89 Mineralogy/Crystallography (Posters)

PETROGRAPHY AND URANIUM MINERALOGY OF SC DHEC WELL CORE, JENKINS BRIDGE ROAD, NEAR SIMPSONVILLE, SOUTH CAROLINA

Paper number: 89-3
Program: Technical Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

High levels of uranium were recently discovered in well water south of Simpsonville, South Carolina. In order to characterize the mineralogical source of the uranium and possible structural controls on its presence, South Carolina Department of Health and Environmental Control (SC DHEC) cored a deep well on Jenkins Bridge Road in early 2003. The recovered core was scanned with a gamma-ray spectrometer to identify areas of high gamma-ray activity. The highest counts are derived from a coarse granitic zone at a depth of approximately 170 feet. A slickenlined fault plane at 159.5 feet and narrow pegmatite layers at depths of 371, 666 and 679 feet also yield high gamma-ray counts. Thin sections were made from the above materials and from subvertical healed fractures at 298 feet and 468-470 feet; a carbonate-rich zone and associated garnet-epidote skarn at 392-393 feet were also sampled.

Uraninite and coffinite are the principal U-rich minerals in the core. Other U-bearing minerals include zircon, monazite and allanite. Discrete small crystals of uraninite occur in the coarse granite and in the pegmatite layers. They contain between 3.4 and 4.4 weight percent PbO. Coffinite is present as tiny crystals dispersed along microscopic fractures in the granite and pegmatite, plus along the slickenlined fault plane. Coffinite is sometimes intergrown with its Th-bearing analogue, thorogummite. Compared to uraninite, coffinite and thorogummite contain lower Pb, often below the detectability limit. Together the textural and compositional evidence point to primary uraninite crystallization with subsequent mobilization of uranium and secondary precipitation along fractures as coffinite.

Calcite occurs in the core as fracture-filling crystals together with epidote + fluorite +/– prehnite, and in the carbonate-rich zone (diopside-scapolite marble). Its presence is consistent with the carbonate signature shown by nearby high-uranium well waters.