Isotopic and geochemical composition of the Georgia kaolins: Insights into formation and diagenetic conditions.

摘要

This study investigates the nature of the original sediments, the depositional environment(s), and diagenetic reactions that have occurred within kaolin deposits located in Georgia, U.S.A. The rare-earth element (REE) chemistry, organic geochemistry, and stable isotope geochemistry were all examined to provide further information on source, groundwater, and microbial influences on the kaolins.;The light REEs are associated with the phosphate mineralogy and have experienced variable degrees of fractionation and mobilization within the sediment during diagenesis. Older kaolins display a greater degree of light REE mobility whereas younger kaolins show very little REE fractionation. In all samples, the heavy REEs are associated with detrital zircons and do not display evidence of diagenetic mobility. Variations in light REE mobility appear to be due to variations in physical properties of the kaolins and the presence of acidic groundwater produced by organic acids.;Two different types of kaolin were discerned based on their total organic carbon contents, namely organic-lean and lignitic kaolins. The initial organic matter of lean kaolins has been microbially decomposed. The organic matter from the lignitic kaolins was derived primarily from terrestrial conifers and microbial lipids.;At least two major periods of iron sulfide crystallization occurred during the deposition and diagenesis of the kaolins. Initial marcasite crystallization occurred during deposition, which was followed by pyrite crystallization that formed overgrowths on marcasite. Marcasite burrows were formed as a result of sulfate-reducing bacteria while a continual supply of sea-water sulfate existed. The pyrite overgrowths, however, formed via disproportionate sulfur-reducing bacteria that utilized sulfur from the oxidation of marcasite.;Many of the original mineralogical and geochemical signatures have been altered via the various diagenetic process involving groundwater and microbial interactions. However, it appears likely that organic acids were important factors in all of these diagenetic alterations.