The Questa Weathering and Stability Study included detailed earth-science characterization of hydrothermally-altered igneous rock-pile and analog materials and associated groundwater. Radiometric dating demonstrates that natural weathering processes and their mineralogical outcomes have been active at Questa over a million years or more. Dissolution of pyrite, calcite, and silicate minerals is the predominant geochemical reaction that results in low-pH, high-TDS seepage and precipitation of gypsum, jarosite, and iron hydroxide minerals. Silicate minerals in the Questa rock piles dissolve congruently, without producing clay minerals.
Seepage waters are thermodynamically under-saturated with respect to all silicate minerals (including clay minerals), sulfides, and carbonates, but are saturated with respect to jarosite, ferric oxides, and gypsum. Clay minerals at Questa formed under hydrothermal, not weathering, conditions. Secondary precipitates predicted from water chemistry are directly observed, and locally they act to cement rock fragments in rock piles and debris flows.
