The Cerro Bayo District, Chilean Patagonia: Late Jurassic to Cretaceous Magmatism and Protracted History of Epithermal Ag-Au Mineralization

The Cerro Bayo low sulfidation epithermal district is located in the Aysen region, Chilean Patagonia, at the western tip of the Deseado Massif epithermal silver-gold province. Mineralization is hosted in the mainly rhyolitic fragmental successions of the Jurassic Ibanez Formation. New biotite Ar-40/Ar-39 and zircon U-Pb ages together with published ages constrain the eruption of this unit to between 158 and 144 Ma. Rhyolitic domes and dikes; including Cerro Bayo proper, aligned on the prominent N-S-striking Cerro Bayo fault and intruding rocks of the Ibanez Formation were dated by U-Pb on zircon at 146.5 +/- 0.2 and 146.3 +/- 0.2 Ma. Dacitic domes of 83.0 +/- 0.2 and 82.6 +/- 0.2 Ma (U-Pb on zircon) are present in the western part of the district at Laguna Verde. Silver and gold mineralization is hosted by steeply dipping, dominantly N to NW and subordinate W-striking quartz veins and, to a lesser extent, in breccias, which, on the basis of new adularia Ar-40/Ar-39 ages, were emplaced in three main episodes: (1) Mallines: ca. 144 to 142 Ma, (2) Bahia Jara and Brillantes: ca. 137 to 124 Ma, and (3) Laguna Verde: 114 to 111 Ma, overall spanning 33 m.y. Most Ag-Au has been produced from veins emplaced in the latter two episodes. Veins at Mallines and Bahia Jara are spatially related to the Cerro Bayo fault. The oldest episode of mineralization in the district is similar in age to the youngest epithermal deposits in the Deseado Massif, whereas the economically most important veins are age equivalent to skarn and polymetallic vein mineralization in the Patagonian Andes of southern Chile. The protracted history of epithermal processes at Cerro Bayo records the evolution from continental-scale extension related to the Gondwana breakup to the establishment of Andean-type arc and back-arc environments. The largely extensional tectonics throughout this period resulted in similar epithermal mineralization styles emplaced episodically over a large time interval.