IN-SITU ACID WEATHERING REACTIONS IN AREAS AFFECTED BY LARGE-SCALE AND PERVASIVE HYDROTHERMAL ALTERATION IN THE SOUTHERN ROCKY MOUNTAINS: IMPLICATIONS REGARDING ACID-PRODUCING MINE DUMP STABILITY

In-situ natural acid weathering reactions were studied in mineralized but unmined areas in the southern Rocky Mountains that were superimposed by pervasive hydrothermal alteration and abundant disseminated pyrite. Although host lithologies vary, the rocks were similarly altered to weak through intense quartz-sericite-pyrite (QSP) assemblages. Pyrite weathering reactions are associated with acidic, metal-rich surface and groundwater. Mineralogical studies from key outcrop sequences characterize the transition from relatively unweathered/unoxidized parent rock through weathered/oxidized daughter material. Quantitative XRD analyses document a decrease in chlorite, illite, pyrite ± plagioclase as weathering progresses with a correlative increase in dioctahedral smectite, kaolinite, and jarosite. SEM observations support quantitative mineralogical data that show an inversely linear decrease in chlorite (89% reduction) and pyrite (100% reduction) with increasing smectite. Decreases in illite (32% reduction) and plagioclase (26% reduction) from parent to daughter rock are still notable but less linear with respect to increasing smectite. SEM textures and clay XRD studies suggest that smectite grew largely by neoformation and dissolution of chlorite, illite, and plagioclase. However, SEM data and elemental microprobe mapping portray zones of reaction around illite grains with a loss of K, Mg, and Fe. These reacted zones show gross inheritance of the original illite morphology and chemical analyses suggest the presence of smectitic interlayers. Although smectite and kaolinite are late paragenetically, smectite always post-dates kaolinite. These data suggest waters related to pyritic weathering were more acidic and precipitated kaolinite. As solutions evolved to more neutral compositions, they equilibrated with smectite. We suggest that pyrite-rich mine waste containing QSP wall rock will undergo similar reactions and form smectite. The production of this highly swelling clay has important implications for mine dump stability.