Caracterización geológica, estructural y de las alteraciones de las vetas del torreón, Andacollo, Neuquen.

Abstract

This work presents new lithological, structural, and alteration descriptions associated with the E-W to ENE-WSW and NE-SW veins, exposed along the southern margin of the Torreon stream (Andacollo, Neuquén), in the southwestern part of Cordillera del Viento (-37°10´46”, -70°38´36.10”). The units that outcrop in the region are structurally conditioned by their periclinal closure and present structures associated with three orogenic cycles: Chanic, Gondwanic, and Andean. Consequently, lineaments of various orientations can be observed, potentially playing a key role as structural controls of the vein systems. In the study area, black tuffs from the Arroyo del Torreon Formation, corresponding to the Cerro San Pedro Member (Mississippian), crop out in contact with its upper member (Sofía Member, Upper Mississippian), overlain by polymictic conglomerates from the Huaraco Formation (Upper Pennsylvanian). Above these units, in erosional unconformity, lie pyroclastic facies of the Cordillera del Viento Formation (lower Jurassic). Aphanitic andesitic-basaltic dikes cut through this formation and Sofia Member. Porphyritic dacitic dikes, assignable to the Naunauco Group (71±1 Ma), cut across the entire stratigraphic sequence. Two distinct groups of veins were identified, differing in their structural, morphological, and textural characteristics. The E-W to ENE-WSW veins are emplaced in sinistral strike- slip faults, previously normal, and are hosted in the Sofía Member and the Huaraco and Cordillera del Viento Formations. On the other hand, the NE-SW veins are extensional in nature and are hosted exclusively in the Sofía Member. Additionally, two kinematics faults populations were identified in the E-W and ENE- WSW orientations. Kinematic population 1 is linked to the Huarpic phase (Upper Triassic-Lower Jurassic) in a N-S to NNW-SSW extensional regime, while kinematic population 2 corresponds to the reactivation of pre-existing (normal) faults under a NNE- SSW to WSW-ENE compressional regime. The geometric and kinematic compatibility between both groups of faults (E-W and ENE-WSW) suggests they could be part of the same system. Using combined techniques (visual estimation, petrographic analysis, X-ray diffraction, and spectrometry), three types of alterations were identified in all the veins: silicification and argillic-phyllic alteration (illite-sericite±quartz) near the veins, transitioning to intermediate argillic alteration (illite±smectite±quartz) in intermediate zones for the E-W to ENE-WSW veins, and propylitic alteration in distal zones, represented by illite±chlorite±quartz in the NE-SW veins, and illite±chlorite±calcite±quartz in the E-W veins. These alterations suggest an increase in pH (5 to 7) and a decrease in temperature (250°C to <200°C) from proximal to distal zones, characteristic of an intermediate to low- sulfidation epithermal system. Spectrometry analyses revealed that the dominant white mica is illite, whose spectral maturity (MSI) varies with proximity to the veins. High MSI values (>1) near the fault-vein zones indicate areas of higher temperatures where hydrothermal fluids circulated, later migrating laterally through the host rocks, interacting with them, and cooling, resulting in lower MSI values (~1). The variation in the Al-OH absorption position suggests compositional changes in the illites, with a trend from paragonitic to normal potassic illite in the NE-SW veins (AlOH 2199 to 2208 nm), reflecting a sodium-rich environment, and an increase in K, Fe, and Mg contents towards more distal areas.