Abstract

Hydrothermal activity genetically related to pluton intrusion during the Incaic tectonic phase affected the Potrerillos district in the Chilean Precordillera at the southern edge of the Atacama Desert which produced acidic fluid-driven alteration lithocaps that were later overprinted by supergene processes. Outcrops with advanced argillic (AA) alteration commonly contain alunite supergroup minerals (e.g., alunite, jarosite, aluminum-phosphate-sulfate (APS) minerals) of both hypogene and supergene origin, indicating overprinting of multiple alteration events over the system’s lifespan. Morphology, textural patterns, and composition of alunites, jarosite, and APS minerals related to hypogene and supergene processes in the Potrerillos district were investigated as proxies for the physicochemical parameters as well as the composition and source of the fluids producing AA alterations in the area. Field observations, optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, short-wave infrared spectroscopy, in situ mineral chemistry (µ-XRF, SEM-EDS, EPMA) and sulfur isotope analyses to define three different types of fluids, each related to a specific depositional environment (Fig. 1): i) A magmatic-hydrothermal environment dominated by acidic fluids of magmatic origin, with sulfur species derived from SO2 disproportionation at depth. This environment is characterized by a mineral assemblage consisting of tabular-shaped alunite, quartz, and traces of Ti oxides, indicative of precipitation at temperatures >200 °C. In shallow and oxidized environments, the magma-derived fluids mix with meteoric water, producing a fluid containing SO42- originating from both disproportionation at depth and H2S-oxidation from the vadose zone, thereby triggering the precipitation of tabular-shaped alunite, quartz, kaolinite, and small amounts of smectite; ii) Steam-heated acidic-fluid dominated environment at a temperature range of 150 to 200 °C with highly oxidized and high-PO43- activity solutions (at a pH of ~ 1 to <5) containing sulfate generated upon oxidation of distilled H2S in the vadose zone. This environment is characterized by fine-grained tabular alunite associated with quartz, kaolinite, and traces of dickite and APS minerals with svanbergite-woodhouseite composition; and iii) Strongly oxidized supergene environment with acid-sulfate meteoric waters (pH ~1), characterized by an assemblage of jarosite, pseudocubic alunite, kaolinite, halloysite, Fe-oxides, smectite, and low amounts of APS minerals. The acidic solutions therein originate from sulfuric acid produced upon oxidation of sulfide minerals and dissolution of other minerals such as alunite and APS minerals at the time of rock leaching, resulting in enrichment of the supergene fluid in Al3+, Fe3+, Fe2+, H+, K+, SO42-, Ca2+, Sr2+, and PO43-. The textural relationships between alunite, jarosite, and APS minerals, and the sem analyses suggest important variations in the physical and chemical characteristics of the fluid in the steam-heated and supergene environments. Multiple events responsible for AA alteration in the studied area have induced complex evolution of an epithermal system that grades from an early magmatic-hydrothermal stage to late leaching processes driven by meteoric waters. Furthermore, the capability of alunite supergroup minerals to record environmental physicochemical changes renders them suitable for the evolution of epithermal systems to be studied, including both, hypogene and supergene processes.