Caracterización litológica, de las alteraciones hidrotermales y mineralizaciones de dos sondajes de las vetas San Pedro y San Pablo, distrito minero Andacollo, provincia de Neuquén

Abstract

This final undergraduate work provides new lithological, mineralogical and textural descriptions of the rocks, hydrothermal alterations and mineralization of the San Pedro and San Pablo veins, which allowed us to characterize the hydrothermal system that gave rise to them. These veins are located in the Andacollo mining district, in the southwestern sector of the Cordillera del Viento, Neuquén province. Near the study area there are several geological units, among them the Arroyo del Torreón Formation (lower Carboniferous), characterized by volcaniclastic and, to a lesser extent, volcanic and sedimentary rocks. The Sofia Dacite intrudes this unit, while the Huaraco Formation (upper Carboniferous) rests in discordance over these underlying units. Porphyritic and aphanitic andesitic dikes intrude the Arroyo del Torreón Formation. Finally, porphyritic dacitic intrusives of the Naunauco Group (71±1 to 68.46±0.31 Ma) cut the entire stratigraphic sequence. The San Pedro and San Pablo veins are located in a complex structural context, with veins and veinlets arranged discordantly in relation to the surrounding rock units. Their main orientation is NE-SW and subvertical inclinations towards the southeast, lodged in extensional structures that have been subsequently reactivated and are located in volcaniclastic rocks of the Arroyo del Torreón Formation. At surface, these veins are up to 300 meters long and have an average thickness of about 3 meters. They exhibit high Ag/Au ratios of approximately 100:1 and are classified as intermediate to low sulfidation epithermal veins. They are composed mainly of quartz and adularia, with the presence of fine-grained Ag sulfosalts and Cu, Ag, Pb, Zn sulfides, native silver and electrum. Detailed analyses by combined techniques on the drill hole samples identified four types of alteration, chronologically ordered as follows: 1) quartz±sericite/illite±pyrite±leucoxene; 2) chlorite±pyrite; 3) carbonatization (carbonates) and 4) supergene (iron oxides and hydroxides). The first one develops close to the main fluid circulation conduits linked to the mineralized structures, while the second and third ones develop distal and superficial respectively. This detailed study allowed us to identify at least 10 pulses in the San Pedro and San Pablo veins, which were grouped in 4 episodes of hydrothermal filling: Episode 1 (E1), Episode 2 (E2), Episode 3 (E3) and Episode 4 (E4). Where the most important Au, Ag and base metal mineralization concentrations occur in quartz veins and veinlets corresponding to E1. These mineralization-bearing veins and veinlets correspond to P1: quartz±adularia±sericite +pyrite+galena+sphalerite+chalcopyrite+polybasite-perseite and P2: quartz±sericite±pyrite+galena+sphalerite+chalcopyrite+acanthite- argentite+electrum+native silver. From the paragenesis of alteration, gangue and ore minerals, it was possible to determine some formation conditions. The characteristics of episode 1 suggest that it was formed from fluids with intermediate sulfur and oxygen fugacity. Episode 2, on the other hand, would have been originated from fluids with alkaline pH and intermediate temperatures (200-300 °C), evolving towards lower temperatures (< 200 °C) and almost neutral pH (6-7), with lower sulfur and oxygen fugacity.