Discovery, Development, and Production of a Blind Ag-Au Bonanza in the Calera Vein, Orcopampa District, Peru

Abstract

Epithermal precious metal veins of the adularia-sericite type in the Orcopampa district were worked during Spanish times and since 1965 by the Buenaventura Group. Ore is hosted by generally ENE- to NE-striking normal faults that cut Miocene volcanic rocks of silicic and intermediate composition. Early-produced ores contained silver as freibergite and had very high Ag/Au ratios. The potential importance of the largely buried Calera structure was recognized during district-scale geologic mapping in 1972. Exploration began in 1975 and ore was discovered by diamond drilling in 1978. A large tonnage of rich ore containing gold as well as silver was discovered in 1983. Reserves of 2,383,240 short tons with an average of 16.2 ounces Ag per short ton and 0.15 ounces Au per ton had been defined by 1986, mostly in one exceptionally large ore shoot. Ore tends to occur in strongly fractured rock at the junctions of footwall splits with the Calera vein. The presence of several ore-grade paragenetic stages contributed ...