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Pierina |
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Peru |
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Au Ag
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Super Porphyry Cu and Au
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The Pierina Au-Ag ore deposit in the Cordillera Negra, in the Department of Ancash in north-central Peru, ~10 kilometers NW of the city of Huaraz, at an altitude of ~4100 m (#Location: 9° 27' 27"S, 77° 35' 18"W).
For details of the regional setting, see the separate Peruvian Andes Cu-Au Province record.
Pierina lies within the Cordillera Negra, which is made up of Jurassic to Cretaceous sedimentary rocks overlain by Eocene to lower Miocene andesites, dacites and rhyodacites. These are cut by a late Tertiary (16.0 to 2.7 Ma) granodioritic batholith. A 70 km belt of alteration in the Cordillera Negra corresponds to a series of eruptive centres and Pb, Zn, Cu, Ag and Au deposits (including Pierina) localised by cross structures. This alteration zone directly correlates with a NNW trending Pliocene-Pleistocene graben filled with pumice and tuffs over an andesite basement. Pierina is a high sulphidation (acid sulphate) epithermal deposit hosted by Tertiary pyroclastics, and was the first of this type discovered in the district.
The hosts to ore are rhyodacitic pumice and lithic tuffs overlying andesitic lavas, while a quartz-feldspar porphyry intrudes on the southern margin and is believed to be either late stage of post mineralisation. The southern section of the graben contained the dome/vent from which the pyroclastics were erupted. There is a strong lithological control to the mineralisation and alteration at Pierina. Residual vuggy silica containing alunite is the main host to ore. This core zone is surrounded by alunite with minor pyrophyllite and dickite, which also locally hosts ore, in turn surrounded by an outer kaolinitic to illitic clay zone. The pumiceous tuffs are more strongly mineralised and altered, containing the bulk of the ore, although lesser mineralised pervasive quartz-alunite is also found in the overlying lithic tuffs. The basal andesites have undergone clay-pyrite alteration with veinlet mineralisation. A sulphide feeder zone has been intersected to the south.
Gold is disseminated in the vuggy-silica altered rhyodacite pumice, present as micron sized native gold grains associated with Fe-oxides, quartz and pyrite. More than 95% of the ore at Pierina is oxidised, which is regarded as both supergene and hypogene, with overall higher grades of up to 90 g/t Au. Remnant, overprinted kernels of pyrite-enargite-covellite-native sulphur are found locally. The pre-oxidation phases of quartz-alunite acid leaching created a porosity for pervasive pyrite-enargite-covellite, and quartz-pyrite-Au veinlet stages with associated lower 0.5 to 2 g/t grades. The final barite-Au veinlet phase contains up to 1.5 g/t Au. Hypogene mineralisation is dated at 14.9 Ma. The ore zone covers an area of 900 x 300 m.
As at December 1997, estimates of the pre-mining proved + probable reserves totalled 112.5 Mt @ 1.96 g/t Au for a total of 220 t of contained Au, with around 1750 t Ag. Peak production was in 2000 with 31 tonnes of gold produced over the 12 month period.
The ore was treated by heap leach processing (80% recovery) to produce 25.5 t (0.821 Moz) Au in 2000 at a cash cost of $US40 per oz. The life of mine cash cost is predicted at less than $US 90/oz. The waste:ore stripping ratio is 1.4:1.
The Pierina operation was controlled by Barrick Gold Corp. Mining commenced in November 1998. Closure activities were initiated in August 2013, but the mine continued to produce gold until late 2016, with an output from the heap leach in 2015 of 1.68 t of gold. Remediation and pit back fill continued into 2018.
The most recent source geological information used to prepare this decription was dated: 2001.
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.
Pierina
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Selected References:
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Benavides A, Vidal C E 1999 - Exploration and mineral discoveries in Peru: in Weber G B (Ed.), Pacrim 99, International Congress on Earth Science, Exploration and Mining Around the Pacific Rim AusIMM, Melbourne pp 187-195
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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.
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Fifarek, R.H. and Rye, R.O., 2005 - Stable-isotope geochemistry of the Pierina high-sulfidation Au-Ag deposit, Peru: influence of hydrodynamics on SO2-4 -H2S sulfur isotopic exchange in magmatic-steam and steam-heated environments: in Chemical Geology v.215, pp. 253-279
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Noble D C, Park-Li B, Henderson W B, Vidal C E 1997 - Hypogene oxidation and late deposition of precious metals in the Pierina high sulphidation deposit and other volcanic and sedimentary rock-hosted gold systems: in Congreso Peruano de Geologia, Resumenes Extendidos Soc. Geol. del Peru Vol Esp. 1(1997) pp 121-127
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Rainbow A, Kyser T K and Clark A H, 2006 - Isotopic evidence for microbial activity during supergene oxidation of a high-sulfidation epithermal Au-Ag deposit: in Geology v34 pp 269-272
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Volkert D F, McEwan C J A, Garay E 1998 - Pierina Au-Ag deposit, Cordillera Negra, north-central Peru: in Walton G, Jambor J (Eds), Pathways 98, Extended Abstracts Volume BC & Yukon Chamber of Mines, Vancouver & SEG, Colorado pp 33-35
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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