The Cerro Bayo epithermal Ag-Au district, Chilean Patagonia: 34 million years in the making

The Cerro Bayo epithermal district, located in the Chilean Patagonia, comprises over one hundred Ag-Au-rich quartz-adularia veins, with a historical production of 27.9 Moz Ag and 0.5 Moz Au. The veins were emplaced in the Upper Jurassic Ibanez Formation (biotite Ar-40/Ar-39 similar to 154-144 Ma), which were intruded by a series NS aligned rhyolitic domes (zircon U/Pb similar to 146 Ma), as well as trachytic domes (zircon U/Pb similar to 83-82 Ma). Mineralization consists of electrum, base-metal sulfides and silver sulfosalts in quartz +/- adularia gangue. Seven new adularia Ar-40/Ar-39 ages from veins reveal a protacted history of mineralization ages between similar to 144 Ma at Mallines, in the southeastern part of the district; to similar to 111 Ma at Laguna Verde, in the western part of the district. The new ages, together with published constraints, allow three mineralization episodes to be defined: (1) Mallines, similar to 145 to 142 Ma; (2) Bahia Jara, similar to 137 to 124 Ma; and (3) Laguna Verde, similar to 114 to 111 Ma; thus hydrothermal activity and epithermal mineralization occurred over 34 m.y. Veins from episode 1 may be related to mineralization events at the Deseado Massif, whereas mineralization from episodes 2 and 3 may be related to hydrothermal activity associated with the magmatism recorded in the Patagonian Batholith, which crops out similar to 60 km west of the Cerro Bayo district.