Alunite supergroup minerals from advanced argillic alteration assemblage in the southern Atacama Desert as indicators of paleo-hydrothermal and supergene environments

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, and 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 as follows: (i) hypogene environment with sulfur originating from H 2 S disproportionation and tabular alunite (> 200 µm); (ii) steam-heated environment with sulfur from oxidation of distilled H 2 S 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 conditions, chemical composition, and elemental concentration in the supergene fluids, these variations also were observed to a lesser extent in steam-heated solutions. The temperature of hypogene alteration is estimated to range from to 120 to 200 °C; lower temperatures are possible for the steam-heated environment.