THE ROLE OF GEOLOGY IN GENERATING CONTAMINATED ACID MINE DRAINAGE IN KAOLINITE DEPOSITS HOSTED BY HYDROTHERMALLY ALTERED VOLCANIC ROCKS, CAROLINA SLATE BELT, USA

Clay deposits that form by weathering of mineralized felsic volcanic and volcaniclastic rocks have the potential to generate acid mine drainage containing significant metal contents. Large kaolinite deposits in the Carolina Slate Belt (reserves of >650,000 tons at Haile and Brewer, SC) are attributed to weathering of crystal tuff, whereas smaller low quality clay lenses are ascribed to alteration of volcaniclastic rocks. Felsic volcanic and volcaniclastic rocks in the region contain extensive deposits of iron-sulfide, gold, and other metal sulfides that formed in ancient hot springs settings. Chemical weathering of the volcanic rocks resulted in a relatively pure clay rock (~35% kaolinite, ~45% quartz, and ~20% sericite) containing iron-silica-rich layers (goethite, kaolinite, chalcedonic quartz). Clay rocks and silicastones formed via weathering contain assemblages of aluminum minerals and poorly crystalline phases (e.g., feldspar, muscovite-illite, kaolinite). Low-grade lenses of disseminated sulfide minerals (pyrite, chalcopyrite, galena) occurring in high-alumina alteration zones are partially weathered to iron oxide concretions and crusts containing goethite, hematite, lepidicrocite and kaolinite. Mining of the clay deposits exposes pyrite to weathering resulting in acidic drainage contaminated with metals found locally in small pits, seepages, and drainage channels within the deposits. Al and Fe-oxyhydroxides and Fe-sulfate minerals actively form along the edges of water bodies as a result of pyrite oxidation and subsequent increases in ferric iron concentration by evaporation. Mineralogy, trace element, and water data suggest that aqueous oxidation of arsenian pyrite containing galena and sphalerite inclusions generated acidic drainage (pH~3.5) containing Fe, Pb, Zn, As, Se, and Hg. Adsorption of Se and As apparently occurs predominantly on ferric hydroxide substrates (60-230 ppm Se and 11-511 ppm As), although coexisting Al-oxides and clay minerals (~7 ppm Se, 22 ppm As) compete for Se and As in this setting. Quantitative mineralogical modeling results suggest that maintaining neutral pH of groundwater and careful waste management may preserve an environmental system that is a relatively stable repository for trace metals.