Hydrothermal alteration and exploration implications of the Laguna Amarilla high-sulfidation epithermal prospect, Los Menucos District, Río Negro Province, Argentina.

Abstract

The Laguna Amarilla prospect, located in the Los Menucos mining district (Río Negro Province), is one of six high-sulfidation epithermal prospects recognized in the central sector of the North Patagonian Massif. It developed over a volcanic and subvolcanic succession of felsic to intermediate composition, comprising phenodacitic lapilli tuffs, phenorhyolitic tuffs, monomictic breccias, and subvolcanic bodies. The concordia age obtained (279.6 ± 6.3 Ma) for the phenodacitic lapilli tuff is consistent with the Gondwanan magmatism recorded in the region, although two grains yielding ages near the Triassic-Jurassic boundary indicate a younger magmatic event, correlatable with the host rocks of the Cerro La Mina and Iván projects.

Integration of petrography, VNIR-SWIR reflectance spectroscopy, scanning electron microscopy, and X-ray diffraction allowed the identification of three hydrothermal alteration assemblages: i) advanced argillic (high-crystallinity kaolinite + dickite + quartz ± pyrophyllite ± diaspore); ii) intermediate argillic (illite + montmorillonite ± quartz ± low-crystallinity kaolinite); and iii) propylitic (montmorillonite ± chlorite ± epidote ± carbonate). Their spatial distribution defines an advanced argillic core with vuggy silica textures, grading laterally into intermediate argillic domains. Au anomalies are concentrated within this core, particularly where dickite and vuggy silica textures are developed. The Kaolinite Crystallinity Index proved to be the most effective spectral tool for vectoring toward mineralization. Finally, the presence of pyrophyllite and diaspore suggests a deep erosional level of the system and potential preservation of economic mineralization at shallow depth.