Abstract

Advanced argillic (AA) alteration produced by hydrothermal activity and subsequent supergene alteration in the Potrerillos district, Atacama Desert, Chile, includes sulfate bearing and aluminosilicate alteration minerals. These alterations commonly contain alunite supergroup minerals (e.g., alunite, jarosite, aluminum-phosphate-sulfate (APS) minerals) of both hypogene and supergene origin. In this contribution, we investigate the mineralogical and geochemical characteristics of these minerals, to decipher the physicochemical parameters of formation, as well as the composition, source, and evolution of the fluids that produced the AA alteration in the area. Field observations, optical microscopy, and geochemical analyses provide insights into the geochemical evolution of the hydrothermal system and its later supergene weathering. Three precipitation environments with different sulfur sources and characteristics of alunite group minerals have been defined at Potrerillos: I) hypogene environment with sulfur originating from H2S disproportionation and tabular alunite (>200 μm); ii) steam-heated environment with sulfur from oxidation of distilled H2S above the water table, tabular alunite (<100 μm) and pseudocubic APS; and iii) supergene environment with sulfur from the oxidation of hydrothermal sulfides and dissolution of alunite and APS in the vadose zone represented by pseudocubic alunite, jarosite (≤5 μm) and APS minerals. Hydrothermal and supergene minerals have endmember and intermediate compositions in the alunite-jarosite solid solution series, respectively. Divalent exchange in alunite and jarosite, and the precipitation of APS minerals reflect variations in the redox and pH of the supergene fluids, their chemical composition, and elemental concentration, and to a lesser extent variations in the steam-heated fluids. The temperature of hypogene alteration is estimated to range from 200° to 120 °C, lower temperatures cannot be discarded for the steam-heated environment.