Estudio petrográfico, espectral y de susceptibilidad magnética de la alteración hidrotermal asociada con depósitos polimetálicos de Pb-Zn-Cu-Ag-Au, Sierra de Comechingones, Córdoba (Argentina)

Abstract

In the metamorphic basement of the Sierra de Comechingones, the southernmost extension of Sierras de Cordoba (Argentina), it is hosted a group of polymetallic Pb-Zn-Cu-Ag-Au veins associated with hydrothermal alteration zones, which are the result of a Late Paleozoic metallogenic period. In this paper, the application of spectral analysis in the SWIR (Short-Wave Infrared) region as well as magnetic techniques, unusual in Argentina for this style of mineralization, proved to be an effective tool in the study of altered areas and also useful to establish targets for mineral prospecting and/or exploration. A sericite-quartz assemblage is the main hydrothermal alteration product of the migmatic host rocks. It is developed into narrow but continuous altered zones which follow the main direction of the mineralized structures and can reach up to 5 meters to each side of the veins. Subordinately and locally, a carbonate siderite-calcite assemblage takes place as a result of altered amphibolites, which are interspersed within the metamorphic sequence. In the reflectance spectra, the wavelength position of the Al-OH absorption feature revealed compositional variations and cation substitutions in the hydrothermal white micas, ranging from potassic (wavelengths near 2200 nm) to micas with a lower octahedral Al content (wavelengths > 2210 nm) towards phengite. These variations and the spatial distribution within the altered areas are primarily controlled by three main factors: 1) hydrothermal fluid temperature, 2) pH and/or 3) Mg-Fe/Al ratio in the fluid-rock system. The crystallinity indexes (CI) calculated from the spectral data, indicate the predominance of high crystallinity illites with a slight increase of this parameter towards the proximal zones to the mineralized veins. The illite/smectite mix-layered clays indicate a fluid temperature decrease towards the distal zones and they can be used as vectors to mineralization. Both sericite-quartz and siderite-calcite hydrothermal assemblages are frequently affected by supergene oxidation processes that led to a conspicuous precipitation of hematite-goethite within the altered areas. The oxidation state of Fe +3 in these minerals is the main cause of the decrease of the magnetic susceptibility parameter in the host rocks. With a regional perspective, these hydrothermal alteration zones characterized by a high abundance of white micas of moderate to high crystallinity (CI=1,21-3,92 and IK=0,21-0,23), compositions ranging from potassic to phengitic (Al-OH wavelengths between 2200-2215 nm) and associated with hematization zones and low magnetic susceptibility values ( -3 SI), may be considered as potential targets for future prospecting and/or exploration of polymetallic veins hosted in the metamorphic environment of the Sierras Pampeanas.