GEOCHEMISTRY AND MINERALOGY OF MINE WASTE AT SULPHUR BANK MERCURY MINE, LAKE COUNTY, CALIFORNIA

Sulphur Bank Mercury Mine (SBMM) is located on the eastern shoreline of the Oaks Arm of Clear Lake in Lake County, California. The site was mined intermittently from 1865 to 1957, producing approximately 4.4 million kilograms of mercury (Hg) from 272,000 metric tons of ore and generating about 1.9 million cubic meters of waste. The U.S. Environmental Protection Agency (EPA) listed the SBMM as a Superfund site in 1990 because of elevated levels of mercury (Hg) in fish in Clear Lake and exposure of local residents to toxic contaminants in mine waste. Since 1990, EPA has taken 9 remedial actions on the site and has proposed a large-scale cleanup plan for the mine site and for off-site contaminated residential soils. Mine waste at the site has elevated levels of Hg, arsenic (As), and antimony (Sb). The planned remediation will consolidate piles of ore, calcined tailings (from retorting), and waste rock, and subsequently cap them in place. The composition of the caps will be determined by factors including the chemistry and mineralogy of the waste. To support EPA’s efforts, the U.S. Geological Survey (USGS) is characterizing the geochemistry and mineralogy of the mine waste. Several composite samples were collected from the major waste piles during 2020–21, and additional composites will be generated from material collected during drilling of groundwater monitoring wells by EPA in 2022 in the Waste Rock Dam area between Herman Impoundment (a flooded mine pit) and Clear Lake. The composite mine waste samples are being analyzed for total metals (60 elements), metals by 5-step sequential extraction, As and Hg bioaccessibility, Hg isotopes, sulfur isotopes, and mineralogy by x-ray diffraction (XRD, semi-quantitative), scanning electron microscopy, Raman spectroscopy, and electron microprobe analysis. Results for material < 0.063 mm from 14 composites indicated concentrations (in milligrams per kilogram, mg/kg) in the following ranges: waste rock As 10–227, Hg 47–3,130, Sb 8–440; tailings As 8–71, Hg 24–80, Sb 12–125; and ore As 8–52, Hg 1,900–9,110, Sb 32–244. Mineralogy based on XRD indicates the following minerals are most abundant (in decreasing order): amorphous silica, kaolinite, plagioclase, quartz, hematite, alunite, mica, gypsum, and jarosite. These and subsequent results will be used to inform EPA’s cleanup actions.