Geology, hydrothermal alteration and mineralogy of the Sofía-Julia-Valencia Veins, Andacollo, Neuquén, Argentina

Abstract

Geology, hydrothermal alteration and mineralogy of the Sofía-Julia-Valencia Veins, Andacollo, Neuquén, Argentina. The Sofía-Julia-Valencia vein system, located in the Andacollo mining district in central west Argentina, is hosted by ENE-WSW oriented strike-slip faults which are the result of reactivation of normal faults affecting Carboniferous to Jurassic rocks during Upper Cretaceous-Paleogene. These veins contain a total resource of 22,900 Oz of gold with 5.5-6.7 g/t AuEq. Geologic mapping and a U-Pb age of 71±1Ma in zircon, obtained in an altered and mineralized dacitic dyke of the district, allowed to associate the mineralizing event to the Naunauco Andesitic belt magmatism (Upper Cretaceous-Paleogene) and to the Cretaceous-Paleogene Metallogenic Belt of the Andes in southwestern Argentina. The ore bodies are made up of multiple veins and veinlets that, from oldest to youngest, correspond to: (1) scarce early quartz+pyrite+molybdenite+iron poor-sphalerite veinlets, (2) quartz+epidote+calcite±albite (apatite+rutile+titanite+light rare earth elements bearing phosphates) associated with quartz+biotite, epidote (actinolite)+chlorite+calcite, with pyrite+pyrrhotite±chalcopyrite±(iron rich-sphalerite), marcasite veins. These veins are cut and reopened by (3) polymetallic veins and veinlets formed by quartz+sericite±carbonates (chlorite), with irongold rich sphalerite+silver rich-galena+chalcopyrite+pyrite, native gold±arsenopyrite±(pyrrhotite, bornite, argentite). Pyrite (4) and (5) carbonate+framboidal pyrite veinlets cuts all the previous ones. Multistage carbonate generation brecciate and cut previous veins and veinlets. Quartz shows granular, comb textures and some calcites developed platy textures. Four hydrothermal alteration types affected the veins host rock: (1) patches of early potassic alteration; (2) widespread propylitic alteration with disseminated sulfides; (3) later phyllic alteration overlapped to the previous ones; and (4) late supergene alteration. The sphalerite and chlorite composition in the veins (1 and 2) along with their mineral assamblages indicates they were formed by initially alkaline fluids (e.g., feldspar stable) with intermediate sulfur and oxygen fugacity and mesothermal temperature conditions (~400-240 °C), that evolved to conditions of lower sulfur (e.g., pyrrhotite stable) and oxygen fugacity, temperature <150 °C, with slightly acidic pH (<5). Fluid inclusions hosted in quartz, calcite and sphalerite from polymetallic veins and veinlets (3) in the western sector of ore deposit show they emplaced at ~1,400 m depths and originated by two discrete pulse of hydrothermal fluids: an early one with highest temperatures (390 to 260 °C) and salinities (4-11% NaCl eq.) and a later one of lower temperature (<220 °C) and salinity (<8% NaCleq.). During this second dilution and cooling event, took place the main mineralization stage. The overlapping of lower temperature veins/veinlets and hydrothermal alteration mineral assemblages to those of higher temperature observed in the SJV vein system, possibly evidence a telescoping process resulting from uplift and erosion of the Cordillera del Viento during the Upper Cretaceous-Paleogene compressional event.