PHOSPHORITE ANALYSIS AND PROVENANCE USING SEM, REFLECTANCE SPECTROSCOPY, AND PETROGRAPHY, CHARLESTON, SC

The Charleston area was a phosphate mining district up until the early 20^th century. The phosphorites themselves are largely uncharacterized. The phosphorites in this area range in color from light brown/tan to dark grey/black, herein referred to as Type 1 and Type 2, respectively. This study focuses on the origin and character of eight cobble to small boulder sized phosphorite nodules and rounded clasts collected from four younger Pleistocene fluvial sand locations.

All phosphorites samples studied are cemented with cryptocrystalline dark phosphate identified as collophane. Type 1 samples displayed a higher content of clay-sized minerals and carbonates, while Type 2 samples were higher in silica with angular quartz clasts. While both shared similar fossil content, Type 2 phosphorite had fewer fossils and less carbonate. Semi-quantitative SEM chemical analyses were hindered by pervasive Ca-phosphate cement. Thus, the cement was removed with a 6-molar nitric acid treatment in order to analyze the residual mineral constituents. The results produced a number of accessory mineral constituents which proved to be useful for provenance analysis, including zircon, ilmenite, pyrite, hematite, monazite, and allanite. The presence of zircon, allanite, and possibly monazite suggest a granitic source rock for most samples. Abundant clay minerals identified using reflectance spectroscopy include kaolinite with minor kaolinite-smectite mixtures.

These samples indicate some reworking of originally marine phosphorite (Paleogene) with terrestrial input indicated by the angular clasts. The presence of zircon and allanite with monazite is consistent with granitic sources as well. The samples also display replacement of carbonate primarily with phosphate and in at least one case hematite, which is seen in the fossils.

Clay mineralogy, quartz and accessory minerals (e.g., zircon) are consistent with a Piedmont provenance, followed by multiple episodes of Ca-phosphate cementation. Based on our analyses, it appears that clasts in Type 1 and Type 2 phosphorite have slightly different histories, with greater terrestrial input for Type 2 samples.