Abstract
The San José mining district in the northwestern Deseado Massif of southern Argentina produces silver and gold from intermediate sulfidation style epithermal veins.The district geology is characterized by a subhorizontal sequence of Jurassic volcanic rocks, dominated by volcanogenic agglomerates and lava flows of andesitic to basaltic andesitic composition of the Bajo Pobre Fm, that are locally overlain by felsic ignimbrites and air fall tuffs of the Chon Aike and La Matilde Fms. Several small hornblende-porphyritic subvolcanic stocks, as well as polymictic and matrix-supported breccias cross-cut the andesite sequence. Geochronology indicates that Jurassic volcanism was active in the district between 151.3 ± 0.7 and 144.7 ± 0.1 Ma, and mineralization formed from 151 to 141 Ma. More than 50% of the San José district is covered by post-Jurassic deposits including Cretaceous sedimentary and volcano-sedimentary rocks, Tertiary plateau basalts and modern periglacial gravel deposits.The Huevos Verdes-Frea vein cluster is the economic heart of the San José district and source of its production with more than 20 km of northwest-striking veins hosted by andesite lava of the Bajo Pobre Fm.West-northwest-striking veins (~N280° and ~N320°) display dextral strike-slip motion and contain ore shoots over strike lengths of up to 1200 m (Kospi, Odin, Frea, Ayelen). By contrast, north-northwest striking veins (>N320°) (Huevos Verdes) show sinistral strike-slip motion and host more discontinuous ore shoots, with strike length up to ~400 m separated by transverse syngenetic faults. Vein mineralization is characterized by three paragenetic stages. Pre-ore barren quartz and chalcedony is followed by the ore stage, consisting of mottled and crustiform quartz-sulfides and late sulfide-rich veinlets. Sulfide paragenesis shows repetitive cycles starting with pyrite, sphalerite, galena, chalcopyrite, minor bornite, and covellite, followed by precious metal-dominated pulses with electrum, acanthite/argentite, and rare silver sulfosalts. Gangue mineralogy is mainly characterized by quartz intergrown with minor adularia, illite, illite/smectite, and preferentially at deeper levels Fe-chlorite (±epidote), indicating precipitation from near-neutral pH hydrothermal fluids at temperatures of >220 °C. Kaolinite gangue accompanies illite/smectite with increasing presence towards higher levels and in later stages suggest fluid mixing with steam-heated fluids resulting in decreased fluid temperatures and pH towards shallower levels and over time.Other mineralized sectors of the district lack economic significance. The El Pluma-La Sorpresa vein cluster comprises narrow quartz veins and silicic ribs hosted by andesitic volcanic rocks of the Bajo Pobre Fm. with illite, illite/smectite and kaolinite alteration. Cerro Saavedra is an alteration system in the vent area of a partially eroded andesitic volcanic edifice with heterolithic explosion breccias where proximal residual quartz with vuggy texture is surrounded by a halo of quartz-alunite and distal illite alteration. At Saavedra Oeste a swarm of northwest-striking, short vein and breccia lenses is hosted by ignimbrites of the Chon Aike Fm. Hydrothermal alteration ranges from proximal silicification grading out to illite, illite/smectite and minor kaolinite.
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