WEATHERING OF HYDROTHERMALLY ALTERED VOLCANIC ROCKS AND THE GENESIS OF KAOLINITE DEPOSITS: CAROLINA SLATE BELT, USA

Deposits of specialty clays in the Carolina Slate Belt contain trace element and radiogenic isotope variations and abundances (REE, Nd-Sr, Ga, As, Pb) that establish the origins of the resource. Major and trace element geochemical data also distinguish processes leading to remobilization, transport, and concentration of elements during weathering (Al, Fe, As, Pb, Se). The clay deposits contain assemblages of aluminum minerals and noncrystalline phases (e.g., feldspar, muscovite, sericite, Al-oxyhydroxide, kaolinite, etc.) that define pathways for mobile elements serving as environmental indicators. The largest clay deposits (e.g., Haile, SC) are attributed to saprolitization of crystal tuff. The tuffs also contain deposits of Fe, Au and other metals in ancient hot springs settings (Ca. 554 Ma). Chemical weathering of the volcanic rocks since the mid-Paleozoic has modified the major and trace element abundances and resulted in a white clay rock (~35% kaolinite, ~45% quartz, and ~20% sericite) with a distinctive trace element signature (e.g., 4-36ppm Ga, 0-7ppm Se, 0-22ppm As). Other rocks including silicastones (>80% quartz), and layers of concretions and crusts (goethite, hematite, lepidicrocite, kaolinite, Al and Fe-oxyhydroxides, and Fe-sulfate minerals). Alkali elements and Fe were removed from tuffs during clay formation, in contrast, REE, Th, Hf, and Zr were preserved. The clay rocks are LREE-enriched at ~50-100x chondrite and have Ce/YbN of ~2.0 to 6.6 and La/SmN=1.99 to 5.6. The HREE portion of the pattern is typically flat and 10x-30x chondrite with Tb/YbN of ~0.71-1.05. In general, negative Eu/Eu* anomalies of 0.62 to 0.75 characterize the clay rocks. The patterns can be explained by superimposition of LREE enriched and HREE enriched minerals (alunite, silica, phosphate minerals). Compared to clay rocks, concretions are enriched by factors of 50 in Se and 25 in As, whereas silicastones are depleted (by factors of 10); Ga is conserved in both rock types. The behaviors of REE and other trace elements, relative to the parent rock, are predictable based on the dominant mineralogy of the residual host (clay rock, silicastones, iron oxide concretions and Al-Fe-crusts) and a combination of acidic hydrothermal and saprolitic processes.