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La Zanja |
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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 La Zanja deposit is at an altitude between 2500 and 3600 m above sea level, located ~55 km NW of the city of Cajamarca, ~45 km NW of the Yanacocha cluster of deposits, and 15 km NW of the Sipán deposit. It predominantly comprises high sulphidation gold-silver mineralisation in the San Pedro and Pampa Verde orebodies (#Location: 6° 49' 40"S, 78° 54' 1"W).
Initital exploration in 1991 by Buenaventura Ingenieros S.A. recognised a zone of hydrothermal alteration discernable on satellite images in a volcanic environment considered to be favourable to the occurrence of mineralisation. Between 1993 and 1998, Newmont Perú Limited undertook exploration that resulted in the identification of two areas, San Pedro Sur and Pampa Verde, with strong gold anomalies. Subsequently, using the Newmont data, from 1999 to 2003, Compañía de Minas Buenaventura S.A. focused on detailed testing of these nomalies (Knight and Piesold Consult., 2004). Two separate pits have been developed to exploit these deposits. The ore is treated by heap leaching and the processing of the enriched solution via an adsorption/desorption/regeneration System (ADR) with activated charcoal for the recovery silver and gold in Doré through a final casting process (Cabos, 2007).
The deposit has been mined since 2010 as a distant satellite of Yanacocha by Minera La Zanja S.R.L., a joint venture between Newmont Mining Corp. [46.94%] and Compania de Minas Buenaventura S.A.A. [53.06%].
Regional and District Setting
La Zanja is one of a number of epithermal and porphyry style deposits that define a broad halo around the Yanacocha cluster.
For details of the regional setting, see the separate Peruvian Andes record, and for the district setting, the Yanacocha, Kupfertal, La Quinua record.
Geology
The Zanja deposit is made up of the San Pedro Sur and Pampa Verde orebodies, hosted by a suite of Cenozoic volcanic rocks that overlie a Cretaceouss succession of clastic and carbonate rocks. The genesis of these deposits is intimately linked to the development of Cenozoic volcanic-magmatic system.
The volcanic complex in the La Zanja deposit area is composed of 3 volcanic centres interpreted to be equivalent to the Porculla Formation. These centres comprise the La Zanja caldera to the NW, and the Hueco Grande and San Miguel stratovolcanoes to the east and south respectively (Cabos, 2007). These volcanic centres overlap pyroclastic, lava and volcaniclastic levels belonging to the Llama Formation. The Llama and Porculla Formations are of Eocene to Miocene age and together constitute the Lower Calipuy Group.
Cretaceous clastic and carbonate sedimentary rocks of the strongly folded and faulted Goyllarisquizga Group discordantly underlie the Calipuy Group sequences and emerge in the northeast sector of the area. The sedimentary sequence was intruded by Upper Cretaceous to Lower Cenozoic granitioritic to granitic stocks contemporaneous with the coastal batholith. These intrusives are predominantly found in the southern sector of the Yanacocha district.
Alteration and Mineralisation
The dominant alteration systems have been developed in the first two volcanic centres, La Zanja and Hueco Grande. In the latter, a system of narrow radial structures occur to the northwest, occurring as strong silicification accompanied by quartz-pyrophyllite-illite-(alunite) alteration towards the flanks. Alunite has been dated at 15.6±0.12 Ma (Noble et al., 2004). These structures have lengths of hundreds of metres and widths that range from metres to tens of metres, and contain anomalous geochemical values in gold, arsenic and mercury. The dacitic dome of Cerro Chicche, on whose northeastern flank the Sipán deposit is located, appears to be part of a system that predates Hueco Grande, since it partially covers it (Cabos, 2007). Rocks from the dome have been dated at 11.9±0.06 Ma (Noble et al., 2004).
La Zanja caldera is the most important morpho-structural feature of the area, with internal and external diameters of 4 and >12 km respectively, and and open northwest flank. An epithermal alteration system has been developed in the internal margin of the caldera, predominantly controlled by northwesterly structures, in the areas of Cerro San Pedro, Cerro Pampa Verde, Cerro La Zanja and Cerro Chinchimal. These structures are enriched in silica passing into a quartz-alunite-dickite-kaolinite alteration fringe, with an outer assemblage of pyrophyllite-illite-kaolinite in the deepest and most distal parts of the structures. In the crests of acidic domes, low-temperature silica (opallocacedonia) has been emplaced (Tanabe and Turner, 2000). Mineralisation occurs both within the structures and as disseminations in neighbouring rocks.
Induced polarisation data clearly marks the deep contacts between siliceous and argillic alteration. Rock geochemistry of reveals a strong correlation between anomalous Au-Ag-Cu values and the presence of silicification and advanced argillic alteration(Tanabe and Turner, 2000).
The exploited deposit predominantly comprises oxidised high sulphidation gold-silver mineralisation on dome margins in the San Pedro and Pampa Verde orebodies. Several peripheral low and intermediate sulphidation veins have also been recognised, and it is part of a larger copper-molybdenum-gold porphyry-type system.
La Zanja and Sipán, 15 km to the SE, differ from Yanacocha in that they are structurally controlled and caldera related, whereas mineralisation at Yanacocha has a greater reliance on brecciation, lithological control and resurgent magmatic-hydrothermal activity (Turner, 1999).
Production, Reserves and Resources
The gold reserves from both disseminated and structurally controlled bodies in the San Pedro Sur and Pampa Verde bodies in 2007 were estimated to contain (Cabos, 2007 - Potencial Minero de la Región Cajamarca, Asociación Los Andes De Cajamarca):
17.5 Mt @ 0.9 g/t Au, 5.5 g/t Ag for 15.5 t Au and 95 t Ag.
Production commenced in 2010 at a rate of ~3 to 4 t of gold per annum.
At December 31, 2016 remaining Ore Reserves and Mineral Resources were:
Proved+Probable reserves - 15.116 Mt @ 0.53 g/t Au, 8.6 g/t Ag, plus
Measured+Indicated+Inferred Resources of 13.14 Mt @ 0.83 g/t Au, 8.75 g/t Ag.
The most recent source geological information used to prepare this decription was dated: 2007.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
La Zanja - San Pedro
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Selected References:
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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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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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