Geologic evolution of the Caylloma epithermal vein district, southern Peru

The silver- and base metal-rich Caylloma epithermal district in the Tertiary volcanic belt of southern Peru has been worked intermittently since the Incaic period and has produced over 100 million ounces (Moz) of Ag. Intermediate-sulfidation mineralization is present in veins hosted by Miocene andesitic volcanic and volcaniclastic rocks, with minor ore in underlying folded Jurassic sedimentary basement. New 40Ar/39Ar dates give a host rock age of 20.30 +/- 0.11 Ma (andesitic volcanic matrix), a hydrothermal alteration age of 18.35 +/- 0.17 Ma (adularia in vein wall rock), and postmineralization ages of 11.8 +/- 0.8 and 12.25 +/- 0.07 Ma (sanidine and biotite from a rhyolite dome). Gangue minerals include quartz, calcite, rhodonite, rhodochrosite, pyrite and minor adularia, illite, barite, and helvite. Ore minerals include sphalerite, galena, chalcopyrite, and tetrahedrite. Hydrothermal alteration is pervasive in lava flows but weak and localized near veins in volcaniclastic rocks. Hydrothermal alteration types include silicification (quartz-adularia and quartz-illite) and propylitization (chlorite + calcite +/- illite). Banded veins show four stages of mineral precipitation: (1) early sugary quartz, chalcedony, pyrite; (2) manganese minerals; (3) quartz + sulfides; and (4) late calcite + quartz. Cyclic bands in the manganese stage (early sulfides, coarse- to medium-grained quartz, late rhodonite + calcite + chalcedony) are a few millimeters to 5 cm thick and form ore bands up to 1 m thick. The silver- and base metal-rich Caylloma epithermal district in the Tertiary volcanic belt of southern Peru has been worked intermittently since the Incaic period and has produced over 100 million ounces (Moz) of Ag. Intermediate-sulfidation mineralization is present in veins hosted by Miocene andesitic volcanic and volcaniclastic rocks, with minor ore in underlying folded Jurassic sedimentary basement. New 40Ar/39Ar dates give a host rock age of 20.30 +/- 0.11 Ma (andesitic volcanic matrix), a hydrothermal alteration age of 18.35 +/- 0.17 Ma (adularia in vein wall rock), and postmineralization ages of 11.8 +/- 0.8 and 12.25 +/- 0.07 Ma (sanidine and biotite from a rhyolite dome). Gangue minerals include quartz, calcite, rhodonite, rhodochrosite, pyrite and minor adularia, illite, barite, and helvite. Ore minerals include sphalerite, galena, chalcopyrite, and tetrahedrite. Hydrothermal alteration is pervasive in lava flows but weak and localized near veins in volcaniclastic rocks. Hydrothermal alteration types include silicification (quartz-adularia and quartz-illite) and propylitization (chlorite + calcite +/- illite). Banded veins show four stages of mineral precipitation: (1) early sugary quartz, chalcedony, pyrite; (2) manganese minerals; (3) quartz + sulfides; and (4) late calcite + quartz. Cyclic bands in the manganese stage (early sulfides, coarse- to medium-grained quartz, late rhodonite + calcite + chalcedony) are a few millimeters to 5 cm thick and form ore bands up to 1 m thick. The silver- and base metal-rich Caylloma epithermal district in the Tertiary volcanic belt of southern Peru has been worked intermittently since the Incaic period and has produced over 100 million ounces (Moz) of Ag. Intermediate-sulfidation mineralization is present in veins hosted by Miocene andesitic volcanic and volcaniclastic rocks, with minor ore in underlying folded Jurassic sedimentary basement. New 40Ar/39Ar dates give a host rock age of 20.30 +/- 0.11 Ma (andesitic volcanic matrix), a hydrothermal alteration age of 18.35 +/- 0.17 Ma (adularia in vein wall rock), and postmineralization ages of 11.8 +/- 0.8 and 12.25 +/- 0.07 Ma (sanidine and biotite from a rhyolite dome). Gangue minerals include quartz, calcite, rhodonite, rhodochrosite, pyrite and minor adularia, illite, barite, and helvite. Ore minerals include sphalerite, galena, chalcopyrite, and tetrahedrite. Hydrothermal alteration is pervasive in lava flows but weak and localized near veins in volcaniclastic rocks. Hydrothermal alteration types include silicification (quartz-adularia and quartz-illite) and propylitization (chlorite + calcite +/- illite). Banded veins show four stages of mineral precipitation: (1) early sugary quartz, chalcedony, pyrite; (2) manganese minerals; (3) quartz + sulfides; and (4) late calcite + quartz. Cyclic bands in the manganese stage (early sulfides, coarse- to medium-grained quartz, late rhodonite + calcite + chalcedony) are a few millimeters to 5 cm thick and form ore bands up to 1 m thick. The silver- and base metal-rich Caylloma epithermal district in the Tertiary volcanic belt of southern Peru has been worked intermittently since the Incaic period and has produced over 100 million ounces (Moz) of Ag. Intermediate-sulfidation mineralization is present in veins hosted by Miocene andesitic volcanic and volcaniclastic rocks, with minor ore in underlying folded Jurassic sedimentary basement. New 40Ar/39Ar dates give a host rock age of 20.30 +/- 0.11 Ma (andesitic volcanic matrix), a hydrothermal alteration age of 18.35 +/- 0.17 Ma (adularia in vein wall rock), and postmineralization ages of 11.8 +/- 0.8 and 12.25 +/- 0.07 Ma (sanidine and biotite from a rhyolite dome). Gangue minerals include quartz, calcite, rhodonite, rhodochrosite, pyrite and minor adularia, illite, barite, and helvite. Ore minerals include sphalerite, galena, chalcopyrite, and tetrahedrite. Hydrothermal alteration is pervasive in lava flows but weak and localized near veins in volcaniclastic rocks. Hydrothermal alteration types include silicification (quartz-adularia and quartz-illite) and propylitization (chlorite + calcite +/- illite). Banded veins show four stages of mineral precipitation: (1) early sugary quartz, chalcedony, pyrite; (2) manganese minerals; (3) quartz + sulfides; and (4) late calcite + quartz. Cyclic bands in the manganese stage (early sulfides, coarse- to medium-grained quartz, late rhodonite + calcite + chalcedony) are a few millimeters to 5 cm thick and form ore bands up to 1 m thick.