Silver, gold, and base metals vein systems at southern part of Cordillera del Viento, Neuquén Argentina

PONS, MARÍA J.; GIACOSA, RAÚL E.; GRECO GERSON A.; GONZÁLEZ, SANTIAGO N.; DICARO, SEBASTIÁN; CONEDERA, MANUEL; NIMIS, PAOLO; BORDESE, SOFÍA

Puerto Madryn

Congreso; XXI Congreso Geológico Argentino; 2022

Asociación Geológica Argentina

The Andacollo district, the most important metalliferous mining district of the Neuquén province, extends over 220 km2 with more than 56 Au, Ag and base metal (Cu, Pb, Zn) veins. This work is focused on cerro Las Minas and San Pedro areas, located at the south periclinal zone of the Cordillera del Viento anticline (CVA). Previous studies (Giacosa 2011, Strazzere et al. 2017 and references there in) have proposed that mineralization in this area is the result of at least two hydrothermal events of Carboniferous-Middle Triassic? and upper Cretaceous-Paleogene age. Here, we present new structural, geochronological, mineral chemistry (EDS and microprobe analyses), and textural data of the altered host-rocks and the mineralized structures of the two main vein systems (Buena Vista-San Pedro group, BVSPG, and Sofía-Torreón group, STG) in the district. Based on this data, we define new characteristics of the hydrothermal systems and constrain the mineralization events in the framework of the tectonic evolution of the area.Volcaniclastic and marine rocks of the lower Carboniferous Arroyo del Torreón Formation (AT) and marine sedimentary rocks of the upper Carboniferous Huaraco Formation (both included in the Andacollo Group) are covered and intruded by ignimbrite and volcanoclastic rocks of the La Premia Formation (LP) and by cogenetic intrusives of the Huingancó Volcanic and Plutonic Complex (Permian-Lower Triassic). Previous units are separated by the regional huarpic unconformity from the overlying volcaniclastics rocks of Cordillera del Viento Formation (CDV, Medium to Upper Triassic in Giacosa 2011 and references there in). Numerous dacitic porphyritic dikes and andesitic-basaltic dikes cut the previous units. During the Gondwanide orogenesis, thrusts and folds structures of the San Rafael compressive phase deformed the Andacollo Group (Giacosa, 2011). Post-orogenic gondwanic structures are normal faults with E-W to NE-SW trend, related to an extensional stage that controlled deposition of a volcaniclastic sequence assigned to LP (Giacosa 2011). At the onset of the Patagonidic orogeny (Late Cretaceous-Paleogene), these faults were reactivated as transtensive and transpressive faults, in the periclinal zone of the CVA.The BVSPG is hosted in the AT Formation. The San Pedro Norte (SPN) veins are situated in the hanging- wall of an inverted N55ºE normal fault named here as El Maicillo fault, meanwhile San Pedro Sur (SPS) veins are located at the footwall of the El Maicillo fault, that shows dextral component of shear. Along this structure is emplaced a dacitic porphyritic dike with numerous mafic enclaves oriented parallel to the contact with wall rock.In the BVSPG total resources of 63800 Oz AuEq and 1:100 Au/Ag ratios are estimated. Vein lengths vary from 50 to 300 m and 0.3 to 6 m, respectively, with vertical extension of 150-200 m. Underground mapping shows the dacitic porphyry dyke cuts the Buena Vista vein system at Cerro Minas area. The SPN veins are composed by multiple quartz, sericite (adularia) pulses. Quartz veins show coarse banded and crustiform textures typical of extensional and dilatational veins. Most of them are brecciated and cemented by gray quartz and late comb quartz. Main mineralization is hosted in the gray quartz and consist of early pyrite1±arsenopyrite and late pyrite2+iron-bearing sphalerite+chalcopyrite+galena, polybasite- pearceite+argentite±electrum (pyrargyrite, silver) and localized tetrahedrite-tennantite. Traces of enargite and covellite replace Cu-bearing sulfides. Preliminary analyses of fluid inclusions in gray quartz of SPN veins have homogenization temperatures between 225 and 255ºC and low salinities (