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

ANA CECILIA MUGAS; GLADIS PALACIO BALDERRAMO; BRÍGIDA, CASTRO DE MACHUCA

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Año: 2024 vol. 135

0895-9811

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-metamorphicunits of lower Paleozoic age. In the El Morado mining district, Maira mine is the only deposit producing goldat the present time. Quartz veins in this mine and surroundings, occur in concordance with their host rocksfoliation and show numerous evidences of brittle-ductile deformation, mainly represented by a pinch-and-swellstructure 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 goldquartz veins. Intracrystalline deformation is ubiquitous and represented by undulose and patchy extinction inrelicts 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 fluidinclusions.Ore mineralization at Maira mine occurred after quartz veins emplacement and deformation in the uppercrustal brittle-ductile transition zone. Galena, chalcopyrite and lesser pyrite and bornite, together with gold andtetrahedrite were identified infilling fractures in quartz veins. Gold grains were also identified closely related tomalachite, cuprite, goethite and lepidocrocite in oxidizing horizons.