GEOLOGIC SETTING AND CHARACTERISTICS OF THE VOLCANIC-HOSTED EXPLORADOR QUARTZ-AG-AU VEIN SYSTEM, SELENE MINING DISTRICT, APURIMAC, SOUTHERN PERU

The Selene mining district, located in the Huanzo Cordillera of Apurimac, Peru, is mostly within a prominent semi-circular feature of 5-6 km diameter, interpreted as a late Tertiary caldera. The Selene district comprises low sulphidation veins as well as high sulphidation epithermal features with pervasive advanced argillic alteration.

Rhyodacitic pyroclastic units of partially to non-welded ignimbrite and ash fall tuff dominate the exposure outside the caldera margin. Andesitic to dacitic lava flows and epiclastic units are exposed preferentially within the ring zone, and are interpreted to represent a higher stratigraphic level and later stages of volcanic activity, indicating a downward movement of the caldera roof. Resurgent domes and flow-dome complexes of rhyodacitic to rhyolitic composition intruded the volcanic strata, and are exposed preferentially in the center of, and along the ring zone of, the caldera.

The Explorador vein system is located parallel to a segment of the caldera margin. Veins are controlled by steeply dipping strike-slip faults striking 050° to 055°. The principal vein being mined has a strike length of ~1,500 m and an average thickness of 1.3 m (maximum ~4 to 5 m). It is a low- to intermediate sulphidation Ag-Au vein, showing banded quartz with sulfide-rich ginguro bands, lattice textured quartz replacing bladed carbonate, massive milky quartz, as well as several syn- to post-mineral breccia textures. Sulfide minerals include pyrite, marcasite, silver-sulfosalts, ruby silver, and locally, traces of chalcopyrite. Gangue minerals are dominated by several species of quartz; adularia is present in trace amounts.