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Baumgartner R., Fontboté L., Jobin Y., 2002. Carbonate hosted zinc-lead high-sulfidation mineralization at the Cerro de Pasco deposit, Peru. Abstract volume. XI Congreso Peruano de Geología, Lima, Resúmenes p.75

Carbonate hosted zinc-lead high-sulfidation mineralization at the Cerro de Pasco deposit, Peru.

Regina Baumgartner1, Lluís Fontboté1, Yves Jobin1

1Department of Mineralogy, University of Geneva, Switzerland

Einaudi (1977) distinguished two main types of sphalerite on the Cerro de Pasco polymetallic deposit (central Peru) on the basis of their FeS content: "sphalerite-2" in the range of 15 to 30 mole percent (average 21%) and "sphalerite-3" between 0 and 10 mole percent (average 3%). Sphalerite-2, which is spatially related to pyrrhotite pipes, was reported as the most abundant, and interpreted to have formed at temperatures around 250-300°C and moderate sulfur fugacity whereas sphalerite-3 was less abundant and formed at temperatures around 200-250°C and high sulfur and oxygen fugacities.

Recent development at the exploitation of the Cerro de Pasco deposit shows that sphalerite-3 type ores are much more abundant than previously assumed. Ore bodies at the eastern part of the open pit, including those at Diamante, Cuerpo Nuevo and Manto V are characterized by partly very fine sphalerite-3 (very poor in Fe, typical values in the range of 0.4-0.7 mol% Fe), alteration assemblages indicative of high sulfidation environment, and marked zoning. Well zoned orebodies show inner cores relatively rich in Cu which grade outwards to pyritic zones rich in Bi-Ag. Zn-Pb ores (sphalerite-3 and galena) occur predominantly in the external parts. Some ore bodies do not develop well the internal Cu and Bi-Ag rich zones. In addition to the mentioned sulfides, the alteration assemblages include mainly quartz, kaolinite, alunite group minerals (including svanbergite and hinsdalite), and barite. In a more marginal position occur magnetite and hematite, as well as, usually directly adjacent to the Pucará-host carbonates, Mn-Fe-Zn carbonates. These characteristics, including the presence of minerals of the alunite group and of Fe-poor sphalerite, are similar to those of the "sulfide rock" described by Fontboté and Bendezú (1999, 2001) and Bendezú and Fontboté (2001) at San Gregorio and Colquijirca, where they are interpreted as high sulfidation cordilleran base metal replacements (Bendezú and Fontboté, this volume).

"Sulfide rock" bodies at Cerro de Pasco are structurally controlled along three main directions (N45°E, N130°E and N170°E) and is also recognized in areas outside the present open pit. The fine-grained character of the "sulfide rock" and its structural control explain that sometimes it has not been recognized as such and instead it has been described as "panizo".

Further, it has been observed at Cerro de Pasco that coarse grained sphalerite-3 occurs also in the central part of the deposit, in part overprinting iron-rich sphalerite-2 ore bodies (e.g. N9860, E8920, level 1200). The common association of sphalerite-3 with kaolinite allows the easy field distinction of both sphalerite generations. Veins of "sulfide rock" also crosscut the silica-pyrite bodies on the deepest western part of the open-pit.

Our working hypothesis is to genetically link the sphalerite-3 ore stage (largely of high sulfidation character) to the same stage that forms the W-E trending high sulfidation enargite-bearing veins observed at the western part of the open pit and which cut the silica-pyrite body. Some of these veins, apparently mainly the southern ones emplaced in the Excelsior phyllites (Ward, 1961), are particularly rich in Au. To this stage would also belong the alunite occurring within silica-pyrite bodies in the western face of the open pit nearby enargite veins (e.g. N10275-E8860). The presence of E-W veins bearing "sulfide rock" in the west-central part of the open pit (N9940-E9000) is consistent with this hypothesis which is also supported by descriptions of Ward (1961) reporting that some veins were mined for enargite on their deeper levels and for galena-sphalerite in their upper portions. It appears that at Cerro de Pasco, similarly as in the Colquijirca District, a hydrothermal stage characterized by high sulfur and oxygen fugacities (high sulfidation) and relatively low temperatures is responsible for zoned Cu (e.g. enargite) – Zn-Pb mineralization. In both places the sphalerite of this stage is Fe-poor. At Colquijirca this stage postdates by about 600.000 years high sulfidation disseminated gold mineralization (Marcapunta) (Bendezú and Fontboté, 2002). At Cerro de Pasco, this stage clearly postdates the silica-pyrite and iron rich, higher temperature sphalerite-2 bodies. The temporal relationship with high sulfidation disseminated gold mineralization (e.g. Venenococha) has not been established yet.

The present investigation is being carried out with the support of the Swiss National Science Foundation (FN 2000-062000.00). Thanks to Ronner Bendezú for his support in the field.

 

Bendezú, R. and Fontboté, L., 2000, Alunite-Kaolinite alteration in carbonate-hosted Zn-Pb±Ag mineralization at Colquijirca and San Gregorio (central Peru) as product of a Cu-Au high sulfidation epithermal system. Geol. Soc. of Am., November 2000 Annual Meeting, GSA Abstracts with Programs, Vol. 32, No. 7, p. A-280.

Bendezú, R. and Fontboté, L., 2002, Au(Ag) epithermal and “cordilleran base metal lodes and replacement bodies” two different high sulfidation mineralization types. Examples from the Colquijirca District, central Peru. .Congreso Geológico Peruano, Sept. 2002, abstracts volume.

Bendezú, R., and Fontboté, L., 2002, Spatial and temporal relations between “cordilleran base metal lodes and replacement bodies” and precious metal high sulfidation epithermal mineralization in the Colquijirca district , central Peru. Global exploration 2002: Integrated methods for discovery, E.E. Marsh and R.J. Goldfarb (eds), p. 63-64.

Einaudi, M.T. 1977., Environment of ore deposition at Cerro de Pasco, Peru. Econ. Geol. 72: 893-924.

Fontboté, L and Bendezú, R., 1999, The carbonate hosted Zn-Pb San Gregorio deposit, Colquijirca District, central Peru, as part of a high sulfidation epithermal system, in Stanley et al., (eds.), Fifth Biennial SGA Meeting, Mineral Deposits: Processes to Processing, v. 1, p. 495-498.

Fontboté, L. & Bendezú, R., 2001, The carbonate-hosted San Gregorio and Colquijirca (Zn-Pb-Ag) deposits (central Peru) as products of an epithermal high sulfidation system. ProExplo 2001, Lima, Perú, Abril 2001, CD-ROM, doc. 18 p.

Ward, H.J. 1961, The pyrite body and copper orebodies Cerro de Pasco mine, Central Peru. Econ. Geol. 56: 402-422.

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