Hydrothermal alteration and mineralization in the Concordia District: first approach to the interpretation of erosional exposure levels in an intermediate-sulfidation epithermal system from the Argentinian Puna

Abstract

Hydrothermal alteration and mineralization in the El Recuerdo, Polvorilla, Matilde, and Concordia mines werecharacterized through the integration of remote sensing, field observations, SWIR–VIR spectrometry, petrography,and chalcography. The Concordia Pb–Ag–Zn–Sn–Cu District comprises a series of veins that belong to a singlemineralized system with different levels of exposure. Based on the ore assemblage (Ag-bearing galena, chalcopyrite,sphalerite, pyrite, and tennantite-tetrahedrite) and gangue minerals (rhodochrosite, calcite, and quartz),along with textural evidence and hydrothermal alteration patterns, the district is classified as an intermediatesulfidation(IS) epithermal system. This study reveals a distinct spatial zonation, transitioning from deeper levelsin the north to shallower conditions in the south. The northern sector represents an earlier, deeper stage of mineralization,characterized by base metal sulfides in carbonate-silica gangue with colloform banding, cockade,and lattice-bladed textures, where phyllic alteration grades outward into propylitic assemblages. In contrast, thecentral sector (Polvorilla and Matilde mines) exhibits features consistent with shallower conditions, includingopen-space filling, comb, massive, and breccia textures, where phyllic alteration grades outward into argillicalteration to the southern sector (Concordia mine). Mineralization and hydrothermal alteration were controlledby two major north-south oriented faults associated with the emplacement of the Concordia dacitic dome, whichhas an intrusive contact to the north, grading soutward block and ash deposits. At least two main mineralizingstages are inferred: (1) an early carbonate-dominated stage associated with boiling conditions and (2) a subsequentsilica-dominated stage linked to episodic fluid pulses, fracturing, and sealing processes. The recognition ofthis vertical-to-lateral zonation, resulting from differential erosion, provides a critical framework for targetingprecious and base metal mineralization in similar Andean epithermal environments.