Abstract

ECHEVESTE, Horacio J; LOPEZ, Luciano; RODRIGUEZ, María E  and  RECIO, Clemente. Alteración hidrotermal en el yacimiento epitermal Manantial Espejo, Macizo del Deseado, Santa Cruz, Argentina. Rev. Asoc. Geol. Argent. [online]. 2016, vol.73, n.3, pp.369-387. ISSN 0004-4822.

Manantial Espejo is a Ag and Au vein type ore deposit that is located in the southwestern area of Deseado Massif, Santa Cruz province, Argentina and is hosted by the volcaniclastic rocks of the Middle to Late Jurassic Bahía Laura Group. This deposit contains 5.2 Mt of ore reserves with 127 g/t Ag and 2.05 g/t Au. The main veins are located in WNW subvertical normal faults related to an extensional tectonic setting. Four hydrothermal alteration zones were identified: silicification, quartz-illite, argillic and propylitic. Silicification usually occurs as narrow (2 cm) bands parallel to veins, and as replacement of permeable rocks (travertine, ash-rich tuffs or breccia), not always related to mineralized structures. The quartz-illite zone surrounds the silicification zone, and within it pumice and lixiviated crystals were replaced by quartz, adularia crystals and illite-sericite. Argillic alteration which is widespread surrounding the above mentioned zones consists of illite-smectite replacement of plagioclase, with occasional veinlets crosscutting feldspar crystals. Propylitic alteration results in moderate chloritization of mafic minerals and sericitization and/or calcitization of plagioclase, affecting andesites and dacites. Geochemical analyses of fresh and altered host rocks and mass balance considerations hint towards potasic metasomatism, resulting in silica, K2O and Rb enrichment, and depletion of Na2O, CaO, MgO, FeO and, particularly in quartz-illite alteration zone. δ18O values of the fluid in equilibrium with vein quartz and kaolinite, and the presence of tourmaline in the argillic alteration zone, suggest the participation of the final fluids derived from the crystallization of shallow K2O-rich rhyolitic bodies in the composition of the hydrothermal fluids.

Keywords : Silicification; Quartz-illite; Argillic; Propylitic; K-metasomatism.

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