GEOLOGICAL SETTING AND CHARACTERISTICS OF VOLCANIC-HOSTED EPITHERMAL QUARTZ-AG-AU VEINS, SAN JOSE MINING DISTRICT, PATAGONIA, ARGENTINA
The 60,000 km2 Deseado Massif consists of Paleozoic basement unconformably overlain by an extensive sequence of andesitic to rhyolitic volcanic and volcanoclastic rocks of the Bajo Pobre and Chon Aike formations, formed during Late Triassic-Early Jurassic extension. These units are overlain by Cretaceous tuffs and siliciclastic rocks. The Mesozoic rocks are exposed in erosional windows in Tertiary flood basalts. The principal host rock in the San Jose district is the Bajo Pobre Formation. Economically mineralized veins are hosted dominantly in competent andesite flows, whereas less competent volcanoclastic rocks with silica-sericite alteration tend to host only poorly mineralized quartz veins.
Most exploration targets in the San Jose district are low sulfidation quartz veins accompanying sinistral-normal faults striking 330o-340o, and conjugate dextral faults striking ~300o. Dips average ~60°, mostly to the east. However, fault slickenlines rake from 0°-90° indicating that fault-veins range from pure strike slip to dip slip. Average vein thickness varies from 0.30m to 2.50m.
The Huevos Verdes vein system, the most important target in the district, is located in a mineralized belt extending 15 km north-to-south and crops out close to the contact of andesitic lava flows with underlying andesitic volcanoclastic rocks. The vein system consists of an array of sub-parallel veins striking N35°W; the principal vein is ~2000 m long, has various bends and jogs, and has an average width of 1.25m. Ore shoots with high gold and silver grades are present along the vein. Quartz vein textures include ginguro banding, lattice textured quartz replacement of bladed carbonate, sacharoidal massive quartz, and breccias. Argentite-pyrite ore is mainly associated with the dark ginguro bands.