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The Tantahuatay 2 high sulphidation epithermal gold-silver deposit is exploited as an open pit mine, located in the Cajamarca Mineral Belt in the high Andes of northern Peru, at an altitude of between 3200 and 3800 m, ~35 km NNW of the Yanacocha mine, 6.5 km NW of Cerro Corona, 50 km NNW of Cajamarca and 650 km north of Lima (#Location: 6° 44' 21"S, 78° 41' 48"W).
Silver was discovered in the Hualgayoc-Tantahuatay district in 1771, during Spanish Colonial times, and became one of the most important silver producers in Peru. The majority of the historic production came from silver-rich veins in the Miocene Cerro Jesus and Cerro San Jose flow domes, whilst most of the more recent production has been from veins and replacement mantos in Mesozoic siltstone-carbonate units. The current open pit mine commenced operation in August 2011, operated by Minera Coimolache S.A., a joint venture between Compañía de Minas Buenaventura (40.1%), Southern Copper Corporation (44.2%) and Espro S.A.C. (15.7%) with production in 2013 of 4.44 t Au and 21.3 t Ag.
Regional and District Setting
For details of the regional setting, see the separate Peruvian Andes Cu-Au Province record.
The country rock in the ~15 x 5 km, NW-SE elongated Hualgayoc-Tantahuatay district comprises a thick Cretaceous succession of largely carbonate sedimentary rock that have been folded, with minor faulting, during several pulses of deformation that extended at least until the middle Miocene. During the Eocene, dioritic stocks and sills were intruded, and in the middle Miocene, a series of dacitic to andesitic domes and associated pyroclastic rocks were emplaced, along with subvolcanic stocks, sills and dykes (Gustafson et al., 2004).
Three main sulphide associations are recognised in the district: i). pyrite-sphaleIite-galena-chalcopyrite-tennantite-tetrahedrite; ii). pyrite-pyrargyrite-proustite-other minor sulphosalts-sphaleIite; iii). pyrite-enargite±covellite. The first two are associated with quartz-kaolinite pyrophyllite alteration (Vidal and Cabos, 1983), whilst the third occurs as veins and replacement bodies to the NW, cutting the San Miguel diorite and at the eastern edge of the Tantahuatay volcanic complex. In addition, the Tantahuatay andesitic volcanic complex hosts a number of high-sulphidation epithermal Au-Ag deposits, with advanced argillic alteration and small oxide gold resources, overlying extensive pyrite-enargite-covellite mineralisation.
The Cerro Corona porphyry Cu-Au deposit has been delineated near Hualgayoc in the SE of the district.
Two other porphyry copper-style quartz veining systems have been recognised in the district, namely: i). El Morino, an extensive, but very low grade, porphyry Cu-Au-Mo system, ~4 km WNW of Cerro Corona, which has moderately strong biotite-alkali feldspar alteration and early 'A-type' quartz veinlets, but low-intensity chalcopyrite-pyrite-magnetite mineralisation; ii). Quijote, a small exposure of intense quartz veining and low grade mineralisation, east of Cerro Corona and a few hundred metres SE of Hualgayoc village. A blind zone of quartz veining with anomalous Mo contents, local skarn, and bodies of hydrothermal breccia has been encountered at Manto Lora, NE of Quijote, but without potassic alteration or anomalous Cu values (Gustafson et al., 2004).
There is no clear evidence that the polymetallic veins and replacement mineralisation represented in the first two sulphide associations above, are linked to the porphyry systems. Nevertheless, Gustafson et al. (2004) suggest the multiple and variably mineralised intrusions and dome centres so closely associated with the polymetallic veins and mantos are indicative of a magmatic origin for all the mineralisation. In the southwestern part of the district, banded quartz veinlets suggest proximity to the top of a buried porphyry system, although Cu and Au values are very low. However, a possible porphyry-style system does occur below the Tantalmatay 2 gold-silver deposit in the northeastern part of the dome complex (see below).
Mineralisation in the district was apparently emplaced between 14.3 and 12.4 Ma (Macfarlane and Petersen, 1990; Macfarlane et al., 1994; Noble and McKee, 1999).
Small-volume pyroclastic andesitic to dacitic eruptions preceded and accompanied emplacement of a series of volcanic domes at Tantahuatay in the northwestern part of the Hualgayoc-Tantahuatay district. These domes are calc-alkaline, clinopyroxene-bearing, fine-grained hornblende- and plagioclase-phyric andesite. There is stratigraphic evidence to suggest the dome field migrated from SW to NE, controlled by NW and NE trending structures, as indicated by the geometry of the eroded complex (Gustafson et al., 2004).
At the northeastern end of the volcanic complex at Cerro Tantahuatay, the dome complex has undergone strong hydrothermal alteration and pyritisation. Alunite from the altered dome has been dated at 12.4±0.4 Ma, and biotite in a post-mineral dyke at 8.6±0.3 Ma (Noble and McKee, 1999), making the Tantahuatay dome field and associated mineralisation a little younger than the mineralisation at Hualgayoc in the SE (Gustafson et al., 2004).
Qualtz-alunite alteration is pervasive across most of the dome field, with multiple stages of brecciation, intense silicification, quartz-alunite-pyrophyllite-diaspore-zunyite alteration, and sulphide mineralisation being concentrated in individual dome centres. Glacial erosion has removed much of the oxidised capping, although sufficient residual oxide remains at two of these centres, La Cienaga and Tantahuatay, to delineate mineable resources. Tantahuatay is similar in many ways to Yanacocha, athough at the latter, there has been less erosion, and at the former there is less silicification relative to advanced argillic alteration (Gustafson et al., 2004).
Deposit Geology and Mineralisation
Tantahuatay 2 is hosted by the largest, best mineralised and most complex centre within the dome complex, characterised by intense silicification, advanced argillic alteration and brecciation. This alteration and brecciation completely obliterates igneous textures except in peripheral areas and where only the early stages of quartz-alunite alteration have occurred. All of the rocks within the mineralised dome are silicified, with zones of total silicification having textures range from massive to vuggy to sandy. The zones of massive to vuggy quartz invariably carry anomalous, but typically <0.1 g/t Au, unless there is superimposed brecciation and sulphide mineralisation (Gustafson et al., 2004).
Multiple, crosscutting stages of brecciation styles are recognised, including i). crackle breccia, with only minor matrix and rotation of fragments; ii). tectonic or fault breccia; iii). hydrothermal breccia, with mineralised fragmental matrix and polymict clasts; and iv). pebble breccia, with fragmental matrix and relatively rounded polymict clasts.
Brecciation is variably pervasive across much of the deposit area, with late-stage gold mineralisation being best developed in areas that are most strongly affected by multiple stages of silicification and brecciation. Below the base of oxidation, the strongest gold mineralisation is associated with pyrite, enargite and other sulphides. Multiple stages of alunite are also recognised (Gustafson et al., 2004).
The ore deposit is closely associated with zones of irregularly developed quartz-pyrophyllite-alunite within a broad 500 x >600 m block of quartz-pyrophyllite alteration, which is surrounded by quartz alunite to the north, south and west. Massive silica zones are found in close association with the gold mineralisation.
A substantial area of what Gustafson et al. (2004) refer to as 'gusano texture' within the quartz-pyrophyllite zone surrounds the main ore zone to the north, south and east, obliterating the original rock texture. This texture is characterised by segregation of soft, white patches of pyrophyllite, typically accompanied by minor diaspore and as much as 100% alunite, in a hard siliceous matrix. The siliceous matrix is hard and comprises granular quartz enclosing ~5 to 30% interstitial aluminosilicate minerals similar those of the soft patches that generally contain <10% quartz. At Tantahuatay 2, pervasive gusano-textured rock covers an area of at least 500 by 600 m and extends to a depth of at least 200 to 300 m. Lesser amounts are found in the other mineralised and advanced argillic-altered dome centres in the district, in places only occurring in structural zones several metres or less in width.
The gusano-textured rock is typically dense, containing disseminated pyrite and generally <0.1 g/t Au. Localised more intense sulphide mineralisation occurs within more brittle and more intensely silicified breccia and vein zones which cut the gusano-textured rock. This relatively late assemblage is mostly pyrite-enargite-covellite. Cu and Au values vary directly in proportion to the amount of enargite, which is by far the most abundant copper mineral. Enargite-pyrite alone rarely contains >0.2 g Au/Cu%, although some zones of semi-massive pyrite reach 0.4 g Au/Cu%. The highest grades of Au with g Au/Cu% ratios as much as >2, invariably occur where bornite, digenite, covellite and/or sphalerite, and rarely barite, are present with, and apparently overprinting, the enargite. These elevated intervals are typically in the same siliceous zones of brecciation and veining containing the most enargite (Gustafson et al., 2004).
Weak supergene chalcocite enrichment locally extends to depths of hundreds of metres in fracture zones. In the overlying oxide zone, the Au remains, with local enrichment very close to surface, whereas Cu has been strongly leached during surficial oxidation (Gustafson et al., 2004).
A larger 500 x 100 to 250 m area of anomalous >50 ppm Mo surrounds/overlaps the gold ore, occurring as 'smears' of fine-grained, anhedral molybdenite in pyritic advanced argillic-altered rock. Testing for a deep underlying porphyry deposit during the 1990s did not reveal any economic mineralisation, although below the Tertiary unconformity, skarn mineralisation comprising pale diopside, epidote and clay with minor pale sphalelite, galena and chalcopyrite was encountered in the limestone that hosts massive pyrite-enargite mineralisation a few hundred metres farther east (Paredes, 1981). In the dome complex, which only contains rare dykes, advanced argillic- to sericitic-altered volcanic rock and pyrite-enargite mineralisation with minor pyrite-bornite-chalcopyrite and pyrite-covellite were intersected at depths of 500 to 700 m. To the NE of the main deposit, a drill hole intersected 500 m containing granular quartz A veins with traces of minute chalcopyrite inclusions, although the bulk sulphides are pyritic, high-sulphidation assemblages with sharply diminishing grades of gold and copper at depth, within a zone dominated by overprinting advanced argillic and sericitic alteration (Gustafson et al., 2004).
Reserves and Resources
The mine commenced production in August 2011.
As of 31 December, 2015, ore reserves and mineral resources (Compañí de Minas Buenaventura 2015 Annual Report) were:
Total oxide ore reserves - 66.197 Mt @ 0.435 g/t Au, 7.154 g/t Ag (=28.6 t Au, 470 t Ag);
Measured + indicated oxide ore - 49.275 Mt @ 0.311 g/t Au, 7.46 g/t Ag (=14.2 t Au, 375 t Ag);
Inferred oxide ore - 11.400 Mt @ 0.271 g/t Au, 12.44 g/t Ag (=3.1 t Au, 144 t Ag);
Inferred sulphide ore - 290.679 Mt @ 0.310 g/t Au, 10.76 g/t Ag, 0.80% Cu (=90 t Au, 3128 t Ag, 2.313 Mt of Cu).
In 2016 the mine was operated by Minera Coimolache S.A., a joint venture between Compañía de Minas Buenaventura (40.1%), Southern Copper Corporation (44.2%) and Espro S.A.C. (15.7%).
The most recent source geological information used to prepare this summary was dated: 2004.
This description is a summary from published sources, the chief of which are listed below.
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