Mineralogy and microthermometry of the Escondida, Gabriela and Margarita mineralized structures from the Cerro Moro epithermal deposit, Deseado Massif, Argentina

Abstract

Cerro Moro deposit is a low sulfidation Au–Ag epithermal deposit located in Santa Cruz province, southern Patagonia Argentina. It is hosted by numerous structurally controlled quartz veins developed in close spatial and temporal relationship with the Jurassic extension and magmatism. The main area in Cerro Moro comprises the Escondida horst block, limited by NW-SE listric faults which provided the channels for the hydrothermal system. The Escondida (including its high grade ore shoot Zoe), Gabriela and Margarita prospects are representative areas documenting the dynamics of the major fault systems. Four stages of mineralization were defined in Cerro Moro. Gabriela and Margarita prospects, at the center and NE of the deposit, respectively, show good development of bladed quartz replacement textures with adularia crystals and scarce translucent/opaque minerals from the stage 1 (pyrite-sphalerite > chalcopyrite ≫ galena-gold). The Escondida prospect, representing the southwest boundary of the Escondida horst block, is the most significantly mineralized structure hosting the base metal-rich and high grade Au–Ag mineralization. In this vein and in the Zoe high grade ore shoot, the translucent/opaque minerals are abundant (sphalerite, galena, chalcopyrite, lesser pyrite, marcasite, gold, silver and different quantities of Ag-bearing minerals) and mainly related to the dark grey, finely-grained quartz-muscovite veins from the stage 3 and to a lesser degree, from the stage 2. The presence of porous massive, crustiform, moss and colloform textures are interpreted to be formed by a sudden pressure release, where hydrofracturing and repetitive boiling occurred and lead to mineral deposition. Whitish quartz-adularia veinlets from the stage 4 may crosscut the previous sequences. The transparent, translucent and opaque minerals in Cerro Moro express a typical low sulfidation system, where the vein system is the result of the infilling and deposition of low temperature (174–265 °C) and salinity (0.4–0.7 wt% NaCl(eq)) hydrothermal fluids.