaceous (Peruvian event), Eocene (Incaic event) and Neogene (Quechua event). Other periods during the Cainozoic, in contrast, are marked by only moderate shortening and locally by extension. Each of the Incaic and Quechua events is marked by a series of episodes of compression, uplift and erosion, followed by magmatic pulses and then by extension and sedimentary deposition (Noble et al., 1990; Benavides-Caceres, 1999).
The Cainozoic part of this period of tectonism in northern Peru involved four main episodes of tectonism and volcanism:
i). The first began after the Incaic I orogeny at ~59 to 55 Ma in the early Eocene, with eruptions of silicic ash-flow tuffs (Llama Formation; Cobbing et al., 1981; Noble et al., 1990; Benavides-Caceres, 1999).
ii). This was followed by the ~43 to 42 Ma Incaic II orogeny and subsequent eruptions of trachytic- and coarsely porphyritic andesitic lavas, interbedded with thin-bedded volcaniclastic siltstones (Huambo Cancha andesite). These rocks are locally tightly folded by a compression not observed in the overlying sequence (Longo et al., 2010). Diorite stocks exposed in the Minas Congas and Michiquillay districts span 4 m.y. in the middle Eocene, and may be roots of stratovolcanoes from which these volcanic rocks were erupted (Noble et al., 1974; McKee and Noble, 1982).
iii). The Incaic IV orogeny at ~23 Ma was the culmination of an interval of volcanic quiescence during the Oligocene. Following this deformation, arc volcanism was reinitiated near Yanacocha, with conglomerates, 23.2 Ma volcaniclastic rocks and air-fall tuffs (Chala sequence), separated by an angular unconformity from the volcanic rocks of the previous cycle (Noble et al., 1990). Miocene volcanic rocks near Yanacocha that are part of this sequence, include up to 415 m of coarsely porphyritic hornblende and hornblende-pyroxene andesites that are divided in two overlapping sequences, the Tual and Chaupiloma lahars, that are separated by a disconformity. Both units are dominated by lahar and hyperconcentrated
flow deposits. The Tual andesite, which is restricted to the SW and west of the district, contains poorly reworked epiclastic rocks, pumice rich debris-flow
and lahar deposits, as well as volcaniclastic sandstones and pebble conglomerates, and has been dated at 19.53±0.13 Ma (Longo et al., 2010). The Chaupiloma lahars, dated at 15.90±0.18 Ma and mainly found in the NE part of the district, is characterised by thick grey lahar deposits of hornblende-pyroxene andesite, interbedded with rare ash-fall tephra and volcaniclastic sedimentary rocks from pebble conglomerates to laminated siltstone. These ages suggest there may have been a significant time gap between the Tual and Chaupiloma andesites (Longo et al., 2010).
These are followed, above an erosional disconformity, by the ~220 m thick Cerro Fraile Pyroclastic Sequence which is restricted to a north to NW trending, 10 x 20 km, basin to the NE of Yanacocha, covering an area of >300 km2. This sequence is composed of a series of 1 to 10 m thick dacitic ignimbrites and unwelded, poorly bedded pyroclastic-surge deposits interlayered with thin-bedded <1 m beds of tuff. 40Ar/39Ar dating of these rocks has yielded ages of 15.51±0.05 to 15.78±0.17 Ma (Longo et al., 2010; Turner,1997). This sequence is followed by an erosional hiatus (Longo et al., 2010). Sequences from the Llama Formation to this hiatus are included in the regional Calipuy Group that are cumulatively up to 600 m thick, and were erupted from numerous volcanic centres over a wide area between ~55 and ~15 Ma (Reyes, 1980; Wilson, 1984; Turner, 1997; Davies, 2002; Longo, 2005). These may be divided into a Lower and an Upper Calipuy Group, separated by a hiatus representing the Quechua I orogeny at ~23 Ma, and by the degree of deformation.
iv). In the Yanacocha area, the hiatus at the top of the Upper Calipuy Group, at ~15 Ma, is followed by younger andesitic and dacitic lavas, domes, ignimbrites, and minor subvolcanic intrusions, dykes, and breccias of the 14.5 to 8.4 Ma Yanacocha Volcanic Complex (Longo et al., 2010). Locally, this complex has been subdivided (after Teal and Benavides, 2010) into the,
• Lower Andesite Sequence, mainly lavas which are 20 to 320 m thick, equivalent to the 14.5 to 13.3 Ma Atazaico Andesite of Llongo et al. (2010), followed after a volcanic gap by the,
• Yanacocha Pyroclastic Sequence, including the explosive Cori Coshpa trachyandesitic ignimbrite and overlying Maqui Maqui dacitic ignimbrite that together constitute the 12.6 to 12.4 Ma, 50 to 225 m thick Colorado Pyroclastic Sequence).
• Upper Andesite Sequence, which comprises intercalated andesitic to dacitic to rhyolitic flows and ignimbrites dominated by multiple
flow dome complexes. It commences with the 45 to 310 m of pyroxene-hornblende andesitic to dacitic lavas (of the regional effusive, 12.1 to 11.6 Ma Azufre Andesite) and the explosive, 40 to 350 m of ignimbrites (the 11.5 to 11.2 Ma, San Jose Ignimbrite). These more substantial units are overlain by a condensed sequence of 10.78±0.05 Ma flow foliated dacitic domes and associated silicified laminated rocks (Corimayo dacitic domes), the Cerro Yanacocha dacite porphyry intrusion, and the post-mineral Yanacocha Lake rhyolitic dyke and coeval welded rhyolitic, 8.43±0.04 Ma Negritos rhyolitic ignimbrite. The Upper Andesite Sequence was, in the past, mapped as part of the Lower Andesite Sequence, due to similar megascopic compositional and textural characteristics, and similar flow and flow-dome horizons within both sequences.
• Intrusions - multiple pulses of late-stage dykes, plugs and stocks have been intruded into the volcanic pile at Yanacocha, ranging in age from 14 to 8.4 Ma, some of which are spatially and temporally associated with deeper Au-Cu porphyry-style mineralisation beneath the gold-dominant high sulphidation epithermal deposits (Turner, 1997; Longo, 2005).
• Breccias - the entire Yanacocha Volcanic Complex has been cut by multiple phases and styles of breccia that are both spatially and genetically associated with episodes of gold mineralisation, as described below (Teal and Benavides, 2010).
At ~8.4 Ma, arc volcanism ceased near Yanacocha, correlated with initiation of flatslab subduction, and development of a volcanic gap between latitudes 2 and 15°S in northern Peru (Longo et al., 2010).
The basement occurs as a mosaic of topographic highs and basins/grabens that resulted in abrupt thickness changes in the volcanic pile that separates basement from the Yanacocha Volcanic Complex, which particularly in the immediate Yanacocha area, mostly rests directly on Cretaceous rocks.
The oldest exposed sequences in the Yanaocha district are folded and thrusted sequences of the Lower Cretaceous Goyllarisquiza Group, which to the far SW comprise sandstones of the Farrat Formation. In the NE and around Yanacocha, the regionally extensive Middle Cretaceous carbonate sequence is represented by the
Pulluicana-Quillquiñan Group and the Cajamarca and Yumagual formations, composed of thick bedded limestone with minor intercalated shale partings. These rocks have been subjected to NE vergent compressional deformation from the pre-volcanic Incaic I and then the Incaic II orogenesis (Rivera, 1980; Wilson, 1985; Benavides-Cáceres, 1999). These sequences form the basement to the Cainozoic succession described above.
Mineralisation within northern Peru is broadly associated with Miocene magmatism. Yanacocha lies towards the southwestern margin of a broad ~30 km wide by >120 km long, NW-SE trending belt of more than eight porphyry Cu-Au and epithermal Au deposits that extends from Michiquillay in the SE to beyond La Granja (Llosa et al., 1999; Diaz et al., 1997) and Cañariaco in the NW, most likely continuous with the Miocene deposits of southern Ecuador, a further 150 km to the NW.
This mineralised corridor straddles the fault controlled boundary between the Pucará Basin (and overlying Cretaceous sequence) and the Divisoria Arch basement high of coastal Peru to the west.
Porphyry deposits tend to predominate on the NE margins of the mineralised corridor, and epithermal mineralisation to the SW. The porphyry deposits are associated with early to middle Miocene granitoid stocks, emplaced over a ~5 m.y. interval from 20.8 to 15.9 Ma. Potassium silicate alteration in these mineralised stocks has ages ranging from 20.6 to 15.4 Ma (e.g., at Misacocha, Amaro, Perol, Chailhuagon, Galeno and Michiquillay) and an age of alunite at Cochañes of 16.1 Ma. Middle Miocene magmatism in a ~25 km long belt from Chailhuagon (15 km east of Yanacocha), northwest to the Tantahuatay district (30 km NW of Yanacocha), is recorded by igneous ages from 14.4 to 8.3 Ma in the Tantahuatay, Cerro Corona, Hualgayoc and La Zanja mineral districts, and by ages of hydrothermal activity that range from 15.6 to 11.0 Ma (Llosa et al., 2010). High-sulphidation epithermal deposits in the Tantahuatay district overlap the magmatism and span a 2.3 m.y. interval from 13.3 to 11.0 Ma, ~0.2 m.y. younger than the porphyry Au-Cu mineralisation ~6 km southeast at Cerro Corona (13.5 Ma).
At Yanacocha, the host Yanacocha Volcanic Complex is dated at 14.5 to 8.4 Ma, whilst late hornblende andesite tuffs and andesite porphyry and dacite to tonalite quartz-eye porphyry intrusions are 12 to 8.4 Ma (Turner, 1997; Longo, 2005). 40Ar/39Ar dating of hydrothermal alunite from mineralised epithermal centres ranges from 11.5 to 8.5 Ma (Teal and Benavides, 2010).
The Yanacocha cluster of >20 high sulphidation oxide and sulphide gold-silver and porphyry Au-Cu deposits and prospects occur within a coherent NE-SW (50°) trending zone of intense hydrothermal advanced and intermediate argillic alteration 20 km long by 2.5 to 8 km wide covering an area of >100 km2.
This zone of alteration and mineralisation can be divided into an East and a West Block, separated by the NW-SE Quinoa basin. The East Block contains the bulk of the significant deposits, including Maqui Maqui, Carachugo, San Jose, Chaquicocha and those of the Cerro Yanacocha Complex (Yanacocha Norte, Sur Yanacocha, Yanacocha Oeste, Encajón and Yanacocha Verde) and the Kupfertal porphyry Au-Cu mineralisation. The West block hosts part of the concealed Corimayo deposit on its eastern margin, and the Cerro Negro and Quilish deposits, 8 km to the west of the Cerro Yanacocha Complex. Cerro Negro is 4 km NW of Quilish. The Quinua basin is part of a larger NW-SE trending, fault bounded valley filled with glacial gravel. The large, gold-bearing, gravel-hosted, fan deposit at La Quinua is divided into the Norte, Central and Sur sections within peri-glacial gravels of this basin. The Tapado and most of the Corimayo epithermal deposits are located beneath 150 to 200 m of barren cover, below the Quinoa Basin, adjacent to, or below the La Quinua deposits.
The Yanacocha deposits are hosted by the mid to late Miocene Yanacocha Volcanic Complex. This complex is composed of three cycles, each of effusive rocks, comprising andesitic to dacitic lavas, flow domes and associated porphyry intrusions, followed by explosive ignimbritic pyroclastics and minor lacustrine sediments. In the third cycle, the effusives and intrusions become more silicic. In the centre of the deposit cluster area, the lower cycle directly and unconformably overlies deformed and peneplained Cretaceous sediments, predominantly limestone, shale and quartzite. The intervening Tertiary Calipuy Group rocks exposed in the surrounding district wedge out into this unconformity towards the centre of the cluster.
The Yanacocha Volcanic Complex has been divided into three principal suites, with associated intrusions and breccias, as follows:
• Lower Andesite Sequence of Teal and Benavides (2010), and Atazaico Andesite of Longo et al. (2010), which is exposed predominantly within the western part of the district. Here it dominantly consists of hornblende andesitic flows and tuffs with flow-breccia facies and rare associated flow domes, intercalated with lesser block and ash flow tuffs, and lahars rich in pyroxene and hornblende crystals. The andesites are characterised by abundant phenocrysts of plagioclase and ferromagnesian minerals. The sequence passes up into ignimbrites at the transition to the overlying fine-grained, laminated epiclastic horizon (Teal and Benavides, 2010; Longo et al., 2010).
The Atazaico Andesite is the 'effusive stage 1' of Longo et al. (2010). Two texturally distinct porphyritic lavas are recognised i). a fine grained groundmass with fine- to medium-grained (<2.5 mm) plagioclase and hornblende phenocrysts, and ii). with medium-grained (1.0 to 5.0 mm) phenocrysts. These lavas were intruded and locally overlain by pyroxene andesite flow domes containing rare quartz phenocrysts and associated with minor amounts of block and ash flows. This period of eruption and subsequent dome building spanned a period of 1.2 m.y. and progressed temporally from SW to NE across the district. The sequence is up to 320 m thick in the centre of the district (averaging 185 m in the west and 90 m in the east), with an areal extent of 340 km2, making it the most extensive of the units in the Yanacocha Volcanic Complex (Longo et al., 2010).
In the immediate Yanacocha area, the Lower Andesite Sequence unconformably and directly overlies folded early Cretaceous basement sedimentary rocks, with individual flows filling palaeochannels along the erosional surface, and passes upward into the Yanacocha Pyroclastic Sequence. At Yanacocha, 40Ar/39Ar dating gave an age of 14.52±0.13 to 14.21±0.16 Ma for lavas in the SW, overlain by an andesitic flow dome with an age of 13.85±0.09 Ma. The youngest ages are in the NE of the district with 13.76±0.17 Ma for an andesitic tuff low in the section and 13.31±0.08 Ma for a stratigraphically higher andesitic lava (Longo et al., 2010).
• Yanacocha Pyroclastic Sequence of Teal and Benavides (2010), which is largely exposed in the central part of the district, where it is the primary host to the majority of the gold deposits, due to the inherent porosity of the rocks that constitute the bulk of the sequence. This porosity also renders the rocks susceptible to moderate to intense, pervasive advanced argillic alteration, resulting in moderate to complete destruction of textures and original chemical compositions.
The Yanacocha Pyroclastic Sequence follows a gap of ~0.7 m.y. after the last eruption of the Atazaico Andesite, and consists of the following units:
Colorado Pyroclastic Sequence, which is principally composed of andesitic to dacitic ignimbrites, and outcrops over an area of ~90 km2, centred on Cerro Yanacocha, filling the fault-bounded Colorado graben in the eastern part of the Yanacocha district, where the unit is up to 225 m or more, but averaging 133 m, in thickness (Longo et al., 2010). It is referred to as 'explosive stage 1' of the sequence, and has been subdivided again into three informal members:
- Basal epiclastic laminated sedimentary rocks, thin-bedded (<1 to 50 mm), commonly strongly hydrothermally altered, wispy fine-grained air fall tuff, or reworked tuff, with a fine crystal component, grading to fine crystal-lithic tuff with local medium-bedded lithic tuff containing fragments of basement quartzite, and local laminated lacustrine sediments. The sequence is discontinuous, confined to local volcanotectonic depressions, with the thickest development being ~20 m, which lies at the base of the Colorado Pyroclastics, although laminated rocks are found in at least two higher stratigraphic positions where they are associated with gold deposits. They are locally crossbedded and are composed of chalcedony with local opal and kaolinite, interpreted to be water lain, derived from deposits of colloidal silica that settled from acidic lakes over fumarole vents in volcanic craters (Edwards, 2000; Longo, 2000, 2005; Longo et al., 2010).
- Cori Coshpa andesitic ignimbrite member, the lower of the two ignimbrites, described as a eutaxitic crystal tuff on the 1:25 000 Yanacocha District geological map (Teal et al., 2007 in Teal and Benavides, 2010). It is 40 to 60 m thick, light grey to white in colour, poorly to non-welded, and contains 20 to 25 vol.% white pumice of trachyandesitic composition. It also includes intercalated poorly welded crystal, crystal-lithic and lapilli tuffs composed of variably eutaxitic and locally crystal-rich sequences. It contains angular to subrounded clasts that include Cretaceous argillite and quartzite as well as porphyritic andesite, and has phenocrysts dominated by plagioclase, but also includes hornblende and biotite, but no pyroxene. Biotite yielded an age of 12.63±0.05 Ma (Longo et al., 2010).
The Quilish dacitic intrusions that were intruded somewhere between ~14.2 and ~12.6 Ma (described below in the Intrusions section) are interpreted to be related to the early Colorado Pyroclastic Sequence Cori Coshpa andesitic ignimbrite member, intruding all rocks older than this unit, but none younger.
- Maqui Maqui hornblende-biotite trachyandesite to dacite ignimbrite member, which overlies the Cori Coshpa ignimbrite along a poorly exposed contact, forming a single cooling unit normally ~90 m thick, although it is up to 230 m thick in the Colorado graben. It has a dark grey to grey-brown colour, is moderately welded, with a eutaxitic texture, and contains abundant flattened lapilli fragments and crystals of broken plagioclase as well as hornblende, but only traces of biotite. In addition it contains minor (i.e., <1 to 2 vol.%) of <1 to 5 cm fragments of black argillite, quartzite and porphyritic volcanic rocks, and fiamme that range from <1 to 10 cm long with aspect ratios of 6:1. Ages of 12.49±0.08 and 12.40±0.10 Ma have been obtained, where sufficiently unaltered. When compared with the Cori Coshpa ignimbrite, the age difference and the field relationships suggest the two ignimbrites result from different eruptions (Longo et al., 2010).
The Yanacocha porphyries and early diatremes, dated at 12.4 to 11.9 Ma (described below in the Intrusions section) have a similar composition and age range to the Maqui Maqui dacitic ignimbrite member, and intrudes mineralised Colorado Pyroclastics ignimbrites (Longo et al., 2010).
- San Jose Ignimbrite - Moore and Saderholm (2002) mapped a fourth traceable, cohesive unit which overlies the Maqui Maqui ignimbrite, and is listed as the San Jose Ignimbrite on mine maps (e.g., Teal and Benavides, 2010). This unit has been strongly altered and no reliable age dates are available, although Teal and Benavides (2010) suggest there is evidence it is pre-12.05 Ma. They describe it as a hornblende andesite lithic-crystal tuff with abundant previously altered tuff clasts and levels of strongly eutaxitic crystal lithic tuff. However, in their stratigraphic column, Longo et al. (2010) show the San Jose Ignimbrite as occurring above the Azufre Andesite in the 'Upper Andesite Sequence', and have dated less altered rocks they correlate with this unit at 11.5 to 11.2 Ma to support this interpretation. Longo et al. (2010) do not show any unit between the Maqui Maqui Ignimbrite and the Azufre Andesite. It is uncertain if there are two ignimbrites of different ages straddling the Azufre Andesite, both mapped as San Jose Ignimbrite, or a single unit that has been interpreted differently. See the more detailed description of the unit below.
• Upper Andesite Sequence of Teal and Benavides (2010), which they refer to as the 'capping sequence', comprises the following:
- Azufre Andesite, is composed of lavas, flow-domes and minor pyroclastic deposits of strongly porphyritic, trachytic-textured, hornblende-pyroxene andesite to dacite. These lavas are up to 310 m (averaging 113 m) thick, flow-layered, locally grading into blocky autobrecciated bands a few metres thick. This is 'effusive stage 2', which either overlies the Colorado Pyroclastics, or an erosional unconformity separating it from older units. It is exposed within an area of ~125 km2, which is 5 to 8 km wide by ~15 km long, extending from Pampa Cerro Negro in the WSW to Alta Machay in the ENE. It is difficult to distinguish from the older Atazaico Andesite, although pyroclastic deposits are relatively more abundant in the Azufre Andesite. Domes associated with the Azufre Andesite have slightly larger and less abundant phenocrysts compared to lavas and typically form equant to slightly elongate bodies over areas of ~0.2 to 2 km2, characterised by prominent ribs elongate parallel to steeply inclined flow-foliations. Many of these domes are composite in age and formed via dome-building during multiple eruptive events. Extrusive domes and feeder dykes commonly have associated pyroclastic deposits of block and ash flows that overlie the lavas and occur as isolated exposures (Longo et al., 2010). Nine age determinations of the Azufre Andesite range from 12.09±0.10 to 11.58±0.18 Ma (Turner, 1997, n=2; Longo, 2005, n=7).
- San Jose Ignimbrite, the 'explosive stage 2' of the complex, which is predominantly composed of dacitic and andesitic ignimbrites, deposited as three distinct members, representing the last voluminous eruptions of the Yanacocha Volcanics. Each of the three ignimbrite members is mineralogically and texturally distinct, whilst there are also vertical variations in mineralogy and degree of welding that together suggest separate pyroclastic flows. The three members have a cumulative thickness of up to 350 m (averaging 138 m) and are found over an area of 175 km2 occupying a 15 km diameter depression (the Otuzco trough) that extends from Cerro Yanacocha southwards to Cajamarca. Ignimbrites in this pile, particularly those of the middle member, contain abundant fragments of quartz-alunite, massive quartz, vuggy quartz and quartz-sanidine-andalusite altered rock locally containing up to 350 ppb Au, suggesting at least some advanced argillic alteration and gold mineralisation predates the San Jose Ignimbrite. The characteristics of the individual members is as follows:
Lower member, the most voluminous, extensively exposed in area of 134 km2, occurring as a compound cooling unit of three or more hornblende-pyroxene-biotite dacitic ignimbrites, each including a zone of poorly sorted and poorly to non-welded lapilli pumice tuff below an upper densely welded ignimbrite characterised by strongly developed eutaxitic texture. These ignimbrites are crystal rich in contrast to the juvenile pumice which has sparse phenocrysts. Five 40Ar/39Ar ages suggest eruption as a series of cooling units over a period of ~0.2 m.y. between 11.54±0.09 and 11.34±0.16 Ma, or alternatively as a single event given by the weighted mean age of 11.46±0.08 Ma (Longo et al., 2010).
Middle member, a white, non- to densely-welded, hornblende-biotite andesitic to trachydacitic ignimbrite containing large, ~1 cm, concentrically zoned hornblendes and lapilli-sized pumice. Several internal cooling breaks are evident. This member is exposed within an area of ~100 km2 from Yanacocha to Cajamarca. Basal lithic concentrations are common where it locally directly overlies the Azufre Andesite or Atazaico Andesite, or elsewhere where it contains up to boulder size fragments of previous altered rock. Six 40Ar/39Ar ages of from 11.30±0.09 to 11.22±0.08 Ma suggest a short interval for eruption. The weighted mean age is younger than the mean age for the lower member, although the oldest age of the middle member is indistinguishable from the youngest age from the lower member, so it is unclear whether or not there is significant temporal gap between the two eruptions (Longo et al., 2010).
Upper member, the most distinctive of the three members, but the smallest by volume and surface exposure, restricted to an area of 16.5 km2, and ranging from 10 to 100 m in thickness. It is a moderately to densely welded simple cooling unit containing abundant <10 to 50 cm long, commonly flattened blobs, and subordinate equant sub-angular blocks, of unvesiculated andesite, supported by a matrix of glass with ash-sized particles (<2 mm), dominated by broken plagioclase phenocrysts and small blobs and blocks. It has the most mafic mineralogy of the San Jose Ignimbrite and contains large poikilitic oxyhornblende and abundant green augite. As a whole, it dips 10 to 15°S, away from an apparent vent. The weighted mean 40Ar/39Ar age of the ignimbrite is 11.25±0.06 Ma (n = 2, MSWD = 0.027), identical to that of the middle member, supporting a two eruption model consistent with the stratigraphic relationships, i.e., either a single continuous eruption produced both the middle and upper members, or the upper member represents a separate eruption <0.1 m.y. after the middle (Longo et al., 2010).
Within the centre of the Yanacocha trend, the San Jose Ignimbrite both overlies and hosts mineralisation at the San Jose gold deposit. The ignimbrite, which was overlain by siliceous laminated rocks, grades abruptly downward from fresh rock at ~30 m depth, typically via a narrow, 1 to 2 m wide zone of hydrothermal opal and smectite, into weakly argillic altered rock by ~60 m depth, then to gold-bearing (5 to 10 g/t Au) granular and vuggy quartz altered rock with cavities that resemble eutaxitic texture (Klein et al., 1997, 1999; Longo, 2005; Longo et al., 2010). Both lower and upper members of the San Jose Ignimbrite are found above the San Jose gold deposit, although locally, advanced argillic and weaker alteration also cuts both the upper and middle members. These observations and the presence of gold-bearing hydrothermally altered fragments within the ignimbrite indicate that this ignimbrite both pre- and postdates phases of gold deposition.
Note: Teal and Benavides (2010) regard the San Jose Ignimbrite as being a strongly altered unit in the 'Yanacocha Pyroclastic Sequence' that is pre-12.05 Ma in age. Those authors place it stratigraphically above the Maqui Maqui ignimbrite, but lower than the andesitic lava sequence at the base of the 'Upper Andesite Sequence', presumed to be the Azufre Andesite of Longo et al. (2010). However, while Teal and Benavides (2010) place the San Jose Ignimbrite below this andesitic lava sequence, they also show an 11.25 to 11.24 Ma welded hornblende andesite tuff-breccia with compacted fragments up to 60 cm in length, the Shacsha Ignimbrite above the andesitic lava, where Longo et al., 2010) place their San Jose Ignimbrite.
- Flow domes, a 6.5 km long, NE trending string of low-silica dacitic dome complexes, which according to Longo et al. (2010) are closely associated in time with the San Jose Ignimbrite. Individual flow domes vary in surface area from <100 m to >2 km in diameter, and are distributed from the SW of the Yanacocha trend, in the vicinity of the Cerro Negro, Cerro Quilish deposits, through Corimayo, surrounding the main Yanacocha-San Jose-Carachugo deposits to Maqui Maqui in the NE. Three of these domes, dated by Longo et al. (2010), have ages of 11.68±0.07 and 11.36±0.09 Ma (Ocucho dome), 11.28±0.09 Ma (Chaquicocha Sur dome) and 11.23±0.07 Ma (Alta Machay dome). The domes are elongated parallel to near vertical, cm-scale flow banding, and are fed by dykes. Older andesitic domes that are hydrothermally altered, are overlain by fresh domes that post date the San Jose Ignimbrite, suggesting progressive, multistage growth, e.g., the 1.6 km2 Ocucho dome complex was emplaced into an older, previously kaolinite and opal altered dome. The lavas that form the domes have porphyritic to seriate textures with flow-foliated phenocrysts within a pilotaxitic groundmass, and are locally capped by carapace autobreccias with foliated clasts. The mineralogy of each dome is similar to a corresponding member of the San Jose Ignimbrite. Of the three examples studied by Longo et al. (2010), the Alta Machay dome is similar to the middle member, containing hornblende, but lacking pyroxene, whilst the Ocucho and Chaquicocha Sur domes are similar to the lower and upper members respectively and contain hornblende and green augite. Longo et al. (2010) concluded that, on the basis of phenocryst mineralogy and age, the Ocucho, Alta Machay, and Chaquicocha Sur domes may occupy the respective vents for the lower, middle and upper members of the San Jose Ignimbrite.
- Coriwachay Dacite-Rhyolite Magmatism - The San Jose Ignimbrite of Longo et al. (2010) is overlain by the Coriwachay Dacite, that is composed of intercalated andesitic to dacitic flows, intrusions and ignimbrites, dominated by multiple flow dome complexes. It represents a shift to more silicic and more highly oxidised dacitic to rhyolitic magmas that accumulated in the centre of the district, and is interpreted to reflect increased crustal contamination (Longo et al., 2010). Although it has a small volume relative to the other units of the Yanacocha Volcanic Complex, most of the gold at Yanacocha was deposited during intrusion associated with this suite. It includes at least four units of small volume, high-silica, dacitic domes and plugs, together with rhyolitic dykes and ignimbrites, that have been dated between ~10.8 and 8.4 Ma, the principal of which are:
Corimayo dacitic flow domes of 'effusive stage 3', which occupy an ~8 km easterly trend from La Quinua to Chaquicocha Norte across the central part of the Yanacocha deposit cluster. Many of these domes have distinctive flow foliations and are spatially associated with silicified laminated rocks similar to those described in the Colorado Pyroclastics. The Corimayo dacites generally have a higher phenocryst content (up to 43 vol.%) compared to other Coriwachay Dacites. Limited dacitic tuffs have been found associated with one of the domes at Corimayo (Gomez and Klein, 2000; Gomez, 2002). Dating of these flow domes is uncertain, with Longo et al. (2010) arguing a 40Ar/39Ar age of 10.78±0.05 Ma the most likely. The slightly older 11.06±0.02 Ma Tapado dacite (U/Pb, zircon; Chiaradia et al., 2009) is grouped with the Corimayo dacitic flow domes.
Cerro Yanacocha dacite porphyry intrusion - see the 'intrusion' section below.
Yanacocha Lake rhyolitic dyke - see the 'intrusion' section below.
Negritos rhyolitic ignimbrite, of 'explosive stage 3' which is confined to the Cerros de los Negritos (~10 km north of the centre of the Yanacocha cluster), a single cooling unit of welded rhyolitic ignimbrite that occurs as a ~50 m thick, ~5 km2, erosional remnant overlying dacitic flow domes. The ignimbrite is pinkish tan to light brown in colour, and has eutaxitic texture. It is crystal-rich, with an assemblage of phenocrysts similar to other Coriwachay dacites. It also has abundant fragments of argillite, quartzite, porphyritic dacite and rare granular to vuggy quartz. It has been dated at 8.43±0.04 Ma, identical to the age of the Yanacocha Lake rhyolite dyke and is interpreted to represent the first post-mineral magmatism.
Multiple pulses of late-stage dykes, plugs and stocks have been intruded into the volcanic pile, some of which are spatially and temporally associated with deeper Au-Cu porphyry-style mineralisation (e.g., Kupfertal between Yanacocha Sur and San Jose, and below the Yanacocha Sur and Maqui Maqui epithermal ore) beneath the gold-dominant high sulphidation epithermal deposits at Yanacocha. The principal intrusions include (after Longo et al., 2010):
• Quilish dacitic intrusions, which occur as a series of plugs exposed along the NE-SW Yanacocha trend in the western part of the district for ~14 km. These plugs intrude the 14.5 to 13.3 Ma Atazaico Andesite of the Lower Andesite Sequence and other older units in the western part of the Yanacocha district, but not the 12.6 to 12.4 Ma Colorado Pyroclastics and younger rocks. Several similar dacite porphyry dykes occur on the NE margin of the Yanacocha epithermal trend, near La Sorpresa, where they strike NW-SE and intrude Cretaceous limestone and Chaupiloma andesite of the Calipuy Group in a 2.3 km long zone near La Sorpresa. The dacite porphyries in both areas contain 30 to 40% phenocrysts of plagioclase, hornblende, slightly embayed quartz, sanidine, biotite and Fe-Ti oxides, set in an aplitic (<0.05 mm) quartzo-feldspathic matrix. Based on the ages of over- and underlying rocks, Longo et al. (2010) conclude these intrusions were emplaced between ~14.2 and ~12.6 Ma, a similar age to the ~14 to 12 Ma hydrothermal alunites in the western parts of the
• Yanacocha porphyries and early diatremes, which include:
- Andesite porphyry dykes and plugs - which have abundant coarse phenocrysts, dominantly of plagioclase and hornblende, with trace biotite and quartz, and trace to absent pyroxene, although they are generally clay altered. On the basis of composition and presence of hornblende-rich phenocryst mineralogy, Longo et al. (2010) regard these porphyries as being similar to the Maqui Maqui dacitic ignimbrite, but dissimilar to the Atazaico and Azufre Andesites. Yanacocha porphyries intrude, are interbedded with or overlie ignimbrites that are strongly altered and host much of the gold at Yanacocha and are inferred by Longo et al. (2010) to belong to the the Colorado Pyroclastics. Three datings of Yanacocha porphyries yield ages 12.4 to 11.9 Ma that overlap in part the age range of the Maqui Maqui ignimbrite and overlying Azufre Andesite.
- Clay-altered diatremes - that are volumetrically minor and associated with the Yanacocha porphyries at a number of localities.
• Cerro Yanacocha dacite porphyry and late diatreme outcrops at a number of locations along the NW trend of the Yanacocha deposit cluster, e.g., in the NE (Yanacocha Norte), SW (Yanacocha Oeste) and south-central (Chaquicocha) part of the Yanacocha district, and is spatially associated with gold ore. Dacite porphyry plugs, which are both altered and unaltered, have a northerly trend through the Cerro Yanacocha deposit, where they are spatially associated with a large, matrix-supported breccia body containing fragments of the dacite porphyry and referred to as a 'diatreme'. Those porphyries that are altered, contain vuggy quartz with halos of advanced argillic alteration, and are cut by hypogene alunite- or alunite-pyrophyllite cemented open-space breccia that contains covellite-enargite-pyrite assemblages (Loayza-Tam, 2002). Quartz phenocrysts are larger and more abundant, and ferromagnesian minerals less abundant, in the Cerro Yanacocha dacite porphyry compared to those of the Corimayo dacite domes (Longo et al., 2010). Biotite from the dacite porphyry at Yanacocha Oeste has an age of 9.91±0.04 Ma, identical to a 40Ar/39Ar age from dacitic porphyry at Yanacocha Norte (9.90±0.05 Ma; Turner, 1997). These ages are younger than the 10.78±0.05 Ma Corimayo dacite domes.
• Yanacocha Lake rhyolitic dykes and plugs, the youngest intrusive igneous rocks in the Yanacocha district. They are high-silica dacitic to rhyolitic intrusions, exposed along a ~4 km long, NW trend, intruding late clay-altered diatreme breccias (Chan Ganaqui, 1995), to Cerro Fraile dacitic pyroclastics. The dyke has a steeply dipping, frequently contorted, flow foliation, with quartz phenocrysts that are smaller and more abundant than in the other units of the Coriwachay Dacite. Locally, angular and contorted fragments of the flow-foliated rhyolite are found in a breccia along the dyke margin. A biotite from the dyke yielded an age of 8.40±0.06 Ma (Turner, 1997), whilst a 40Ar/39Ar age of biotite from a similar rhyolitic intrusion at Yanacocha Oeste was 8.59±0.14 Ma (Chiaradia et al., 2009).
Multiple phases and styles of hydrothermal breccia have been recognised within the Yanacocha Volcanic Complex that are both spatially and genetically associated with episodes of gold mineralisation, although each style commonly grades into another. There are differences in emphasis ion the types of breccia in different breccia classifications, which will be described below (after Bell et al., 2004; Teal and Benavides, 2010; Longo et al., 2010):
• Phreatomagmatic breccias, which according to Bell et al. (2004) are locally termed diatremes, and are the most prominent and distinctive type, containing rounded to subangular heterolithic clasts that commonly reflect the adjacent wallrocks, set in a matrix rich in broken and ground plagioclase crystals. These breccias occur as steeply dipping dykes and steeply plunging pipes up to several hundred metres in diameter.
• Phreatic breccias, which Bell et al. (2004) describe as containing subangular to rounded monolithic to occasionally heterolithic clasts, set in a strongly abraded, homogenous sand-sized rock fragment matrix, varying from matrix dominated and supported to clast supported. They also occur as steeply dipping dykes, as individual pipes, and as composite pipes with phreatomagmatic breccias. These are also referred to as 'sandy matrix breccias'. According to Bell et al. (2004), these breccias are also mapped as 'diatremes' by mine geologists, and appear to contrast with those described as 'diatreme breccias' by Longo et al. (2010), as described below.
Longo et al. (2010) describe breccias referred to as phreatic breccias by mine geologists as heterolithic, matrix- to clast-supported breccias with a fine-grained 0.1 to 0.5 mm granular quartz matrix. These breccias contain no juvenile fragments, are chaotic and poorly sorted, and are interpreted to have resulted from eruptions of superheated ground waters at a shallow depth. They crosscut all lithologies and alteration types, and contain fragments of nearly all rock types found at Yanacocha. Multiple steeply and gently dipping fluidised pebble dykes and sills are a late component of these events that crosscuts the main body of phreatic breccia. These pebble dykes and sills have a characteristic core of coarse, well-sorted, rounded pebbles (2 to 6 mm in diameter) of massive quartz that grade outward to sand-sized (<1 mm) particles with both planar and crossbedded features. Phreatic breccias are generally barren, but may locally host gold ore.
The largest development of phreatomagmatic and phreatic breccias, referred to as 'the diatreme', covers a NE-SW elongated area of ~1100 x 400 to 800 m area, with NW-SE trending apophyses, located in the northern sections of the Cerro Yanacocha Complex mine. It comprises a larger core of phreatomagmatic breccia occupying ~60% of the total 'diatreme', sandwiched by smaller accumulations of phreatic breccias to the east, SW and west. The Yanacocha Norte, Sur and Oeste deposits are located on the NE, southern and SW margin of the 'diatreme' respectively. These breccias are closely associated with intrusions of plugs, domes and dykes of the Cerro Yanacocha dacite porphyry from 200 to 800 across. Similar sized to smaller breccia bodies are closely associated with the Maqui Maqui, Carachugo-Chaquicocha, Corimayo, Cerro
Quilish and Cerro Negro deposits and deposit groups.
• Diatreme breccias Longo et al. (2010), which are rare and intrude the volcanic section. They are usually spatially associated with feldspar and quartz porphyry intrusions and expand upwards, with a clay-rich matrix supporting heterolithic clasts that include quartz-rich rock, clay-altered Yanacocha porphyry, volcanic and porphyritic rocks, and basement quartzite and black argillite. Rarely juvenile fragments of plastically deformed Yanacocha porphyries are observed, suggesting contemporaneous intrusion and brecciation (Abarca and Harvey, 1997; Turner, 1997; Longo, 2000; Loayza-Tam, 2002). Quartz- and clay-altered fragments infer hydrothermal activity prior to breccia emplacement. Although these breccias are not internally strongly mineralised, gold ore commonly occurs along their margins, suggesting passage of ore fluids along the permeable breccia margin (Changanaqui, 1995; Abarca and Harvey, 1997; Turner, 1997; Rota, 1997; Harvey et al., 1999; Longo, 2000). Bell et al. (2004) describe a similar breccia, which they say is less common compared to the phreatomagmatic and phreatic varieties. It has subangular to subrounded monolithic and rare heterolithic clasts in a porphyritic matrix of andesite to dacite, which occur at Cerro Yanacocha as subvertical pipes above dacitic intrusions, and represent the transition between intrusive rocks and phreatomagmatic breccias.
• Hydrothermal breccias, which Bell et al. (2004) describes as containing angular to subangular clasts, set in a fine grained, sometimes fluidised texture quartz matrix, flooded by iron oxide/sulphide, chalcedony and alunite. The clasts are commonly monolithic reflecting the enclosing wallrock lithology. These breccias also occur as dykes, cutting almost all rock types. Longo et al. (2010) describe a progression of hydrothermal breccia, that include:
- Early hydrothermal breccia, occurring as matrix-supported, heterolithic and monolithic breccias with dense quartz cement, hosted in a variety of quartz-rich and quartz alunite altered wall rocks (Abarca and Harvey, 1997). These breccias appear to predate gold ore, commonly with grades of <0.03 g/t Au, but are locally cut by fractures containing barite and coarse native gold.
- Later, more intense hydrothermal breccias that are heterolithic and monolithic with quartz, quartz-alunite and alunite matrices also containing fragments of massive quartz and early quartz-cemented breccia. These breccias are spatially associated with gold mineralisation, and locally carry high gold grades of >25 g/t Au. Cryptocrystalline quartz flooding in these breccias produced a dense massive and microcrystalline textured (generally <0.05 mm, but rarely up to 0.2 mm) quartz matrix, enclosing angular to subrounded clasts. In the sulphide environment, the quartz is accompanied by fine-grained pyrite ±alunite, barite, enargite, tennatite, covellite, native sulphur, native gold, with late chalcocite and trace gold. In the oxide zone, gold occurs in highly fractured, poorly cemented, massive quartz, with jarosite, hematite, goethite or mixed limonite cement. High grade gold is accompanied by creamy microcrystalline (<0.3 mm) quartz with rutile. Breccia zones are typically subvertical, but are also sometimes subhorizontal bodies.
• Vuggy quartz, matrix supported fragmental rocks, as described by Longo et al. (2010), which are common in zones dominated by quartz-rich altered rocks. Fragments are composed of massive or vuggy quartz, interpreted to represent leached and replaced pre-alteration pyroclastic rocks. The original rock texture is partly preserved, but the breccia is very porous with abundant vugs representing leaching by acidic hydrothermal fluids of phenocrysts, pumice and lithic rock fragments from the original lithology. Three textural types of groundmass quartz are recognised i). dense fine-grained (<0.1 mm) quartz grains in aggregates; ii). overprinting of this fine-grained quartz by slightly coarser (~0.1 mm) granular-textured quartz; iii). recrystallisation of finer grained quartz to ~0.3 mm quartz, with drusy quartz lining of vugs. Vuggy quartz is interpreted to have developed in fractured and/or porous crystal-rich volcanic/pyroclastic rocks and provided fluid-flow pathways with high porosity and permeability. Subsequent acidic hydrothermal fluids from later hydrothermal pulses was channeled into the early vuggy quartz and laterally into the volcanic stratigraphy progressively expanded outward as stratabound bodies of vuggy quartz. Vugs and late fractures host the gold-bearing sulphide and oxide ore, with >400 m of oxide in some fracture-dominated structural zones. In the sulphide zone, fine pyrite, enargite, tennantite, covellite and native sulphur, with late chalcocite and rare acanthite, galena, sphalerite and orpiment occurs in vugs and fractures, whilst in the oxide zone, poorly cemented, highly fractured vuggy quartz contains jarosite, hematite, goethite or other limonite cement occurs with high to bonanza grade gold, including coarse visible gold associated with rutile, cassiterite and barite (Longo et al., 2010).
• Breccia relationships - Crosscutting relationships are complex, with multiple generations of brecciation in all deposits. In individual breccia complexes/deposits, phreatomagmatic breccias are usually cut by phreatic breccia, and all are cut or replaced by hydrothermal breccias. Breccias commonly contain clasts of earlier breccias, implying multigeneration brecciation (Bell et al., 2004; Teal and Benavides, 2010; Longo et al., 2010).
As detailed in the Regional Setting section above, the alignment of the Yanacocha gold deposits are interpreted to have been controlled by the intersection of a middle Tertiary, NE trending crustal break, the Chicama-Yanacocha structural corridor (Turner, 1997), one of a series of parallel trans-arc structures regionally superimposed upon the older Paleocene-Eocene, Incaic I and Incaic II NNW to NW Andean parallel deformational fabric of the basement terrain (Benavides-Cáceres, 1999). This produced a structural lattice pattern at both a district and deposit scale that controlled the location of breccias and emplacement of shallow intrusions into the volcanic pile, and subsequently also localising multiple phases of gold mineralisation. The same pattern produced a mosaic of horst and graben subdomains that have controlled the distribution and thickness of units within the overall volcanic pile, resulting in abrupt internal volume and thickness changes, and localisation of volcanic facies and units, sometimes over short distances.
On a deposit scale, discontinuous east-west oriented faults/fracture zones are interpreted as purely extensional and are important controllers of higher grade gold mineralisation.
A detailed structural synthesis of the district (Rehrig and Hardy, 2001) suggested i). the primary σ1 direction of near east-west compression was rotated anticlockwise by 10 to 20° over time, creating complementary NE translational shears and extensional faults and splays (Yanacocha trend parallel) generated along, and between, sinistral NW-striking transpressional faults; and ii). formation of pure tensional, generally east-west striking faults and joint systems were widespread and are evident throughout the cluster of deposits.
Pervasive acid-sulphate alteration within the Yanacocha district occurred in multiple coalescing centres. The distribution of alteration is complicated by telescoped multiple stages of high sulphidation alteration in some deposits, as well as from the superposition of high sulphidation, gold-copper porphyry related and later intermediate sulphidation alteration. In addition, there is a strong lithologic and elevation control on alteration, resulting in a highly variable distribution of facies. The same alteration zoning that may occur over kilometres horizontally, is seen over tens to hundreds of metres vertically. In addition, lavas, dykes and breccia bodies are commonly less altered than more porous enclosing pyroclastic rocks, resulting in local argillic, propylitic and even fresh zones within large silicified bodies (Bell et al., 2004).
On an individual deposit scale, mineral assemblages are zoned outward from a pervasive, massive siliceous, generally gold-bearing core, which grades abruptly, both laterally and downward, into an intense, acid-leached vuggy quartz envelope, and where preserved, caps of granular quartz, which are interpreted to represent near-surface, vapour-dominated horizons.
The silica-rich core is progressively enveloped by ~10 to ~100 m wide, or even broader, envelopes of advanced argillic quartz-alunite±pyrophyllite±diaspore±dickite±kaolinite and intermediate argillic illite-smectite±pyrite, and then grades into a distal and weakly developed halo of propyllitic chlorite-illite-smectite-calcite alteration (Longo et al., 2010; Teal and Benavides, 2010).
At depth, quartz-pyrophyllite altered hosts grade down into a NE trending diaspore-bearing zone, which with greater depth becomes a quartz-diaspore dominant keel that is spatially associated with a quartz eye tonalite porphyry of the Cerro Yanacocha intrusive (Longo et al., 2010; Pilco, 2011).
Alteration types - In more details each of these alteration types are characterised as follows:
• Massive silica - In the centre of the Yanacocha trend, ~10 km2 of overlapping intense silica alteration centres are exposed within a NW-SE elongated 6 x 3 km area, extending from north of the Cerro Yanacocha mine complex (i.e., the Yanacocha Oeste, Sur and Norte deposits) through the San José, Carachugo and Chaquicocha deposits to the SE, with NE trending elements. It consists of rock that is nearly 100% altered to quartz (Longo, 2000, 2005), with a variety of textures that include massive, vuggy and granular quartz as described above. The granular silica cap may be up to 100 m or more in thickness, with a very irregular base, occurring as fingers penetrating downward into the massive and vuggy quartz below which appears to be structural controlled and superimposed on quartz-alunite. The average quartz content at all three Cerro Yanacocha deposits is 68% silica. Massive silica does not persist to depth, but has a gently sloping base that deepens to the north, varying in thickness from ~200 m at Yanacocha Oeste (to an elevation of 3650 m), up to 400 m at Yanacocha Sur and ~300 m at Yanacocha Norte (to an elevation of 3820 m), while the upper surface occurs at shallow depths throughout Cerro Yanacocha. Massive silica also occurs as structurally controlled subvertical bodies that are locally >450 m thick, and crosscut the stratigraphy. Turner (1997) determined that granular silica in the Yanacocha district presently consists of α-quartz and cristobalite. Silicification is one of the earliest stages of mineralisation as illustrated by the presence of silicified clasts in phreatomagmatic breccia (Longo et al., 2010; Teal and Benavides, 2010; Pilco, 2011). Similar, but smaller, generally NW-SE trending zones of silica alteration are exposed to the NE at Maqui Maqui and to the SW at Tapado-Corimayo, and further to the SW again enclosing the Cerro Negro and Quilish deposits.
Barren, amorphous opaline silica occurs as rootless pods near silica-kaolinite and montmorillonite at the margin of and ~200 m above gold ore and may have either a supergene of steam heated origin (Longo et al., 2010).
• Advanced argillic - This style of alteration is zoned progressively outwards from the massive silica core, as follows (after Pilco, 2011):
- Silica-alunite, occurs as a thin ring composed of quartz-alunite±pyrophyllite, with traces of kaolinite, with as much as 25 wt.% alunite [KAl3(SO4)2(OH)6], but averaging 6.5 wt.%. In the Cerro Yanacocha Complex deposits it occurs as an envelope that is as much as 10 to 200 m vertically and >200 m laterally. In parts of the complex, the silica-alunite shell is absent silica-pyrophyllite alteration assemblages abuts the massive silica core and surrounds islands of silica-alunite. In the Cerro Yanacocha Complex deposits, at least four pulses of alteration and mineralisation are evidenced by crosscutting alunite relationships. Alunite also occurs as patches in a texture known locally as 'patchy silica-alunite' and may fill fractures in massive silica in the 'massive silica-alunite texture' (e.g., Turner, 1997; Pinto 2002).
- Silica-pyrophyllite, comprising a shell of quartz-pyrophyllite±alunite± minor diaspore± occassional zunyite, which at Cerro Yanacocha surrounds the massive silica and silica-alunite alteration zones, extending vertically to depths of at least 700 m below the present surface to an elevation of ~3200 m. In the Cerro Yanacocha Complex deposits, there is up to 34 wt.% pyrophyllite [Al2Si4O10(OH)2], averaging ~12 wt.%, which occurs in pits, filling fractures and replacing phenocrysts, and as patchy alteration at depth. Occassionally up to 2 wt.% zunyite is observed related to pyrophyllite. Dickite and kaolinite have been observed together in Yanacocha Norte, although the dickite does not appear to be closely related to pyrophyllite assemblages.
- Silica-diaspore, occurring as quartz-diaspore±pyrophyllite, with an average of 7.5 wt.% diaspore [AlO(OH)], that may be enriched to 18.wt %. Diaspore first appears at ~3600 m elevation and increases downward apparently related to an underlying Cerro Yanacocha intrusive porphyry. At depth, diaspore occurs in veinlets with pyrophyllite and traces of alunite, but also fills available open spaces, and locally apparently replaces quartz as a patchy alteration. Subhedral and euhedral crystals of diaspore are up to 0.4 mm across, and tends to have an antithetic relationship with quartz (Hemley et al., 1980).
Note: Whilst Pilco (2011) subdivides the advanced argillic alteration into these three zones, Teal and Benavides (2010) have mapped quartz-alunite and quartz-clay subdivisions that are zoned around the massive silica-dominant replacement cores. The clays the latter recognise include pyrophyllite, montmorillonite, illite and kaolinite.
• Intermediate argillic, which at the Cerro Yanacocha Complex deposits consists of illite-smectite with minor montmorillonite and dickite, that in the central part of the deposit complex is restricted to phreatomagmatic breccias (Pilco, 2011). However, this zone also forms a broad outer lower temperature halo to, and locally overprints the advanced argillic alteration and according to Teal and Benavides (2010) comprises montmorillonite±illite-sericite, grading to propyllitic alteration. Typically it contains disseminations and veinlets of pyrite with or without sphalerite and galena. A band of intermediate argillic alteration also occurs below the phyllic zone that is found above the Kupfertal porphyry Au-Cu mineralisation. See the Porphyry Mineralisation section below for more detail.
• Propylitic, alteration is generally peripheral to intermediate argillic assemblages and includes chlorite ±calcite ±illite ±smectite ±pyrite. Original rock textures are preserved and it grades outwards into fresh country rock. Some of the clays may be supergene products (Longo et al., 2010).
• Phyllic, comprising an assemblage of muscovite/sericite ±chlorite ±dickite ±topaz ±anhydrite ±K feldspar. Muscovite and/or sericite are widespread in the matrix of the high-sulphidation advanced argillic system peripheral to gold mineralisation, and also occurs as fine-grained white mica replacing phenocrysts (Pilco, 2011). However, while not mapped as a distinct phyllic zone (e.g., Teal and Benavides, 2010), muscovite-sericite seems to form a discontinuous ring-like shape surrounding the pits of the Cerro Yanacocha Complex (Pilco, 2011). Where muscovite-sericite is the dominant alteration, the mineral assemblage can contain >30 wt.%, averaging ~20 wt.%, with ~2 wt.% chlorite, and ~7 wt.% pyrite, 6 wt.% dickite and ~4 wt.% gypsum (after anhydrite). Topaz occurs from trace amounts to 1 wt.% (Pilco, 2011). See the Kupfertal description in the Porphyry mineralisation section below where a phyllic zone is recognised and is described in more detail. Phyllic alteration related to the porphyry system may have been overprinted by subsequent telescoped advanced argillic alteration.
• Potassic alteration has been detected in deep drill holes into the Kupfertal porphyry Au-Cu mineralisation, in the form of enhanced levels of biotite which gives the rock a dark brown discolouration. The highest elevation at which this alteration is encountered at Kupfertal was ~3350 m. See the Porphyry Mineralisation section below for more detail.
Temporal distribution - The alteration zonation types and pattern described above, repeated in multiple overlapping alteration centres, forms a mosaic within a broad envelope superimposed on the volcanic pile along a major axis ~20 km NE-SW by up to 8 km NW-SE.
Longo (2005) demonstrated a general WSW → ENE progressive migration of multiple alteration and mineralised centres over the period from 13.6 to 8.2 Ma, by dating (40Ar/39Ar) hydrothermal alunite at multiple mineralised centre. Five stages of hydrothermal activity were defined on the basis of alunite stages. This progression of hydrothermal stages spanning the district is interpreted as follows (Longo et al., 2010):
• Stage 1 alunite, dated at 13.56±0.24 to 12.64±0.61 Ma is associated with the Atazaico Andesite and intrusions of Quilish dacite, and is found at the Cerro Negro Oeste deposit on the southwest margin of the district. No alunite alteration related to gold mineralisation has been identified from the overlying Colorado Pyroclastic Sequence, although laminated tuffaceous rocks intercalated with ignimbrites are locally strongly hydrothermally altered.
• Stage 2 alunite, alteration at 11.46±0.15 to 11.41±0.89 Ma is spatially associated with, and likely closely followed, eruption of the Azufre Andesite domes from ~12.1 to 11.6 Ma. It produced the widespread alteration, mostly to the east the rocks affected by stage 1, occurring both in the western central part of the district at Cerro Negro Este and Pabellon Sur, and to the east, north and west of the San Jose gold deposit near the centre of the Yanacocha trend. Stage 2 predated the San Jose Ignimbrite, whilst stage 3 followed soon after (Longo et al., 2010).
• Stage 3 alunite occurs as veinlets and in hydrothermal breccia containing gold ore that crosscuts intense quartz-rich alteration along the margins of the Corimayo (10.78±0.05 Ma) dacitic flow domes (Gomez, 2002). Alunite of this stage has ages of 11.0 to 10.7 Ma and extends widely for 10 km from Corimayo in the centre of the trend, to Maqui Maqui on the NE end. Stage 3 alteration zones contain gold in a ~14 km2 area that includes the San Jose, Carachugo, Quecher and Arnacocha deposits. The stage 3
alunites have the same age within analytical uncertainty as 10.73 Ma hydrothermal biotite in the deep Kupfertal porphyry Au-Cu deposit in the centre of the Yanacocha trend, from to 2 to 6 km from the coeval epithermal mineralisation. Longo et al. (2010) conclude the shallow stage 3 epithermal gold deposits formed immediately after, but in close association with the deeper porphyry-style Au-Cu mineralisation during the same magmatic-hydrothermal event.
• Stage 4 alunite is apparently associated with the Cerro Yanacocha dacite porphyry which intrudes a Corimayo style porphyry intrusion (the Punta Negra dacite at Encajon), a Yanacocha porphyry, early diatreme bodies and breccias, as well as the previously altered Colorado Pyroclastics at Cerro Yanacocha, near the centre of the Yanacocha trend (Loayza-Tam 2002; Longo, 2005). The 9.91±0.04 Ma age of the Cerro Yanacocha dacite porphyry is analytically indistinguishable from the ages of stage 4 alunite from the eastern Yanacocha district (near the centre of the Yanacocha trend), which range from 10.29±0.31 to 9.95±0.14 Ma (Longo et al., 2010).
• Stage 5 alunite ranges from ~9.3 to ~8.2 Ma and is broadly spatially and temporally associated with intrusions of Coriwachay Dacite, and is restricted to the Cerro Yanacocha area in the centre of the Yanacocha trend. 40Ar/39Ar ages and the geologic evidence support division of stage 5 into an earlier substage 5A at 9.3 to 9.1 Ma and a later substage 5B at 8.8 to 8.2 Ma, that is temporally associated with the 8.59±0.14 Ma Yanacocha Lake rhyolites.
Via these 5 stages, the focus of hydrothermal activity progressively moved 18 km ENE, from the Cerro Negro Oeste deposit on the SW margin of the district to the Maqui Maqui deposit on the NE end of the trend by 10.2 Ma at the end of Stage 3. By ~9.25 Ma hydrothermal activity had moved back to be centred exclusively at Cerro Yanacocha near the middle of the Yanacocha trend, and finally, further major pulses produced a separate late phase in the central Yanacocha Norte and Sur deposits in the middle of the cluster by 8.5 Ma. As such, the epithermal systems forming the Yanacocha deposit cluster persisted for a minimum of ~3 m.y., contemporaneous with the span of a series of porphyry intrusions (Longo et al., 2010).
The ore related quartz-rich altered rock largely occurs as gently dipping, tabular bodies that are 100 to 200 m thick and are broadly conformable with the volcanic stratigraphy (Harvey et al., 1999; Longo, 2000; Bell et al., 2004), although it also occurs as structurally controlled subvertical bodies, that are locally >450 m thick, crosscutting stratigraphy (e.g., Chaquicocha Norte; Longo, 2000; Goldie, 2000).
Epithermal and Supergene Mineralisation
Gold mineralisation was introduced in multiple pulses as a late stage event, superimposed on all alteration types. It most commonly occurs in fractures in massive, vuggy, and granular quartz, with higher grades of >3 g/t Au mostly associated with a late intermediate-sulphidation event that is characterised by 'creamy silica' (Teal and Benavides, 2010).
Gold mineralisation is found throughout the Tertiary volcanic column, including pyroclastics within the Atazaico Andesite, ignimbrites of the Colorado Pyroclastic Sequence and the San Jose Ignimbrite, as well as breccias throughout the column. It also occurs at depth as porphyry style Au-Cu mineralisation associated with intrusive quartz-eye porphyries.
Porosity is the major control of the localisation of mineralisation. Ore is commonly localised around the margins of less permeable breccia bodies as well as around the fractured margins of andesite and dacite domes. The latter is the result of rheological contrasts along flow-dome margins in the more silicic Upper Andesite sequence, shallow dioritic intrusions and late stage diatreme breccia margins which created structural traps for hypogene gold-silver±copper sulphide and sulphosalt precipitation. Deposits having both stratabound and discordant feeder and/or contact margin geometries. In addition, the higher porosity and permeability of ignimbrites of the Yanacocha Pyroclastic sequence, these lithologies were the preferred host rocks of the dominantly stratabound gold deposits.
The gold ore commonly has higher grade cores of >2 g/t Au in steeply dipping structural zones of fractured massive and vuggy quartz, enveloped by larger tabular zones of quartz-rich alteration and lower gold grades of ~0.2 to 1 g/t Au (Harvey et al., 1999; Longo, 2000; Myers and Williams, 2000; Teal et al., 2002). Ore with typical medium grades of 1 to 2 g/t Au is found within quartz-rich altered rocks which originally contained hypogene sulphides that have been subsequently oxidised by supergene processes. Within some structural zones, supergene oxidation reached depths of >400 m (Harvey et al., 1999; Goldie, 2000, 2002; Myers and Williams, 2000; Longo, 2000; Bell et al., 2004). The central oxidised, quartz-rich zones generally have the highest average gold grades (>2.5 g/t) and host local bonanza-grade gold ore (up to 540 g/t Au), where paragenetically late, coarse visible gold is intergrown with barite or quartz in vugs and along high-angle fractures that crosscuts quartz-rich rock. These bonanza grades are also associated with massive botryoidal limonite, rare Sn-rich hydrous iron oxide, and locally cream-coloured porcellaneous chalcedony with rutile.
Hypogene sulphide mineralisation - Below the base of supergene oxidation, hypogene sulphide mineralisation contains covellite-enargite-pyrite±chalcocite (Harvey et al., 1999; Goldie, 2000; Bell et al., 2004). Higher sulphide gold grades of >5 g/t are associated with pyrite-tennantite-covellite-gold±barite±orpiment±native sulphur, apparently formed during the final stage of hydrothermal mineralisation (Longo, 2000). Coarse visible gold grains of up to 1 mm across are occassionally found adjacent to pyrite veinlets replaced by tennantite in zones with >25 g/t Au. The ore-related sulphide minerals commonly also fill cavities and fractures in the quartz-rich zones, and therefore, locally postdate the principal wall-rock alteration (Longo et al., 2010).
Low to moderate amounts of covellite, pyrite and minor enargite are found in silica-pyrophyllite alteration outboard from the main mineralised body and at the margins of intrusive rocks and phreatomagmatic breccias. Small amounts of chalcopyrite are found at depth with silica-diaspore (±pyrophyllite), while variable amounts of pyrite, covellite, chalcopyrite and molybdenite, with very rare digenite, are associated with muscovite-sericite alteration above the Kupfertal porphyry Au-Cu mineralisation. The occurrence of the main sulphides in the hypogene sulphide zone within the Cerro Yanacocha Complex deposits is as follows (after Pilco, 2011):
• Pyrite (FeS2) occurs as anhedral to subhedral crystals, typically filling open spaces, especially in vuggy silica, but also occurs in fractures and as disseminations, and can be 5 to 40 wt.% of the host. At least two generations of pyrite have been documented, i). early, coarse grained, euhedral pyrite grains, and, ii). a late, fine grained variety, rimming early coarse pyrite. Small blebs of chalcopyrite, bornite and pyrrhotite are common in euhedral pyrite. Shallow pyrite usually lacks inclusions of chalcopyrite and pyrrhotite, which are usually only found below 3700 m elevation. At Yanacocha Norte, four stages of pyrite have been observed. Pyrite is commonly replaced and rimmed by digenite.
• Chalcopyrite (CuFeS2) has been encountered in veins at depth (below 3250 m elevation), in some cases accompanied by to specularite and associated with covellite and with unidentified sulphosalts. Molybdenite (MoS2) has also been rarely observed at depth in veins.
• Enargite (Cu3AsS4) occurs as euhedral crystals and as massive aggregates that fill open spaces. It postdates pyrite and is replaced by covellite and sometimes digenite. Zonally, it overlaps and occurs below the chalcocite blanket.
• Chalcocite (Cu2S) has been found as fracture fillings and also as massive aggregates at shallow levels. It has also rarely been observed rimming enargite. Chalcocite mostly occurs at the base of oxidation as supergene enrichment blankets, commonly within massive silica (±alunite±kaolinite) and pyrophyllite alteration.
• Sphalerite and galena assemblages are associated with pyrophyllite and illite-smectite alteration, and occur as a late assemblage in veinlets, generally peripheral to the main mineralisation. These minerals usually rim enargite, but are then rimmed by covellite.
• Covellite (CuS) occurs as tabular crystals and massive aggregates, and usually fills open spaces, occurs as veins, and is disseminated in the groundmass of breccias with other sulphides. Covellite occurs to greater depths than either chalcocite and enargite.
• Digenite (Cu9S5) commonly rims pyrite, sphalerite, galena, and in some cases covellite.
• Gold occurs in association with massive and vuggy silica, and with silica-alunite alteration, extending into the silica-pyrophyllite zone. In the Cerro Yanacocha Complex deposits, high grade gold is related to barite and creamy silica. Visible native gold is not generally present, although some occurrences have been recorded at Yanacocha Sur as late veins within a north-south structure accompanied by pyrite and alunite. In other deposits, such Chaquicocha, high-grade, and in some cases visible gold, occurs at depth, related to fine-grained pyrite, covellite (±enargite), and creamy to grey silica. At shallow oxidised levels, gold has been observed with pyrite and creamy silica. Late fine-grained pyrite can be arsenian and gold bearing (McComb, 2009).
Paragenesis - The district-wide hydrothermal activity, with multiple episodes of magmatism, alteration, gold and gold-copper deposition, which spanned a period of ~3 m.y., was responsible the size and metal endowment at Yanacocha.
Bell et al. (2004) interpreted five distinct paragenetic phases of mineralisation in the Yanacocha district. There does not appear to be a direct relationship between these five mineralisation periods and the five pulses of alunite alteration recorded by Longo, (2005) and Longo et al. (2010) and summarised above (in the 'Alteration' section). However, if the phases of mineralisation do have a temporal spread and are related to different magmatic events, it would be expected that the intensity of each mineralising pulse would be more pronounced in the vicinity of the progressing centres of intrusion and alteration, and vary along the Yanacocha trend. These five periods of mineralisation may be summarised as follows (after Teal and Benavides, 2010; Bell et al., 2004):
• Phase 1 is characterised by pervasive silicification and coeval deposition of fine disseminated Au-bearing pyrite. With depth, this phase produced patchy textured silicification, grading to 'wormy', and at deeper levels A-type veinlets, some of which are banded, suggesting a transition from high sulphidation epithermal to a Au-Cu porphyry system (Pinto, 2002). Clasts of this style of mineralisation are incorporated into deeper level diatreme breccias at Yanacocha. Fluid inclusion data indicate temperatures of from 200 to 500°C and salinity ranges to >40 wt.% NaClequiv. in some deeper samples (Loayza, 2002). Secondary biotite from potassic alteration at the deep Kupfertal deposit yielded an age of 10.72±0.09 Ma (40Ar/39Ar: Longo, 2005).
• Phase 2 is the main gold event, postdating the initial pervasive silicification, and is characterised by fine-grained pyrite with minor enargite and covellite. Sulphides occur as disseminations and void and fracture fillings. Gold of this phase occurs as submicron grains usually closely associated with Fe-oxides along fracture networks in the oxide zone (Turner, 1997; Bersch, 1999).
• Phase 3 is a higher grade gold event, averaging >1 g/t Au, and is apparently associated with intermediate sulphidation. It is recognised by the occurrence of coarse gold associated with late-stage barite or creamy chalcedony. The creamy chalcedony crosscuts previously silicified pyroclastic rocks and phreatic breccias, but occurs as the matrix in some hydrothermal breccias. Mineralisation of this phase occurs in all deposits, and is especially important at the Chaquicocha Alta, El Tapado and Corimayo deposits in the central part of the Yanacocha trend.
• Phase 4 is a late copper-(gold) pulse, closely associated with dacitic intrusions and phreatomagmatic breccias, and is characterised by an enargite-covellite-gold bearing pyrite with quartz-alunite alteration at shallow levels, and pyrophyllite-diaspore alteration at depth. Alunite related to this phase has been dated at 9.12±0.32 Ma (40Ar/39Ar; Longo, 2005), the Stage 5A alunite of Longo et al. (2010). This phase is evident at the Cerro Yanacocha deposits in the middle of the Yanacocha trend.
• Phase 5 occurs as sparsely distributed veinlets of rhodochrosite-dolomite and base metal sulphides, interpreted to represent a transition from acid to a more neutral pH fluids, accompanied by a decrease in the sulphidation state of the system. This latest phase has been observed at the Cerro Yanacocha deposits.
• Supergene, the final phase, related to a district-wide weathering profile generally ranging from 50 to 300 m in depth. This was responsible for supergene oxidation of sulphides, principally covellite-enargite±trace chalcopyrite, leaching of copper from the weathering profile to form a copper depleted capping of iron oxides and remnant liberated gold, and the formation of the copper enrichment blanket below the base of oxidation. This resulted in changes in mineralogy and physical nature of the mineralisation, making the oxide gold ores readily amenable to economic processing via cyanide heap leaching (e.g., Bell et al., 2004), which across the district, has a life-of-mine gold recovery of >70% (Teal and Benavides, 2010). In the cases of the Yanacocha Sur and Maqui Maqui deposits, the copper redeposited in supergene blankets at the base of oxidation contained ~1 wt.% Cu as secondary covellite and chalcocite.
An example of supergene mineralisation is the Yanacocha Verde deposit, which lies beneath the Yanacocha Sur high sulphidation epithermal gold deposit. Mineralisation is within the Lower Andesite sequence and younger Yanacocha dacite porphyries and breccias, mostly below the Yanacocha Pyroclastic Sequence which hosts the epithermal ore. It comprises a supergene sulphide assemblage of covellite + chalcocite intermixed with hypogene enargite. This assemblage grades downward into hypogene covellite and eventually into a chalcopyrite dominant-assemblage in the underlying intrusive Yanacocha dacite porphyries and breccias. Mineralisation occurs as a number of anastomosing near horizontal layers distributed over a vertical thickness of up to 250 m and width of 1 km, with cores and layers of >1% Cu, encapsulated within larger halos of >0.5 and >0.3% Cu.
Distribution of metals in the Cerro Yanacocha mine complex deposits (after Pilco, 2011) may be summarised as follows:
• Gold distribution in this group of deposits, is controlled by both NW and NE trends, with the 0.2 g/t Au contour extending downward from the current surface elevation of ~3900 m, to ~3600 m, although locally feeder zones may extend to ~3400 m. Higher grade >1 g/t Au occurs to elevations of 3650 m and frequently has north-south trends, mostly associated with massive silica, and at deeper levels with silica-alunite and silica-pyrophyllite. The main gold mineralisation occurs from the current surface downwards, to directly above and overlapping the copper mineralisation between elevations of 3600 and 3650 m, although some gold is related to structures that can persist for 200 m deeper (Pilco, 2011).
The gold deposits of the Cerro Yanacocha Complex are largely localised within massive silica and silica-alunite altered porous pyroclastic host rocks and phreatic breccias that surround the margins of the less permeable, composite phreatomagmatic breccias of the 'diatreme' in the centre of the mine complex (Teal and Benavides, 2010). Silver distribution closely follows that of gold, although zones of >50 g/t Ag grade are found at varying levels, from deep to shallow, across the complex.
• Copper generally overlaps the base of the main gold zone, but persists to greater depths, e.g., in the north-south section 14900mE through the Yanacocha Sur and Yanacocha Norte deposits, the 0.5 g/t Au grade contour encloses much of the gold deposit from near the surface, downwards for ~400 m, to overlap the 0.1% Cu contour by ~50 to 200 m (Pilco, 2011).
The top of the preserved copper mineralisation in rocks with a high silica and low mafic content is largely coincident with the base of oxidation, above which copper has undergone supergene leaching.
The 0.1% Cu grade contour locally persists for >500 m below the upper limit, although the >0.5% Cu and >1% Cu mineralisation occupies the upper 200 m (to a maximum of 350 m) of the >0.1% Cu envelope. The >0.5% Cu zone largely only overlaps the lower sections of the >1 g/t Au envelope, which on this section has a vertical extent of as much as 280 m. The >0.5% Cu and >1% Cu mineralisation is represented by both a supergene chalcocite blanket and the underlying enargite dominant zone with lesser covellite. Although the supergene chalcocite has a limited vertical extent of generally <150 m, the enargite zone extends to depths of 150 to >400 m lower, where covellite becomes the dominant Cu sulphide. Cu grades of >1% are largely in the supergene chalcocite blanket developed within the Colorado Pyroclastic Sequence, whilst the underlying high grade hypogene enargite and covellite assemblages are restricted to Coriwachay Dacite phreatomagmatic and hydrothermal breccias and related structures. These structures may locally host higher grade >1% Cu 'feeder zones' to depths of ~300 m lower (Pilco, 2011).
The overlap of the Cu and Au mineralisation is largely within the lower massive silica alteration envelope and to a lesser extent the fringing silica-alunite and silica-pyrophyllite shells and laterally in the phyllic alteration. Unlike gold, copper mineralisation persists into the 'diatreme' in the Cerro Yanacocha deposits, copper mineralisation generally follows NE trends, with subsidiary NW controls.
• Arsenic is widespread in the Cerro Yanacocha complex deposits, and follows the main NE trend. Arsenic is widely distributed in all three pits laterally following the main NE trend, although a broad, NW trend is also evident between Yanacocha Oeste and Yanacocha Sur pitls. Arsenic generally accompanies copper but extends to higher levels than the preserved copper mineralisation, extending into the oxide zone. In oxide it occurs as scorodite (FeAsO•2H2O) and in the middle sulphide zone downward, as enargite (Cu3AsS4). At Yanacocha Oeste, values of >1000 pm As are found above an elevation of 3520 m, related to phreatomagmatic and hydrothermal breccias, and is associated with feeder structures. At Yanacocha Sur and Yanacocha Norte, its distribution overlaps that of Au and Cu, above 3650 m elevation, with the As:Cu ratio decreasing downwards into the covellite zone (Pilco, 2011).
• Molybdenum is best developed at Yanacocha Oeste, where it appears to occur in two different bodies at depth and at shallow levels. At depth, grades of >60 ppm, in some cases reaching 120 ppm Mo, occur external to the >0.5 wt,% Cu (±As), persisting downwards as veins. At shallow depths, grades of >200 ppm Mo occur in the oxide zone of Yanacocha Oeste and appear be displaced to the SW and separated from the main arsenic mineralisation in the central part of Yanacocha Oeste. In the others pits molybdenum is rare and lacks a defined pattern (Pilco, 2011).
• Zinc and lead. Zinc is distributed along a well established NE trend in the central part of Yanacocha Oeste to Yanacocha Norte deposits, and is localised peripheral to, both laterally and below both the 1000 ppm As and 0.3 g/t Au shells. It is present as sphalerite and can locally reach >5000 ppm Zn. Anomalous Zn also occurs above and peripheral to muscovite-sericite alteration zones. Lead has a close relationship and distribution pattern to that of Zn. At shallow levels it may reach grades of as much as >2000 ppm Pb, but diminishes in concentration with depth (Pilco, 2011).
The main and late stage epithermal gold mineralisation of the Yanacocha cluster overprints, and is telescoped onto, multiple centres of generally low grade, 'pre-main gold stage' porphyry Au-Cu deposits and the higher grade gold-copper rich lithocaps of those same porphyry accumulations.
Porphyry mineralisation includes the Kupfertal deposit, which is exposed as a small outcrop of quartz stockwork in the deeply incised (~300 m) glacial Encajón gorge between the San Jose and Yanacocha Sur epithermal deposits, and similar mineralisation below the Yanacocha Sur, and Maqui Maqui epithermal deposits.
Supergene Au-Cu sulphide blankets were also formed at the base of oxidation in the lithocaps and upper porphyry systems, e.g., Yanacocha Verde below Yanacocha Sur (Bell et al., 2004; Teal and Benavides, 2010), as described above.
The Kupfertal mineralisation is exposed over a small area in the floor of the NE-SW Encajón valley, at ~3815 m elevation, 300 m vertically below the quartz cap that forms the top of Cerro Yanacocha. It is located between the San Jose and Yanacocha Sur deposits, 500 m to the south and 1 km to the north, respectively, and the Carachugo-Chaquicocha and Corimayo-Tapado deposits to the east and west respectively.
It is interpreted to represent a shell of porphyry style alteration, veining and Au-Cu mineralisation, hosted within Miocene volcanic rocks, above a causative porphyry intrusion at depth, which has not yet (to 2011) been intersected (Pinto, 2002; Gustafson et al., 2004; Bell et al., 2004; Longo et al., 2010; Teal and Benavides, 2010; Pilco, 2011).
• Geology - The 1:25 000 Yanacocha District geological map (Teal et al., 2007 in Teal and Benavides, 2010) shows the outcrop of 'patchy' alteration and 'wormy' veining on the upper periphery of the Kupfertal porphyry system as being within the eutaxitic crystal tuff unit that constitutes the Cori Coshpa ignimbrite member of the Colorado Pyroclastic Sequence. Higher on the walls of the Encajón gorge, this sequence is overlain by ignimbrites of the Maqui Maqui ignimbrite member, and the San Jose Ignimbrite, both of which host epithermal gold ore at the San Jose and Yanacocha Sud deposits to the south and north respectively. Within the Kupfertal area, the host sequence is intersected by numerous NW-SE normal faults, offsetting the shallowly dipping sequence by <10 to >50 m. In the centre of the deposit, the uneroded Cori Coshpa ignimbrite member is only a few tens of metres in thickness, and locally outcrops, but is as much as 100 m thick within 500 m to the NE and SW along the valley floor, reflecting an apparent fault controlled updoming (Pinto, 2002).
The Cori Coshpa ignimbrite member is underlain by andesitic lavas of the Atazaico Andesite, comprising an upper 120 to 250 m of porphyritic andesite to dacitic lavas with medium to coarse phenocrysts of plagioclase, hornblende and biotite, and minor quartz, and a lower ~250 m thick suite of porphyritic andesitic lava flows with finer phenocrysts of hornblende and coarse biotite (Pinto, 2002).
Limited deeper drilling has penetrated up to 60 m into an underlying sequence of 'crumbly' feldspathic sandstone with ~80% quartz, accompanied by clay altered feldspars. This sequence is apparently belongs to the Cretaceous basement. These rocks have undergone weak potassic alteration and contain 'A-type' stockwork quartz veinlets (Pinto, 2002).
Although no causative porphyry intrusion has been intersected, a weakly altered and mineralised late intermineral intrusive granodioritic porphyry has been cut in two diamond drill holes at an elevation of ~3280 m, over 500 m below the Encajón valley floor (Sillitoe, 2000; Pinto, 2002; Gustafson et al., 2004). It is in fault contact with enclosing more heavily altered and mineralised andesite lavas, and has a poorly developed chlorite-sericite assemblage, superimposed on earlier weak K silicate alteration, manifested by biotite replacing hornblende phenocrysts. This alteration is accompanied by sparse 'A-type' quartz veinlets, constituting ~2 vol.% of the rock, with sporadic vein hosted and disseminated chalcopyrite and molybdenite assaying 0.14 % Cu and 0.1 g/t Au (Sillitoe, 2000; Pinto, 2002; Gustafson et al., 2004).
The mineralised zone is also cut by numerous late andesitic to dacitic dykes that vary from 0.1 to >50 m in thickness, dip steeply, and dominantly strike at 120°, parellel to the main fault direction. These dykes do not contain veining and are barren, although most have been largely altered to clays (Pinto, 2002).
• Alteration - The porphyry mineralisation and 10.73±0.05 Ma hydrothermal biotite alteration deep in the Kupfertal system is indistinguishable in age from the widespread Stage 3 quartz-alunite alteration of Longo et al. (2010), including that associated with epithermal mineralisation ~4 km west at Corimayo and ~1 km NW at Punta Negra in Encajon. This relationship is interpreted to indicate the shallow epithermal mineralisation is part of a lithocap to the Kupfertal porphyry system, resulting from fluids that escaped upwards from the latter, and were then channelled through the surrounding porous pyroclastic wall rocks to shallower depths to be precipitated as high sulphidation epithermal Au-Cu mineralisation (Longo et al., 2010; cf. Lepanta-Far Southeast in the Philippines; Hedenquist et al., 1998).
However, stages 4 and 5 quartz-alunite alteration from samples closest to the Kupfertal porphyry are 0.8 and 1.5 m.y. younger, indicating they post-date the porphyry-hydrothermal pulse reflected by the potassic biotite alteration date. These samples are of Stage 4 alunite from the San Jose Sur deposit, ~500 m to the south of, and topographically higher than Kupfertal with an age of 9.95±0.14 Ma, and Stage 5 alunite from a zone of 'patchy' alteration, directly above the porphyry mineralisation, with an age of 9.25±0.10 Ma. As such, these younger phases of epithermal alteration and mineralisation appear to have been telescoped onto and overprinted the upper sections of the Kupfertal porphyry system and its lithocap (Longo et al., 2010).
The interpreted uppermost extremities of the Kupfertal alteration system is revealed by deep erosion in the bottom of the Encajón gorge as a small outcrop of a mosaic stockwork of amorphous, patchy textured cryptocrystalline quartz and clay, and 'wormy' textured quartz veinlets. Rock outcropping above the Encajón valley floor, that is altered to alunite-pyrophyllite with volcanic texture still visible, passes down into a grey, quartz-rich rock with a 'patchy' texture.
This 'patchy' alteration zone occurs as irregular to rounded 'whitish' patches, mostly composed of pyrophyllite, alunite, kaolinite and diaspore, superimposed on a light grey siliceous matrix. The upper or outer periphery of this 'patchy' alteration is described, as 'gusano texture' by Gustafson et al. (2004) where it occurs as small rounded blobs or patches of soft, white alunite ±pyrophyllite±diaspore in a moderately hard matrix of granular quartz with interstitial pyrophyllite±diaspore±alunite. The silica in the matrix of both variants appears as be largely recrystallised (Pinto, 2002). These textures are the product of extreme acid leaching (Gustafson, 2000) and an advanced argillic alteration front, and appears to form a zone peripheral to porphyry mineralisation across the Yanacocha district (Pinto, 2002). As the alteration increases in intensity, it becomes progressively texture destructive, and passes into a rock that is largely alunite and pyrophyllite, with irregular ragged remnant quartz patches that are more or less isolated in pyrophyllite and locally linked to form very irregular clots and sinuous, 'wormy' vein-like shapes, known as 'wormy' veins (Klein et al., 1999; Pinto, 2002).
This intense pyrophyllite-alunite±kaolinite alteration assemblage passes downward into sericite with decreasing amounts of pyrophyllite and increasing kaolinite, which may be after chlorite that is abundant at deeper levels. It is uncertain if this transition is a gradation, or if the advanced argillic alteration overprints the phyllic assemblage. Sillitoe (2000) favours the latter.
This phyllic zone is generally fine-grained sericite-kaolinite altered rock in which protolith textures have been obliterated down to at least 595 m below the Encajón valley floor. It forms a shallowly dipping, well defined, up to 300 m thick zone, below the ~3750 m elevation, covering an area in excess of 1 x 0.8 km. This largely texturally destructive alteration, is superimposed over an earlier secondary potassic assemblage, evidenced by pseudomorphed feldspars and relicts of biotite replaced by sericite and pyrite, accompanied by quartz and leucoxene or titanite. Opaque minerals include pyrite, covellite, chalcopyrite and molybdenite, with very rare digenite. The protolith appears to be largely fine grained (<2 mm) andesitic volcanic rock (Pinto, 2002).
The phyllic layer is underlain below the ~3400 m elevation (~500 m below the valley floor) by an up to ~200 m thick band of intermediate argillic alteration developed over the transition between phyllic and potassic zones. It covers an area of ~600 x 400 m and is characterised by an assemblage of quartz-illite-chlorite-kaolinite, superimposed over the potassic alteration of an andesitic protolith (Thompson, 2001). This zone has an increase in pyrite content, with an opaque assemblage that also includes chalcopyrite and magnetite, which in many cases has been altered to hematite (Pinto, 2002).
As the intermediate argillic assemblage fades with depth, the dominant alteration is potassic, which has only been detected in deep drill holes. This potassic assemblage takes the form of enhanced levels of biotite occurring as fine disseminations and veinlets, giving the rock a dark brown discolouration. The highest elevation at which this alteration is encountered is ~3350 m, 500 m below the Encajón valley floor. It is developed in fine grained andesitic volcanic and sedimentary sequences. Under the microscope, biotite veinlets are seen to be cutting plagioclase (albite), whilst primary hornblende has been replaced by secondary biotite (Thompson, 2001) in a rock with a low feldspar content. Other secondary minerals include sericite and vivianite (Fe3(PO4)2•8(H2O), accompanied by pyrite, chalcopyrite, magnetite and molybdenite (Pinto, 2002).
The advanced argillic alteration described above was imposed on the pyroclastics of the Colorado Pyroclastic Sequence, and continued downward, locally for >100 m, into the upper member of the Atazaico Andesite lavas. The 'patchy' alteration zone within the advanced argillic domain, appears to be predominantly within the Cori Coshpa ignimbrite, decreasing in intensity in the upper sections of the underlying andesitic lavas of the Atazaico Andesite. The phyllic alteration sheet, in the area drilled, appears to be internal to the Atazaico Andesite lavas, cutting obliquely across stratigraphy. The underlying, overprinting intermediate argillic zone is also transgressive, and predominantly within the lower member of the Atazaico Andesite. The underlying potassic alteration is within the Cretaceous basement sedimentary rocks and lower member of the Atazaico Andesite (as illustrated by diagrams in Pinto, 2002).
• Veining - The shallowest veining at Kupfertal is represented by 'wormy' (or 'pseudo A-type') quartz veinlets, which are restricted to the zone of 'patchy' alteration described above (Pinto, 2002). This 'wormy' veining develops as the intensity of 'patchy' alunite-pyrophyllite alteration of the volcanic protolith increases in intensity, leaving irregular ragged remnant quartz patches that are more or less isolated in alunite-pyrophyllite and locally linked to form very irregular clots and sinuous, worm-like shapes, known as 'wormy' veins (Klein et al., 1999; Pinto, 2002).
Quartz veins that are sinuous but more continuous appear next, the more planar of which are typical of 'A-vein' stockworks commonly found in many porphyry Cu deposits, and are clearly younger than the sinuous and discontinuous 'wormy' veins (Gustafson et al., 2004).
In drill core, away from the outcrop, the 'wormy' quartz and 'A veins' appear at ~25 and ~50 m respectively below the valley floor, although the former abruptly decrease and disappear at depths that vary from ~90 to 150 m below the valley floor, whereas the 'A veins' continue downwards, decreasing in abundance to ~520 m depth. The upper limit of both vein types deepens to both the NE and SW (Pinto, 2002). In general, the 'wormy' veins occur between 3815 and ~3700 m elevation, overlapping the top ~100 m of the relatively continuous and planar 'A veins' (Pinto, 2004; Gustafson et al., 2004). According to Pinto (2004), these 'wormy' veins, which contain microscopic bornite, chalcopyrite and a little anhydrite, were originally developed in association with potassic alteration, which has been overprinted by phyllic and then advanced argillic alteration.
The 'A veinlets', which are not typically symmetrical, are gradational with the earlier 'wormy' veins, and are best developed below the 3700 m elevation. They are principally composed of fine equigranular quartz, with locally disseminated alkali feldspar and commonly contain fine relict disseminated anhydrite, chalcopyrite and bornite, and in some cases may have a porous centre line. They generally fill planar fractures and typically occur in multiple pulses, with the earliest being less planar and less continuous. Any potassic selvage has been obliterated by overprinting phyllic and late advanced argillic alteration (Gustafson et al., 2004).
Granular quartz, which is grey in colour and translucent, in both 'patchy' alteration and 'wormy' veins is essentially identical to that of 'A veins' in this zone of overlap and transition, and contains the same high concentrations of fluid inclusions, which are overwhelmingly vapour dominant at this elevation (Gustafson et al., 2004; Sillitoe, 2000). However, while the wormy veins are generally composed of hard, granular quartz with only traces of interstitial pyrophyllite, siliceous matrix to the 'patchy alteration is softer because of abundant interstitial pyrophyllite.
'B veinlets' are not common at Kupfertal, and have only been recognised in some drill holes, where they are apparently later than 'A veinlets'. They contain coarse-grained columnar quartz shape and have a centre line sometimes filled by pyrite (Pinto, 2002).
Small 'D veins' are found below 3350 m, first appearing in the intermediate argillic zone, where they overprint 'A veins' (Pinto, 2002; Gustafson et al., 2004). They are described as sulphide veins with sericitic alteration halos and typically little quartz (Gustafson et al., 2004).
• Mineralisation - Mineralisation at Kupfertal is dominantly composed of sulphide minerals hosted within the Neogene volcanic rocks of the Lower Andesite and Yanacocha Pyroclastic sequences, and the underlying Cretaceous sedimentary rocks. These sulphides occur as both disseminations and in veins, including 'wormy', 'A', 'D' and the rare 'B' types, and are zoned downwards, closely related to zoning of hydrothermal alteration (Gustafson et al., 2004).
Locally, weak supergene chalcocite replaces primary sulphides in the top 100 m below the surface, but contributes only a small proportion of the total Cu over that interval (Gustafson et al., 2004).
In the upper ~350 m below the Encajón valley floor, the advanced argillic to phyllic alteration zones are characterised by a hypogene assemblage of pyrite enargite and covellite, in decreasing order of abundance. Pyrophyllite, accompanied by pyrite-enargite veinlets extends to greater depths in structural zones. Except in these structural zones, enargite does not usually persist below ~325 m, in the upper phyllic zone, although the copper grade remains steady at about 0.25% deeper into the phyllic zone where the dominant sulphides are pyrite and chalcopyrite, with traces of covellite (Pinto, 2002; Gustafson et al., 2004). Gold grades are erratic and mostly from 0.2 to 0.5 g/t to just below the bottom of the enargite zone, increasing downward to higher, but still erratic values, mostly >0.5 g/t Au in the phyllic zone (Gustafson et al., 2004).
Within the underlying intermediate argillic and potassic alteration zones, sulphides are typically chalcopyrite and pyrite, with a higher pyrite content accompanying the former. The best mineralisation accompanies potassic alteration, with grades of 0.3% to 0.4% Cu and ~0.6 g/t Au (Gustafson, 2000). Gold is apparently associated with chalcopyrite (Odekirk, 2000).
Magnetite is absent <420 m below the Encajón valley floor, with no evidence of sulphidisation of magnetite to form pyrite, whereas it is abundant as disseminations and rare veinlets at a depth of >475 m below the valley floor. Within this lower zone of chalcopyrite-pyrite-magnetite below 475 m, Au values are somewhat less erratic than in the phyllic zone. In the 50 m above 475 m, where minor residual magnetite remians, there is a more or less sympathetic variation in Au grades with magnetic susceptibility, which suggests Au was removed as magnetite was destroyed.
Bornite is not a significant component of the sulphide assemblage but is relatively common as minute traces locked in vein quartz (Gustafson et al., 2004).
In the intermineral granodioritic porphyry, disseminated chalcopyrite and pyrite account for ≤1 vol.% of the rock, with variable amounts of magnetite, but contain ±0.1 g/t Au is associated with the sulphides (Gustafson et al., 2004).
Sphalerite is a minor accessory in different assemblages throughout the alteration halo. Small D veins in the upper levels at which they occur, may contain pyrite-tennantite-galena-sphalerite and sericitic alteration halos, and locally contain spikes of >1 g/t Au, but greatly reduced Cu values (Gustafson et al., 2004).
Strongly anomalous Mo levels of >50 to 250 ppm, are widespread. An irregular zone of anomalous Mo generally correlates with the 'patchy' and 'gusano' alteration at surface in the NW and east of the Kupfertal Cu-zone (Pinto, 2002). Molybdenite typically occurs as smeared anhedral flakes in pyritic advanced argillic-altered rock near surface, but as subhedral to euhedral crystals in quartz veins within the porphyry system at depth (Gustafson et al., 2004).
Pinto (2002) proposes a paragenetic sequence involving:
- Early hypogene porphyry phase of bornite, chalcopyrite, gold, pyrite and lesser covellite, accompanied by magnetite, biotite and molybdenite; followed by
- Late hypogene high sulphidation stage of silica, chlorite, pyrite, covellite, enargite, tennantite, gold, and lesser molybdenite, accompanied by pyrophyllite, sericite, alunite, diaspore and kaolinisation of chlorite; and
- Supergene remobilisation to form covellite and chalcocite blankets that post-dates deposition of enargite.
Near surface fluid inclusions in quartz from all vein types and from 'patchy' quartz alteration are abundant and dominantly vapour filled. However, at depth, saline inclusions with halite and less common unidentified anisotropic daughter crystals increase, as do two-phase liquid-vapour inclusions. The few 'A veins in the intermineral granodioritic porphyry only contain moderate amounts of fluid inclusions containing a suite of vapour-dominant, liquid-vapour, and brine inclusions. 'A' and 'wormy' quartz veins near surface, and increasingly with depth, contain traces of 'locked' chalcopyrite (±bornite) and rare anhydrite. In contrast, the granular quartz in the matrix of the gusano alteration above the appearance of the 'wormy' veins and 'patchy' alteration, contains mush less fluid inclusions, all vapour dominant, and only extremely rare relict chalcopyrite grains (Gustafson et al., 2004).
Other porphyry and related epithermal mineralisation - Progressively deeper drilling beneath the base of the high sulphidation caps throughout the Yanacocha cluster has exposed the transition into concealed porphyry systems beneath a series of outcrop gold deposits, including the corridor connecting the El Tapado and Yanacocha Sur, Yanacocha Sur and Norte, and Maqui Maqui. At the latter, a pyrite-covellite-enargite assemblage, associated with pyrophyllite, alunite and/or kaolinite alteration, 'A veins' and intersections of ~0.25% Cu and 0.93 g/t Au are interpreted as a lithocap to a porphyry system (Sillitoe, 2000).
Other porphyry Cu±Au±Mo deposits within a 50 km radius of Yanacocha include the Michiquillay Cu-Au-Mo; El Galeno Cu-Mo-Au-Ag; La Carpa; Minas Conga - Perol, Chailhuagon, Amaro Au Cu Ag; and Cerro Corona Au-Cu; and the high sulphidation Tantahuatay Au-Ag, La Zanja Au-Ag and Sipán Au deposits described in separate records.
The smaller Colpayoc and Chamis prospects are located ~15 km to the SW of Yanacocha and are part of an inferred Au-Cu-Mo porphyry system expressed as a 6 km2 area exhibiting various styles of alteration and mineralisation that coincides with a broad, semi-circular, >2 km in diameter magnetic feature. Within this area there is evidence of intrusive-related replacement and skarn styles of precious and base metals, as well as limited exposures of oxidised gold-copper porphyry mineralisation. One of the latter is exposed over an area of 300 x 350 m with a JORC compliant Inferred Resource of 19.28 Mt @ 0.5 g/t Au (Turner 2011, NI 43-101 Report for Estrella Gold Corp, quoted by Wild Acre Metals Limited ASX release, 2013).
La Quinua transported mineralisation
The La Quinua mineralisation is located in the western portion of the Yanacocha district, immediately to the west and downslope of the Yanacocha Sur and Yanacocha Oeste volcanic hosted epithermal deposits. It is hosted by gravels scoured by Pleistocene glaciers from the highland where these two volcanic hosted deposits outcropped, transported downslope and deposited under peri-glacial conditions in two fault controlled basins separated by the locally east-west trending Andean continental divide. Both basins, La Quinua and La Pajuela, are bounded to the east by the NNW trending La Quinua Fault and contain up to 320 and 350 m of sediment respectively. The former hosts the La Quinua Central and Sur gold deposits, while the La Quinua Norte mineralisation is in the upper sections of the La Pajuela Basin. The coarse siliciclastics of the La Quinua Basin were derived from over the Yanacocha Sur and/or Oeste epithermal deposits, while the coarse sediments of the La Pajuela Basin originated from the Yanacocha Oeste area.
The succession within the La Quinua Basin comprises, from the base:
Regolith - monolithic gravel and saproilite.
Lower-sequence gravel - up to 220 m of unsorted, massive to weakly bedded cobble gravel (silicified, silica-clay and silica-alunite altered volcanic clasts) in proximal facies, grading laterally into interbedded gravel, sand and silt lateral facies. Overall 57% gravel, 23% sand and 20% clay with an average grade of 0.67 g/t Au.
Iron and organic deposits - around 30 m of bog-iron, organic rich mud and palaeosols carrying, on average 0.53 g/t Au.
Ferruginous gravels - up to 90 m in thickness of goethite and hematite superimposed on the basal parts of the upper-sequence gravels, containing lenses up to 16 m thick of ferricrete. The gravels average 68% gravel, 20% sand and 12% clay with a grade of 0.99 g/t Au.
Upper-sequence gravel - up to 250 m of of unsorted, massive to weakly bedded cobble gravel (silicified volcanic clasts) in proximal facies, grading laterally into moderately sorted pebble gravel, gravelly sand and silt in distal facies, with lenses of silty sand throughout. Overall composition is 69% gravel, 17% sand and 14% clay with an average grade of 0.75 g/t Au.
Moraines associated with this glaciation crop out at >3700 m elevation and contain cobbles of advanced argillic altered volcanic rocks, commonly including massive quartz and quartz alunite boulders. Gold mineralisation within the La Quinua deposits is disseminated throughout, and mostly occurs as micron sized particles, liberated within the mud matrix and disseminated within the mineralised gravel clasts. Grade transitions are gradational, with no rich placer streaks and <25% dilution of grade compared to the source volcanics. The Ag:Au ratio is 6:1, compared to >10:1 in the Yanacocha Sur and Oeste deposits.
The ore deposits have a fan shape, related to the environment of deposit, with the bulk of the ore grade being in the Upper-sequence gravel and Ferruginous gravel of the La Quinua Central deposit, and is related to the erosion of higher source grades in the volcanic hosted epithermal mineralisation.
Pre-mining 'reserves' at La Quinua in 2003 amounted to 424 Mt @ 0.75 g/t Au for 317 tonnes of contained gold (Mallette, et al., 2004). These are included in the Yanacocha reserves and resources quoted below.
Evolution of mineralisation, alteration and magmatism
Magmatism in the Yanacocha district lasted for ~11 m.y., with the Yanacocha Volcanic Complex persisting over the last ~6 m.y., representing a prolonged period of magmatic-hydrothermal activity, accompanied by episodic pulses of gold mineralisation (Longo et al., 2010). The Yanacocha calc-alkaline suite was oxidised, water and sulphate rich, and evolved from early pyroxene → hornblende andesite to late titanite-bearing dacite and minor rhyolite. Several of the dacites contain both high- and low-aluminum hornblendes, interpreted to have crystallised in the middle and upper crust, respectively (Longo et al., 2010). According to the same authors, Mg, Ti, P, Sr and Ba variations of these rocks are consistent with a complex magmatic origin via both cooling, fractional crystallisation, periodic recharge of deeply derived hydrous basaltic or andesitic melts, and mixing with silicic melts derived by crustal melting. The low eruption rates, high phenocryst contents of the volcanic rocks, and widespread hydrothermal alteration are consistent with the interpretation that most of the magmas at Yanacocha crystallised in shallow chambers as granitoids that passively degassed ore fluids (Longo et al., 2010).
Mid to late Miocene hydrothermal activity, alteration and associated gold deposition in the Yanacocha district increased as eruptive output of the Yanacocha Volcanic Complex decreased, reaching a peak in the end stages of magmatic cycles when eruption volumes were at a minimum. Conversely, the periods of minimum gold distribution appear to correspond to the highest rates of eruption. These observations apply to both individual cycles and the complex as a whole. Similarly there is an overall increase of SiO2 content with time, including minor reversals, with the bulk of the gold being deposited in association with the more silicic magmatism during the Upper Andesite Sequence (Longo et al., 2010).
These observations are illustrated by the following relationships (from Pinto, 2002; Gustafson et al., 2004; Bell et al., 2004; Longo et al., 2010; Teal and Benavides, 2010; Pilco, 2011; and references cited therein):
• Atazaico Andesite of the 'Lower Andesite Sequence' ('effusive stage 1') progressed from west to east over an ~1.2 m.y. period from ~14.5 Ma, and include ~36 km3 of andesitic lavas. Melt SiO2 contents increased gradually during eruption, averaging 60.0%. The Quilish dacite was intruded into the Atazaico Andesite in the western parts of the district, but is more or less coeval with the lavas, and is spatially associated with the Stage 1 alunite alteration of Longo et al. (2010). This alteration is coeval with the oldest gold deposits at Quilish and Cerro Negro in the western sections of the Yanacocha trend, containing ~180 t of Au (i.e., ~10% of the gold endowment at Yanacocha).
• Colorado Pyroclastics, the 'explosive stage 1' that comprises much of the 'Yanacocha Pyroclastic Sequence', succeeded the Atazaico Andesite, and spanned 0.23 m.y. from 12.63 Ma, to deposit ~12 km3 of andesitic and dacitic ignimbrites, and marked an increase in melt SiO2 content to 63.2% at ~12.4 Ma. No significant alteration or gold deposition accompanied this stage, although these porous pyroclastics are host to much of the epithermal mineralisaiton.
• Azufre Andesite, the lowest member of the 'Upper Andesite Sequence', representing 'effusive stage 2', which comprises ~14.1 km3 of andesitic and dacitic lavas and subordinate associated domes and pyroclastic deposits, erupted over ~0.4 m.y. from 12.1 to 11.6 Ma, with a slight reversal in melt SiO2 content, down to 62.4%. These effusives are coeval with the 12.4 to 11.9 Ma Andesite porphyry dykes and plugs of the Yanacocha porphyries.
The quiescent interval following the main phase of Azufre Andesite volcanism, but prior to the San Jose/Shacsha Ignimbrite, was characterised by the widespread, intense Stage 2 quartz-alunite alteration of Longo et al. (2010) from 11.46±0.15 to 11.41±0.89 Ma, which preceded the main gold mineralisation. This alteration was mostly developed in the central to eastern section of the Yanacocha trend, and also corresponds to the phase 1 mineralisation event of Bell et al. (2004) which comprises fine disseminated Au-bearing pyrite that accompanied the district wide silicification. No significant gold deposits were formed during this interval, although Gustafson et al. (2004) suggest ~15 tonnes of gold were emplaced in minor occurrences.
• San Jose (or Shacsha) Ignimbrite, also part of the 'Upper Andesite Sequence', representing 'explosive stage 2', comprised the eruption of ~24 km3 of andesitic to dacitic ignimbrites with an average melt SiO2 content of 63.4%, between ~11.46 Ma (lower member) and ~11.25 Ma (middle and upper members). No significant alteration or mineralisation is known from this period.
Over the ~3.3 m.y. to the end of the San Jose Ignimbrite deposition, ~88 km3 of Yanacocha Volcanic Complex rocks were emplaced, covering an area of 500 km2, with an associated ~230 t of Au in a small number of deposits. This represents just over 10% of the total endowment of the Yanacocha trend, almost all of which was at the beginning, associated with the Atazaico Andesite/Quilish dacitic intrusions and Stage 1 alteration at ~13.5 to 12.6 Ma.
• Coriwachay Dacite-Rhyolite Magmatism, represents the final phases of the 'Upper Andesite Sequence', deposited as small volumes of more highly oxidised magmatic rocks over a >2.4 m.y. period between 11.1 and 8.2 Ma. These rocks total only ~1.5 km3, ~2% of the total Yanacocha Volcanic Complex, and are high-silica dacitic lavas, domes and plugs, together with rhyolitic dykes and ignimbrite layers, although only remnants of the latter are preserved. The average melt SiO2 content of these rocks increased markedly to 69.2%. The remaining >85% of the gold in the district was deposited during the deposition and emplacement of these rocks.
During this 2.4 m.y. period, there were three brief episodes of felsic magmatism at 10.8, 9.9 and 8.4 Ma, the first two of which produced the flow domes of Corimayo dacite and dykes of the Cerro Yanacocha dacitic porphyry respectively, while the third corresponds to the Yanacocha Lake rhyolitic dykes and the Negritos rhyolitic ignimbrite.
The 11.0 to 10.7 Ma Stage 3 alunite alteration of Longo et al. (2010) accompanied the 10.78±0.05 Ma Corimayo dacitic flow domes. This also corresponds to the 10.73 Ma hydrothermal biotite in the underlying Kupfertal porphyry Au-Cu deposit (Longo et al., 2010).
The 10.3 to 9.9 Ma Stage 4 alunite alteration of Longo et al. (2010) broadly correspond to the emplacement of the ~9.9 Ma Cerro Yanacocha dacitic porphyry.
Although not specifically stated in the available literature, timing constraints would make it likely that these two phases of alunite alteration broadly coincide with the phase 2 main high sulphidation gold mineralisation event of Bell et al. (2004), characterised by fine-grained pyrite with minor enargite and covellite, and the overprinting phase 3 high grade gold event, apparently related to an intermediate sulphidation phase, recognised by the occurrence of coarse gold associated with late-stage barite and/or creamy chalcedony.
The phase 4, late Cu-Au enargite-covellite-gold bearing pyrite mineralisation of Bell et al. (2004) is spatially associated with dacitic intrusions and phreatomagmatic breccias, and is accompanied by Stage 5A quartz-alunite alteration of Longo et al. (2010) dated at 9.12±0.32 Ma.
It is likely then, that the remaining phase 5 mineralisation of Bell et al. (2004), comprising sparsely distributed veinlets of rhodochrosite-dolomite and base metal sulphides, is associated with the final 8.8 to 8.2 Ma Stage 5B alunite alteration of Longo et al. (2010), and the late 8.59±0.14 Ma Yanacocha Lake rhyolites.
Gold was progressively added to the individual deposits to varying degrees during each of these alteration and mineralisation stages. Longo et al. (2010) estimate that deposition of the contained gold of the Yanacocha cluster during their hydrothermal alunite alteration stages 1 to 5 inclusive, was as follows:
- Stage 1, high sulphidation gold event = ~220 tonnes of Au, as two separate early pulses at 13.6 and ~13.0 Ma that preceded district wide alteration,and was concentrated to the west only, in the Cerro Negro and Cerro Quilish clusters of deposits;
- Stage 2, high sulphidation gold event = ≤15 tonnes of Au, in smaller occurrences associated with the district wide silicification event;
- Stage 3, high sulphidation gold event = ~390 tonnes of Au, in deposits such as Carachugo, Chaquicocha Norte, Maqui Maqui, Corimayo and San Jose;
- Stage 4, intermediate sulphidation pulse = ~310 tonnes of Au, in deposits such as San Jose Sur, Tapado and Chaquicocha Sur;
- Stage 5A, late copper-gold event = ~780 tonnes of Au, in the deposits of the Cerro Yanacocha Complex, which include Yanacocha Norte, Yanacocha Sur and Yanacocha Oeste.
Synthesis - based on the detail outlined above and throughout this description, it may be inferred that:
• A large (>300 km3), deep (>5 to 15 km) composite parental magma chamber formed during the early to middle Miocene, localised at a site of neutral buoyancy (e.g., Sillitoe, 2010) in the middle to upper crust. Much of volume of the chamber was occupied by molten magma for ~6 m.y., due to low heat dissipation because of its depth and volume, and recharge by repeated pulses of hydrous basaltic or andesitic melts from the 'MASH zone' (e.g., Richards, 2003; 2004) at the interface between the asthenosphere and the base of the crust.
It was most likely elongated NE-SW, parallel to the mineralised corridor at Yanacocha, and was focussed at the intersection of a major, long-lived NW-SE trending structural feature that had controlled Mesozoic basin architecture, and a crosscutting NE-SW structural corridor related to the SE termination of Neogene magmatism.
• Subsequently, but prior to ~14.5 Ma, the parental magma underwent differentiation, and a segregated fraction ponded in the roof of the chamber, sufficiently light to continue its upward buoyant passage. This fraction formed the 14.5 to 13.3 Ma Atazaico Andesite extrusions and the coeval to marginally younger Quilish dacite intrusion. Differentiation of the parental magma also released gold rich hydrothermal fluids which had accumulated in the chamber roof, and accompanied the upward passage of magmas to form the 13.5 to 12.6 Ma 'Stage 1' quartz-alunite alteration in the SW section of the Yanacocha trend, and as the intensity of magmatism waned, the coeval Cerro Negro and Cerro Quilish high sulphidation epithermal gold deposit clusters.
These magmas and fluids may have been derived directly from the parental chamber or via additional segregation in a shallower staging chamber.
• This period of quiescence was terminated by the release of differentiated, gaseous magma that had built up in the deep chamber during the hiatus, resulting in the deposition of the voluminous ignimbrites and crystal tuffs of the Colorado Pyroclastics at ~12.63 Ma.
• The parental magma chamber continued to differentiate, most likely fed by repeated pulses of basaltic or andesitic melts from the base of the crust. Escape of further magma from the chamber roof ensued, but this time from its central to eastern sections, to produce the 12.4 to 11.9 Ma intrusive Yanacocha porphyries and 12.1 to 11.6 Ma Azufre Andesite extrusions. The slight decrease in SiO2 content within these units probably reflects addition of fresh, undifferentiated melt from depth into the parental magma chamber, possibly also triggering this new phase of activity. The waning stages of shallow magmatism was accompanied by the upward passage of fluids responsible for the 11.46±0.15 to 11.41±0.89 Ma 'Stage 2' quartz-alunite alteration through the same sections of the Yanacocha trend, accompanied by finely disseminated Au-bearing pyrite.
• Renewed explosive release of large volumes of accumulated gaseous magma from the parental chamber followed, to be deposited as the 11.45 to 11.25 Ma San Jose (or Shacsha) Ignimbrite, with no significant accompanying alteration or mineralisation.
• The final multiple pulses of magma release was predominantly from the central section of the parental chamber to produce the lavas, flow domes, intrusions and finally pyroclastics that together constitute the Coriwachay Dacite-Rhyolite Magmatism between 11.1 and 8.2 Ma. This phase of magmatism was much less intense and more siliceous, reflecting both continued segregation and assimilation of country rock.
- The first pulse of this low volume, more siliceous magmatism produced the 10.78±0.05 Ma Corimayo dacitic flow domes at and near surface, and at a depth, an inferred felsic intrusion that was the focus of the Kupfertal porphyry Au-Cu mineralisation. A number of other similar mineralised porphyry systems are indicated at depth along the Yanacocha mineralised corridor, below or adjacent to epithermal gold deposits, while exposed porphyry Au-Cu deposits form the Minas Conga cluster, ~15 km to the NW along the same structural trend. The Kupfertal intrusion was emplaced into the Cretaceous 'basement' sedimentary sequence at a palaeo-depth most likely between ~2 and ~4 km (e.g., Sillitoe, 2010), surrounded by a >500 m halo of chalcopyrite-pyrite mineralised veins, fractures and disseminations, within the potassic, and later phyllic alteration of the Cretaceous sedimentary and overlying Miocene andesitic country-rocks.
- However, while the bulk of the porphyry system remained sufficiently confined to allow the development of a low grade porphyry Au-Cu deposit, the outermost front of this mineralised halo progressed upward, with time, to penetrate into the basal Colorado Pyroclastics, allowing hydrothermal fluids to leak into the porous ignimbrites. The released hydrothermal fluid migrated upward along this unit to form an extensive, tabular, advanced argillic pyrophyllite-alunite-quartz lithocap, the 11.0 to 10.7 Ma 'Stage 3' alteration, accompanied by a high sulphidation, pyrite enargite and covellite assemblage, that extended for as much as ~4 km west to the Corimayo deposit. At shallower depths, these fluids precipitated the main stage high sulphidation Au mineralisation in many of the deposits, characterised by fine-grained pyrite with gold and minor enargite and covellite. The hydrothermal fluids responsible for this mineralisation
- The next pulse of magma formed the shallower ~9.9 Ma Cerro Yanacocha dacitic porphyry intrusions which penetrated into the ignimbrite members, accompanied by hydrothermal fluids responsible for the 10.3 to 9.9 Ma 'Stage 4' quartz-alunite alteration and the intermediate sulphidation high grade gold pulse, recognised by the occurrence of coarse gold associated with late-stage barite and/or creamy chalcedony.
- Phreatomagmatic/hydrothermal brecciation was subsequently developed, spatially associated with the Cerro Yanacocha dacitic porphyry intrusions, providing a pathway for the ingress of a late pulse of hydrothermal fluids from the parental chamber. This fluid precipitated a Cu-Au enargite-covellite-gold bearing pyrite phase over earlier mineralisation and alteration at a number of deposits, most intensely in the centre of the Yanacocha trend. Mineralisation of this pulse is closely associated with the 9.12±0.32 Ma 'Stage 5A' quartz-alunite alteration, and was responsible for ~30% of the gold endowment at Yanacocha.
- The final hypogene mineralised pulse was associated with the 8.8 to 8.2 Ma 'Stage 5B' alunite alteration, coeval with the intrusive late 8.59±0.14 Ma Yanacocha Lake rhyolites, and comprised sparsely distributed veinlets of rhodochrosite-dolomite and base metal sulphides.
Progressive thermal decline, and/or uplift and erosion during the life of the magmatic/hydrothermal system, resulted in the overprinting (telescoping) of the lithocap and then the younger advanced argillic alteration zones onto the porphyry Au-Cu mineralisation at Kupfertal.
• Glaciation during the Pleistocene scoured the upper sections of the hypogene Cerro Yanacocha mineralised system and deposited the detritus en masse as moraines and winnowed periglacial gravels in the La Quinua and La Pajuela basins to produce the large, unconsolidated, La Quinua deposits.
• Supergene oxidation and leaching to depths of between 50 to 300 m was responsible for supergene oxidation of sulphides, principally covellite-enargite±trace chalcopyrite, leaching of copper from the weathering profile to form a copper depleted capping of iron oxides and remnant liberated gold, and the formation of copper enrichment sulphide blankets of covellite-chalcocite intermixed with hypogene enargite below the base of oxidation.
Exploration Criteria - Loayza and Reyes (2010) suggest favourable exploration criteria and ore control factors include:
• a favourable permeable pyroclastic unit to permit the passage of hydrothermal fluids and to act as a host;
• multiple pulses of brecciation and intrusion accompanying and facilitating the ingress of hydrothermal fluids;
• the presence of structural traps beneath and/or marginal to flow dome complexes, diatremes and late-stage high level intrusives;
• structural traps include overlying fresh, impervious andesite units and/or flow domes with ore in silicified rocks hosted by pyroclastic units of the Yanacocha Pyroclastic Sequence, sandwiched between the Upper and Lower Andesite sequences.
Reserves, Resources and Production
The Yanacocha volcanic hosted oxide ore, which is porous, is ideal for heap leaching and does not require crushing.
The estimated oxide plus sulphide drill-indicated gold endowment of the Yanacocha district is >2200 tonnes (Teal and Benevides, 2010).
The combined reserves, resources and production in the district have been estimated to total >1500 t of gold in oxidised ore from >20 hard rock high-sulphidation epithermal deposits and the two glacial gravel fan deposits at La Quinua (Teal and Benevides, 2010). These deposits overlie a large, hypogene high sulphidation gold and copper sulphide resource, and multiple Cu-Au porphyry systems (Myers and Williams, 2000; Teal et al., 2002; Bell et al., 2004).
From the commencement of mining in 1993 until the end of 2009, the total production of gold had exceeded 1180 tonnes (Longo et al., 2010).
The published reserve at Yanacocha in Dec 2000 was of the order of 1135 t (36.6 Moz) of contained gold at grades of 0.9 to 1.6 g/t Au. Production in 2000 amounted to 55 t (1.8 Moz) Au, extractable at a cash cost of $US 88/oz. Single-year production peaked in 2005 at 103.57 t (3.33 Moz) of gold.
Published remaining attributable ore reserves and mineral resources at Yanacocha, including La Quinua, at 31 December 2015 for [Newmont's 51.35% share] of the deposit (Newmont reserve and resource report, 2016), and the totals were:
Proved + probable reserves - [121.2] 236 Mt @ 0.67 g/t Au for  158 t of contained gold.
Measured + indicated resources - [20.9] 40.7 Mt @ 0.50 g/t Au for [10.25] 20.35 t of contained gold.
Inferred resources - [88.6] 172.5 Mt @ 0.85 g/t Au for  147 t of contained gold.
The mine exploiting this cluster of deposits is operated by Minera Yanacocha, which is 51.35% owned by Newmont Gold Company and 44% by Minas Buenaventura S.A.A.
The most recent source geological information used to prepare this summary was dated: 2011.
Record last updated: 3/6/2016
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.
Anonymous 2001 - Minera Yanacocha: in Extract from Newmont web site http://www.newmont.com 1p|
Bell P D, Gomez J G, Loayza C E and Pinto R M, 2004 - Geology of the gold deposits of the Yanacocha District, Northern Peru: in Hi Tech and World Competitive Mineral Success Stories Around the Pacific Rim Proc. Pacrim 2004 Conference, Adelaide, 19-22 September, 2004, AusIMM, Melbourne, pp 105-113|
Bissig, T., Clark, A.H., Rainbow, A. and Montgomery, A., 2015 - Physiographic and tectonic settings of high-sulfidation epithermal gold-silver deposits of the Andes and their controls on mineralizing processes: in Ore Geology Reviews v.65, pp. 327-364.|
Deditius, A.P., Utsunomiya, S., Sanchez-Alfaro, P., Reich, M., Ewing, R.C. and Kesler, S.E., 2015 - Constraints on Hf and Zr mobility in high-sulfidation epithermal systems: formation of kosnarite, KZr2(PO4)3, in the Chaquicocha gold deposit, Yanacocha district, Peru: in Mineralium Deposita v.50 pp. 391-428|
Gustafson, L.B., Vidal, C.E., Pinto, R. and Noble, D.C., 2004 - Porphyry-epithermal transition, Cajamarca region, northern Peru: in Sillitoe, R.H., Perello, J. and Vidal, C.E., 2004 Andean metallogeny: new discoveries, concepts, and updates Society of Economic Geologists, Denver, Special Publication 11, Ch. 15, pp. 279-299.|
Harvey B A, Myers S A and Klein T, 1999 - Yanacocha gold district, northern Peru: in Weber G B (Ed.), Pacrim 99, International Congress on Earth Science, Exploration and Mining Around the Pacific Rim AusIMM, Melbourne pp. 445-459|
Loayza, C. and Reyes, J., 2010 - Geologic Overview of the Yanacocha Mining District, Cajamarca, Northern Peru: in SIMEXMIN 2010 - IV Brazilian Symposium of Mineral Exploration, Ouro Preto, Minas Gerais - Brazil, May 23rd to 26th, 2010, pdf from www.adimb.com.br/simexmin2010/palestras, 24p. (landscape slideshow)|
Longo A A, Dilles J H, Grunder A L and Duncan R, 2010 - Evolution of Calc-Alkaline Volcanism and Associated Hydrothermal Gold Deposits at Yanacocha, Peru : in Econ. Geol. v.105 pp. 1191-1241|
Longo, A.A. and Teal, L., 2005 - A summary of the volcanic stratigraphy and the geochronology of magmatism and hydrothermal activity in the Yanacocha Gold District, northern Peru,: in Rhoden, H.N., Steininger, R.C. and Vikre, P.G., eds., Geological Society of Nevada Symposium 2005: Window to the World, Reno, Nevada, May 2005, pp. 797-808.|
Mallette P M, Rojas R E and Gutierrez A R, 2004 - Geology, mineralization and genesis of the La Quinua gold deposit, Yanacocha District, Northern Peru: in Sillitoe R H, Perello J and Vidal C E 2004 Andean Metallogeny: New Discoveries, Concepts and Updates SEG Special Publication 11 pp 301-312|
Pilco, R., 2011 - Hypogene alteration, sulfide mineralogy, and metal distribution at Cerro Yanacocha high-sulfidation epithermal deposit, northern Peru: in A manuscript submitted in partial fulfillment of the requirements for the Degree of Professional Science Masters in Economic Geology, Faculty of the Department of Geosciences, in the Graduate College of the University of Arizona, 89p.|
Sillitoe R H 1995 - Yanacocha, Peru: in Sillitoe R H, 1995 Exploration and Discovery of Base- and Precious-Metal Deposits in the Circum-Pacific Region During the Last 25 Years Metal Mining Agency of Japan pp 64-66|
Teal, L. and Benavides, A., 2010 - History and Geologic Overview of the Yanacocha Mining District, Cajamarca, Peru: in Econ. Geol. v.105, pp. 1173-1190.|
Turner S J T, 1997 - The Yanacocha epithermal gold deposits, northern Peru: High-sulfidation mineralization in a flow dome setting: in http://www.dregs.org/abs997.html 1p|
Turner S J, 1999 - Settings and styles of high-sulphidation gold deposits in the Cajamarca region, northern Peru: in Weber G B (Ed.), Pacrim 99, International Congress on Earth Science, Exploration and Mining Around the Pacific Rim AusIMM, Melbourne pp. 461-468|
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