GEOCHEMICAL SIGNATURES OF HISTORIC METAL MINING IN THE VIRGINIA GOLD-PYRITE BELT

 

 

Spatial associations of geochemically distinct types of mineral deposits are common in the Virginia Gold-Pyrite Belt, where both kuroko-type massive sulfide and low sulfide gold-quartz deposits were mined within individual watersheds in the early 1900s.  The Greenwood gold mine is located at the headwaters of Quantico Creek. Eleven km downstream, the creek flows past the reclaimed site of the Cabin Branch pyrite mine, a massive sulfide deposit hosted by metavolcanic rocks of the Chopawamsic Formation, which caused an AMD problem.  Both mines are within Prince William Forest Park. The National Park Service discovered anomalous concentrations of Hg at Greenwood, presumably related to Hg brought in for gold processing.  USGS studies show that Quantico Creek headwaters are near neutral and chemically unremarkable, although significant methyl Hg is present in waters ponded in shaft depressions and methyl Hg bioaccumulates in fish and amphibians. Rock and stream sediments were sampled at both sites and a soil survey was conducted to document the extent of Hg soil contamination at Greenwood, where a 41,600 m² area was sampled on a 20 m grid. Soils were collected below the root zone, typically at a depth of ~18 cm; profiles through leaf litter, humus, and soil were sampled at 10 sites.  Soil Hg concentrations range from <0.02 to 692 mg/kg Hg, of which 8% exceed 1 mg/kg Hg and define an anomalous area of 8,000 m²  that overlaps known and suspected mine workings.  Leaf litter Hg ranges from 0.02 to 1.53 mg/kg and correlates with Mn.  Quartz veins contain <0.02 mg/kg Hg. Hg tends to decrease and Al tends to increase with increasing soil depth. Hg concentrations in Quantico Creek stream sediments at Greenwood range from 0.37 to 3.67 mg/kg Hg, exceeding freshwater stream sediment quality guidelines (0.18 mg/kg threshold effect).   Massive sulfide ore at the Cabin Branch site contains 1% Cu, 6% Zn, 0.2 % Pb, and 3.4 mg/kg Hg.  Sediments upstream and downstream of Greenwood, at Cabin Branch, and at background sites all contained £ 0.09 mg/kg Hg. Sediments at Cabin Branch exceed guidelines for Cu, Pb, and Zn. Distinct chemical signatures for different mine wastes have implications for watershed reclamation because some approaches for treating AMD, such as constructed wetlands, may exacerbate methylation of Hg that was used for on-site gold  processing.