Sedimentary Rock-Hosted Disseminated Precious Metal Mineralization at Purisima Concepcion, Yauricocha District, Central Peru

Sedimentary rock-hosted disseminated gold ores lithologically and chemically similar to those of Carlin-type deposits of the western United States are present in the Yauricocha district, central Peru. The Purisima Concepcion deposit is located in the core of a steeply plunging anticline several hundred meters beyond large pipe-shaped Cu-Zn-Pb-Ag-Au replacement orebodies in limestone bordering a late Miocene granodiorite stock. The central part of the stock is potassium-silicate altered and contains high-salinity fluid inclusions.

Mineralized impure limestone is generally decalcified and silicified and appears to have been carbonaceous. Original laminated bedding and other sedimentary structures are locally preserved. Characteristic minerals include quartz, rhodochrosite, pyrite, calcite, sericite, and barite. Pyrrhotite, arsenopyrite, plagionite (Pb5Sb8Si7), and altaite (PbTe) have been recognized locally. Although the chemical composition of the ores is generally similar to that of the Carlin deposit, rocks of Purisima Concepcion have higher Fe, Mn, Te, Ag, and Tl and lower Hg contents. Most of the Au-bearing rocks contain between 3 and 6 wt percent Fe, and Te («4 ppm) and Mn («4 wt %) are about two orders of magnitude higher than at Carlin. These elevated elemental concentrations are all epigenetic; there is no regional or local geologic or textural evidence for a syngenetic and/or diagenetic origin. The Ag/Au ratio of the ores is about 2.5. Gold correlates positively with Ag, As, Hg, Mo, Sb, Te, and Tl and Ag strongly with Mn. Porous, leached, but not silicified, beds tend to carry higher gold values than silicified rocks.

The presence of enargite, tetrahedrite, and high S copper sulfide minerals in the replacement ores and of pyrophyllite, diaspore, alunite, anhydrite, and high-temperature (^400°C), com-positionally complex, hypersaline fluid inclusions in altered rocks bordering enargite veins in the stock, and the intense dissolution of limestone during deposition of the polymetallic ore-bodies suggest that the fluids initially had low pH and high/o2 and/s2 values. The magmatic hydrothermal system may have been similar to that of Julcani and other enargite-tetrahedrite districts in Peru, with the differences in ore types at least in part reflecting the carbonate composition of the host rock at Yauricocha.

The elemental chemistry of the Purisima Concepcion ores can be qualitatively explained by a process of hydrothermal differentiation in which selective precipitation of base metal and Ag sulfides and sulfosalts depleted the fluids in these components and produced a decrease in Cu/Zn, Cu/Pb, As/Sb, base metal/precious metal, and Ag/Au ratios. The moderate pH and low/o2 and/s2 values implied by the mineral assemblage of Purisima Concepcion presumably reflect in part the presence of carbonaceous material.

Similar deposits can be expected elsewhere in the Western Cordillera of central and northern Peru where hydrothermal fluids related to Neogene stocks encountered impure carbonaceous limestone beds of the Pariatambo and Jumasha Formations and other units. Certain sedimentary rock-hosted, disseminated Au-Ag deposits elsewhere may have a similar genetic relation to porphyry, skarn, and/or limestone-replacement systems.