La Josefina Au-Ag deposit (Patagonia, Argentina): A Jurassic epithermal deposit formed in a hot spring environment

Abstract

The La Josefina Jurassic epithermal Au-Ag deposit located in Patagonia, Argentina, developed in an extensional setting of a back-arc environment, associated with a widespread Middle-Late Jurassic calc-alkaline volcanism. Block faulting has juxtaposed shallow level features evidenced by hot spring manifestations, hydrothermal eruption breccias and Au-rich veins, which suggest that mineralization in these veins, could extend far below the depths already tested by core drilling. Veins are filled by quartz, chalcedony, opal and minor adularia and barite with massive, comb, cockade, colloform-crustiform bandings and lattice-bladed textures. Ore minerals include electrum, Ag-rich sulfosalts (freibergite), pyrite, galena, sphalerite, chalcopyrite and specular hematite with minor arsenopyrite, marcasite, tetrahedrite and bornite. Four mineralizing stages have been identified, the first two (S1 and S2) are Au and Ag-rich, with temperatures ranging from 225 to 290° and salinities from up to 15 wt.% in S1 decreasing to ~1 wt.% NaCl in S2. The third stage (S3) displays higher base metal contents at lower temperatures (~200 °C). Finally, the last stage (S4) is barren with temperatures lower than 100 °C. Veins are surrounded by a proximal alteration halo of quartz + pyrite ± adularia ± illite followed outwards by illite/smectite interstratified clays and smectites (with less chlorite) to a propylitic zone. Stable isotope values calculated for the fluids show a mostly meteoric origin for mineralization fluids. Such distinct features place the La Josefina deposits in a hot spring environment with evidences of being formed at a proximal position of the Jurassic paleosurface and paleowater level.