Developing lifecycle models for speciality clay deposits: applications of trace element geochemistry to kaolin deposits

摘要

Geochemical processes controlling the fate of trace elements have been modelled using geological and environmental features to help design better mining and waste management practices for clay deposits. Geological characteristics and environmental behaviours of kaolin deposits vary according to precursor host rock and climatic setting. Extensive deposits of speciality kaolin clays being mined in the Eastern United States are hosted by regionally extensive volcanic rocks that contained ancient hot springs, although the clay deposits represent high-alumina alteration zones attributed to more recent weathering of crystal tuffs. Examples of such deposits occur at Haile and Brewer, South Carolina. Weathering of the volcanic rocks resulted in a relatively pure, high-quality, white clay (c. 35% kaolin, c. 45% quartz, c. 20% sericite) with a distinctive trace element signature (e.g. <0.2-7ppm Se, <0.6-22ppm As). The clay rock contains remnants of hydrothermal sinter (thin beds of quartz and pyrite) and concretions 1-4 cm in length (2-5% by area) consisting of goethite, hematite, lepidocrocite, and kaolin formed by weathering of sinter. The concretions are local sinks for trace metals (e.g. 60-230 ppm Se and 11-511 ppm As). Geochemical modelling based on empirical data indicates that surficial processes acted to form local point sources highly enriched in potentially toxic elements relative to the clay rocks and that as long as the pH of groundwater is neutral, the system is a relatively secure repository for the trace metals.