Alteration Mineralogy at the Cerro La Mina Epithermal Prospect, Patagonia, Argentina: Field Mapping, Short-Wave Infrared Spectroscopy, and ASTER Images

Abstract

The Cerro La Mina prospect, located in the Los Menucos area, Argentina, is currently being explored for epithermal gold mineralization. Triassic-Jurassic hydrothermal activity produced intense alteration of rhyolites, andesites, ignimbrites, and tuffs of the Late Triassic Los Menucos Group, in the Somun Cura Massif. A detailed surface map of the principal alteration assemblages at Cerro La Mina was produced employing field mapping, supported by short-wave infrared (SWIR) field spectroscopy, petrography, X-ray diffraction (XRD), and scanning electron microscopy (SEM). Advanced argillic, argillic, and silicic alteration zones and their internal mineral assemblage variations were recognized at Cerro La Mina. The silicification consists of massive bodies of fine-grained quartz and minor vuggy quartz, locally containing disseminated aggregates of pyrite, and rutile. Advanced argillic alteration of magmatic-hydrothermal origin occurs adjacent to the silicic alteration zones and can be subdivided into three mineral assemblages: (1) stage 1 alunite + quartz ± dickite ± kaolinite ± pyrophyllite ± diaspore ± rutile ± pyrite ± barite; (2) dickite + kaolinite ± pyrophyllite ± diaspore ± pyrite ± barite; and (3) dickite + quartz ± diaspore. The advanced argillic and silicic alteration grades to argillic alteration, comprising three mineral assemblages: (1) kaolinite ± dickite ± quartz ± illite ± smectite; (2) kaolinite ± illite ± smectite ± ferroan clinochlore; and (3) illite ± quartz ± muscovite ± kaolinite. Supergene alteration, which locally overprints the prior alteration zones, consists of jarosite + hematite + goethite ± stage 2 alunite ± aluminum phosphate sulfate (APS) ± poorly crystalline kaolinite. Consistent discrimination of the main alteration minerals of this prospect was also achieved by means of satellite multispectral remote sensing. The application of image processing techniques, specifically designed for mineral mapping, to selected spectral bands of the Terra/ASTER multispectral sensor resulted in a detailed alteration map for the entire prospect. Comparison between the results achieved through remote sensing with field data on the Cerro La Mina confirmed the accuracy of the former. The silicification and minor vuggy quartz, the abundant dickite, and the coarse-grained hypogene alunite, among other advanced argillic alteration assemblages, suggest that Cerro La Mina corresponds to a volcanic-hydrothermal leached environment, possibly controlled by permeable lithological units. These characteristics led us to interpret this prospect as a possible lithocap similar to those that typically host subsequent high-sulfidation mineralization.