Cordilleran Epithermal Cu-Zn-Pb-(Au-Ag) Mineralization in the Colquijirca District, Central Peru: Deposit-Scale Mineralogical Patterns

Abstract

At Colquijirca, central Peru, a Miocene diatreme-dome complex is associated in space and time with several large epithermal polymetallic (Cu-Zn-Pb-Au-Ag) deposits of the Cordilleran class. Of these deposits, Smelter and Colquijirca, located in the northern sector of the district, are part of a continuously mineralized northsouth corridor that extends for nearly 4 km outward from the diatreme-dome complex and to depths of 1 km below surface. This corridor is zoned from Cu-(Au) ores in its inner parts (Smelter deposit) to peripheral Zn- Pb-(Ag) ores (Colquijirca deposit). The Smelter-Colquijirca corridor has undergone minor erosion, providing a good example of a nearly intact paleo-epithermal system of the Cordilleran class. This description of the hypogene mineralogical patterns of the Smelter-Colquijirca corridor leads to the proposal that they are the result of superimposition in time and space of three main stages. During an early quartzpyrite stage, in which basically no economic ore deposition occurred, carbonate rocks surrounding the Marcapunta diatreme-dome complex were replaced by quartz and pyrite. This was followed by the main ore stage, which was largely superimposed on the quartz-pyrite replacements and produced zonation of ore minerals and metals along much of the Smelter-Colquijirca corridor. The zoning from Cu ores to Zn-Pb ores is complex and comprises a number of distinct and well-defined zones that display abrupt or gradual interfaces between zones. From internal to external parts, these zones consist mainly of the following mineral associations and assemblages: (1) enargite ± (luzonite, pyrite, colusite, tennantite, goldfieldite, ferberite, gold-silver tellurides, bismuthinite, gold, alunite, zunyite, kaolinite, dickite, smectite, illite, sericite, quartz); (2) enargite ± (pyrite, quartz, bismuthinite, alunite, dickite, kaolinite, smectite, illite, sericite); (3) bornite ± (pyrite, quartz, alunite, dickite, kaolinite, barite); (4) tennantite, barite ± (dickite, kaolinite, chalcopyrite, Bi- and/or Ag-bearing minerals); (5) chalcopyrite, sphalerite, galena ± (pyrite, quartz, dickite, kaolinite, barite); (6) sphalerite, galena, pyrite ± (hematite, kaolinite, siderite, magnetite, marcasite); (7) Zn-bearing carbonate zone; and (8) a barren outer zone consisting mainly of calcite. The crosscutting relationships and zoning patterns indicate that during the main ore stage, the inner Cu zones progressively overprinted the Zn-Pb zones. Evidence for the subsequent contraction of the mineralization front is also recognized. A mainly fault-controlled late ore stage was the last recognized episode of mineralization in the Smelter-Colquijirca corridor, related to the deposition of chalcocite, digenite, and covellite as veinlets cutting and replacing the different Cu-bearing zones of the main ore stage; it is not economically important. The different mineral associations and assemblages provide insights into the chemistry of the hydrothermal fluids, particularly that of the main ore stage. The acidity and oxidation state of the main ore-stage fluids varied considerably prior to significant interaction with the host rock. This is best observed in the enargite-bearing zones in which most acidic fluids formed alunite-zunyite-enargite−bearing assemblages and less acidic fluids formed smectite-illite-enargite and muscovite-enargite−bearing assemblages. The common kaolinite-dickite-enargite assemblage was related to a fluid with intermediate character between these two end-member styles. The significant fluctuations in the acidity of the ore fluids in the central parts of the mineralizing system are interpreted to reflect mixing between variable amounts of acidic and oxidized magmatic vapor-derived fluids and less acidic low to moderate saline ore-forming fluids of magmatic origin.