Hydrothermal alteration of the Cerro La Mina gold prospect, Los Menucos, Patagonia Argentina : geology, remote sensing and stable isotopes / Alteração hidrotermal do prospecto aurifero Cerro La Mina, Los Menucos, Patagonia Argentina : geologia , sensoriamento remoto e isotopos estaveis




The epithermal Cerro La Mina prospect, located in the Los Menucos region, Patagonia Argentina, is currently being explored for epithermal precious-metal mineralization. Triassic hydrothermal activity produced intense alteration of rhyolites, andesites, ignimbrites, and tuffs of the Middle- to Late Triassic Los Menucos Group, in the Somún Curá massif. The use of techniques, including geological mapping, multi- and hyperspectral remote sensing, reflectance spectroscopy and staple isotope analysis, alIowed the characterization of five hydrothermal alteration stages in the study area. Event 1 resulted in a low-sulfidation epithermal system with quartz veins, some of them with gold grades between I and 10 g/t and sericitic alteration, which affected riolitic ignimbrites and sedimentary rocks of the Vera Formation. Alteration associated to stages 2 and 3, the most important from the volumetric viewpoint, is related to a high-sulfidation alteration system, which affected rhyolites, dacites and andesites of the Sierra Colorada Formation. These volcanic rocks comprise a complex of resurgent domes, located within a likely oval-shaped volcanic caldera, formed in a transtensive zone placed between two regional dextral strike-slip faults. Some of the most important characteristics of this epithermal system include the presence of hydrothermal breccias, some with gold occurrences with grades locally reaching up to 60 g/t, pervasive silicification and vuggy quartz, together with advanced argillic alteration with hypogenic alunite-l, woodhouseite, svanbergite, minamiite, pyrophyllite, diaspore, dickite and kaolinite. Alunite-l was formed at 267°C, indicating a hydrothermal-magmatic origino Values of δ ‘POT. 34’S of alunite-l (4.4 to 10.3 %0) are higher than those obtained for contemporary pyrite (-14.8 to -10.8%0), suggestirig that aqueous sulfate was derived of the S ‘O IND. 2’ disproportion reaction in a hydrothermal-magmatic system. δ ‘POT. 34’S and δ ‘POT. 18’ O values from alunite-l suggest that the vapor that carried S ‘O IND. 2’ condensate in meteoric water with a ratio of ‘H IND. 2’ S/’SO IND.4’ <1. Isotopic values of oxygen and hydrogen for kaolinite and dickite are ais o compatible with a hydrothermal-magmatic origino Alteration related to event 4, characterized by intense silicification blanket with chalcedony and opal, locally grading at depth to advanced argillic alteration with alunite-2 plus kaolinite ± dickite, was originated in a steam-heated environrnent. Alunite-2 is enriched in S SOB. 34 in relation to sulfide, reflecting a partial isotopic exchange of sulfur between the aqueous components ‘H IND. 2’ S e ’SO IND.4’. This alteration localIy overprints those from stages 2 and 3, suggesting a fall in the level of the water table, probably adding up to syn-hydrothermal erosion of the upper portion of the rocks. Event 5 is related to the formation of fluorite veins, comparable to low-temperature quartz-fluorite veins. These fluorite mineralizations are apparently not linked with the processes related to the gold epithermal. systems of the prospect. The results of this study contribute significantly in the understanding of metallogenic aspects of this complex epithermal system, constituting important parameters to the formulation of a mineral exploration model for the region. Consistent discrimination of the main alteration minerais of this prospect was also achieved by means of satellite remote sensing. The application of processing techniques on images of the multispectral- Terra/ASTER and hyperspectral Hyperion sensors resulted in different lithologic, mineral and structural maps. Satisfactory correlations between these results and field data demonstrate the key importance of the remote sensing in the work of an exploration program, in the initial stages of regional scale and also in more advanced stages, at prospect or deposit scales.


região (argentina) remote sensing isotopes spectrum analysis alteração hidrotermal isotopos sensoriamento remoto hydrothermal alteration analise espectral geology geologia - patagonia

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