Genesis of Silver-Rich Mineralization in the Yacumina Zone, SW Part of the Porphyry-Centered Miocene Polymetallic Morococha District, Central Peru

In the SW part of the Morococha district, located in the Miocene polymetallic belt of central Peru, two Ag-rich veins, Buenaventura and Galera, host the main part of the economic mineralization. The two structures are hosted by Aptian-Albian carbonate rocks, partially covered by rhyolitic volcanic breccias and tuffs, and intruded by a large Early-Miocene intrusion, crosscut by Middle-Miocene porphyries. The polymetallic mineralization in both veins is dominated by sphalerite and galena and contains various Ag-bearing Cu-Sb and Pb-Sb sulphosalts, including freibergite, polybasite, pearcite, geocronite, and jordanite, as well as native silver. The gangue consists mainly of Ca-Mg-Fe-Mn carbonates; quartz is rarely present. A fluid inclusion study suggests ore-formation in a near-surface environment (300-500 m paleo-depth) at moderate temperatures (250-270 degrees C) from low-salinity fluids, resulting from successive boiling events, accompanied by fluid mixing. A stable isotope study (O, C, S) supports a mixed magmatic-meteoric origin of the fluids and dominantly magmatic origin of the sulfur. The Buenaventura and Galera veins show all characteristics of late epithermal polymetallic veins, typical for the distal parts of porphyry-centered magmatic-hydrothermal systems.