Geology, geochronology and structural reconstruction of the Cerro Bayo epithermal district, Chilean Patagonia
Jaime A. Poblete, MSc, 2011
The Cerro Bayo low-sulfidation epithermal district is located in the Aysén region, Chilean Patagonia. The oldest rocks that crop out in the district correspond to mainly rhyolitic fragmental successions of the Jurassic Ibáñez Formation which erupted based on biotite ⁴°Ar/³⁹Ar and zircon U-Pb ages, between ca. 154 and 144 Ma. The veins at Cerro Bayo are spatially associated with N-S aligned rhyolitic domes dated at 146.50±0.21 and 146.3±0.2 Ma (zircon U-Pb), intruding the volcanic rocks. Dacitic domes dated at 83.0±0.21 and 82.6±0.2 Ma (zircon U-Pb) are present to the west of the district at Laguna Verde. Silver and gold mineralization is hosted by steeply dipping N-S to NW trending quartz veins, that were emplaced in three main episodes based on adularia ⁴°Ar/³⁹Ar ages: (1) Mallines: ca. 144-142; (2) Bahía Jara and Brillantes: ca .137-124 Ma; and (3) Laguna Verde: 114-111 Ma. Clay alteration mineralogy around the veins consists of illite±smectite±kaolinite–chlorite. At Mallines the illite+kaolinite–smectite mineral assemblage combined with the high illite crystallinity index indicates that these veins are not deeply eroded and the surface outcrops correspond to high levels within the low-sulfidation epithermal deposit model. At Bahia Jara the illite ± smectite – kaolinite and at Brillantes the illite–chlorite–kaolinite assemblages, and overall lower illite crystallinity indices indicate that the veins in these areas are more deeply eroded than at Mallines. Illite crystallinity index is lower for Brillantes than at Bahía Jara indicating that the level of exposure at Brillantes is deeper than at Bahía Jara. At Laguna Verde the mainly illite – smectite clay alteration assemblage plus the lowest illite crystallinity indices of the entire district indicate higher temperature of formation and, by inference, the most deeply eroded veins of the entire district. The level of exposure of the veins has no relation with age of mineralization and the veins suffered differential degree of uplift and erosion after the Late Cretaceous. Moreover, glacial erosion has not uniformly affected the area. The observed textures and clay alteration assemblages at Mallines indicate important potential at 150–250 m depth if classic published deposit models are considered.