The Cerro Negro epithermal district, northwestern Deseado Massif (Patagonia, Argentina): New insights from telescoped volcanic-hydrothermal systems

Abstract

The Cerro Negro district, northwestern Deseado Massif (Patagonia, Argentina), is one of two world-class Au–Ag epithermal mines in the Patagonian region, with combined reserves, resources, and production of about 8 Moz of Au equivalent. The epithermal deposits are hosted by volcanic and intrusive rocks and are related to the evolution of distinct Jurassic magmatic events. Major WNW trending depocenters were filled by a thick sequence of andesitic-dacitic flows and ignimbrites. In the western part of the district, the sequence is intruded by subvolcanic bodies of the same composition and is grouped into the 159-156 Ma Eureka Volcanic Subvolcanic Complex (EVSC). Hydrothermal activity took place during the last stages of the EVSC, producing epithermal Au- and Ag-rich veins of low to intermediate sulfidation style in the Eureka West (156 Ma) and the Marianas-San Marcos veins (155-154 Ma) that were emplaced in NW striking extensional faults and in WNW to E-W trending strike-slip faults. In the eastern part of the district, basal units of the EVSC are overlain by a thick package of 154 Ma rhyodacitic ignimbrites, hosting the NW to EW-striking Vein Zone deposit and the NW-striking Bajo Negro, Silica cap and Gato Salvaje veins. NNE-trending block faulting formed horsts in the western and eastern parts of the district, separated by a central graben. Uplift of the western and eastern horst blocks lowered the water table and caused erosion, exhumation, and telescoping of the vein systems. Volcanism culminated in a late-to post-mineral rhyolitic event composed of lava-domes and volcaniclastic deposits grouped into the Eureka Rhyolitic Complex (ERC). Continued hydrothermal-volcanic explosive activity produced breccias which cap some of the Au–Ag vein deposits. The high geothermal gradient during ERC volcanism generated renewed hydrothermal activity, and hot springs discharged along the boundary faults of the central graben and within the graben in a fluvio-lacustrine environment. The Au–Ag epithermal veins associated with the EVSC (Eureka and Marianas-San Marcos) show a complex paragenetic history. High Au–Ag grades are related to early sulfide-rich ginguro bands formed at 290–230 °C from meteoric and volcanic fluids due to boiling/flashing mechanism at ~600–800 m below the paleowater table. Low-grade to barren quartz stages followed the early Au–Ag stage, and late calcite was deposited due to the collapse of steam heated CO2 waters. The highest Au–Ag grades are related to a late supergene event extending to 400 m depth. The eastern group of epithermal deposits (Vein Zone, Bajo Negro, Silica cap and Gato Salvaje) were emplaced ~300–400 m below the paleowater table indicated by a silica cap on Cerro Negro hill. They are dominated by silica replacement after lattice calcite texture with high Au grades related to low temperature quartz and late hematite-stained brecciation. We conclude that all deposits are genetically related to intrusive activity, and that additional Au–Ag mineralization may be concealed beneath post-mineral volcanic and sedimentary rocks in the central graben.