STREAM SEDIMENT GEOCHEMISTRY AT THE COLES HILL URANIUM DEPOSIT, VIRGINIA

A study to characterize baseline geochemistry of stream sediments around the unmined Coles Hill uranium deposit in Pittsylvania County, Virginia, was conducted. The deposit is located immediately west of the Chatham Fault, a structural feature dividing the crystalline rocks of the Virginia Piedmont to the west from the sediments of the Danville Triassic Basin to the east. This study encompassed stream reaches both upstream and downstream of the deposit. Because upstream and downstream reaches were located in different but adjoining geologic units, reference watersheds outside of the deposit footprint but within the same rock types as the downstream sites were also included.

Composite sediment samples were collected from 22 stream reaches and analyzed for mineralogy, element composition, carbon speciation, and clay content. Stream water was co-sampled for composition and field parameters (e.g. pH, specific conductance, discharge). A subset of these sites was also analyzed for radionuclides. Sediment composition was statistically analyzed with respect to spatial variations, and correlations between different elements were examined.

Sediments were mainly composed of quartz, feldspars, and micas, with clays, amphiboles, and aluminosilicates (mainly sillimanite) present locally. Carbon content was almost entirely organic, with values ranging from 0.02 – 1.79 wt.%, and showed a positive correlation with many metals, including U. Total U concentrations were lognormally distributed with a range of 0.6 – 3.4 mg/kg and a median of 1.45 mg/kg. Concentrations were statistically significantly higher in downstream sediments relative to upstream. Other relevant constituents included Th (1.9 – 10.8 mg/kg, median 4.1), Pb (7.8 – 75.6 mg/kg, median 9.6), V (12 – 110 mg/kg, median 22), Zn (4 – 69 mg/kg, median 13), As (<1 – 21 mg/kg, median 2), S (<0.01 – 0.03 wt.%, median <0.01), and Ra (0.4 – 2.8 pCi/g, median 0.9). Values were compared to sediments from the larger surrounding region collected by the National Uranium Resource Evaluation (NURE) program. Results provide data on sediment composition around an undisturbed ore deposit and identify compositional anomalies associated with the deposit that are critical to understanding potential environmental effects should the deposit be mined in the future.