Genetic implications of vein texture and mineralogy at the Au-bearing El Morado mining district, Sierras Pampeanas, San Juan, Argentina

Abstract

El Morado mining district is located in Sierra de La Huerta, in the Sierras Pampeanas of San Juan province, Argentina. It comprises numerous Au-bearing quartz veins hosted by intensely deformed igneous-metamorphic units of lower Paleozoic age. In the El Morado mining district, Maira mine is the only deposit producing gold at the present time. Quartz veins in this mine and surroundings, occur in concordance with their host rocks foliation and show numerous evidences of brittle-ductile deformation, mainly represented by a pinch-and-swell structure and extended development of breccia bodies.The application of scanning electron microscopy cathodoluminiscence in quartz samples lead to the recognition of homogeneous texture, which is evidence of recrystallization, commonly observed in orogenic gold quartz veins. Intracrystalline deformation is ubiquitous and represented by undulose and patchy extinction in relicts of large old grains where bulging and sub-grain rotation recrystallization microstructures are well represented. These dynamic quartz deformation processes indicate temperatures going up to ∼350 °C. Fluid inclusion post-entrapment modifications were recognized in quartz veins as numerous collapsed, small-sized fluid inclusions.Ore mineralization at Maira mine occurred after quartz veins emplacement and deformation in the upper crustal brittle-ductile transition zone. Galena, chalcopyrite and lesser pyrite and bornite, together with gold and tetrahedrite were identified infilling fractures in quartz veins. Gold grains were also identified closely related to malachite, cuprite, goethite and lepidocrocite in oxidizing horizons.