FIELD OBSERVATIONS AND STABLE ISOTOPE ANALYSIS OF ALUNITE AT THE SULPHUR BANK MERCURY MINE, CLEAR LAKE, CALIFORNIA

The Sulphur Bank Mercury Mine (SBMM) is a historic mine located in Lake County, California. Initial mining of sulfur occurred from the 1850s to the early 1870s. Underground mining of mercury took place from 1873 to 1906 and open-pit mining of mercury was done from 1927 to 1957. In 1990, the U.S. Environmental Protection Agency listed SBMM and the adjacent Clear Lake as a Superfund site due to mercury contamination at the mine site and in the sediment and food web of Clear Lake. Previous studies have described the mineralogy of vein material and wall-rock alteration at SBMM and the composition of water in the flooded mine pit (Herman Impoundment). This study expands on these studies by investigating the stable isotopes of alunite, kaolinite, cinnabar, amorphous silica, and native sulfur. A working hypothesis is that previous and new field observations are consistent with an updated low-sulfidation steam-heated deposit model. For this study, a set of vein and wall-rock samples from SBMM were analyzed using powder X-ray diffractometry to determine mineralogy. These samples were separated into mineralogically pure concentrates for stable isotope analysis via mass spectrometry. Preliminary results have yielded δ³⁴S values showing small differences between sulfate minerals (averaging +1.0 ±1.3 per mil on the Vienna-Canyon Diablo troilite, or VCDT, scale) compared with native sulfur and cinnabar (averaging 0.0 ±1.2 per mil VCDT); the differences may represent S-isotope fractionation during mineral formation. Further results will provide a stable isotope suite including δD, δ¹⁸O_OH, δ¹⁸O_SO4, and δ³⁴S. These data will be compared to previous stable isotope studies of sulfate and silicate minerals in other steam-heated hot-spring environments. Further analysis will provide insights into the origin of the elements that formed the hydrothermally altered rocks at SBMM. This work is being done in parallel with ongoing studies that are looking into active weathering processes and current groundwater and surface-water geochemistry of mercury and sulfur at SBMM. This study will provide insights into the past hydrothermal activity that formed the deposit and help to provide more comprehensive geochemical characterization of the site and its evolution.