Field Observations and Stable Isotope Analysis of Alunite at the Sulphur Bank Mercury Mine, Clear Lake, California
The Sulphur Bank mercury mine is a historic mine located in Lake County, California, first exploited in the 1860s. Underground mining of mercury took place from 1873 to 1906 and open-pit mining of mercury was done from 1927 to 1957. The U.S. Environmental Protection Agency listed the Sulphur Bank mine and the adjacent Clear Lake as a Superfund site in 1990 due to mercury contamination at the mine site and in the sediment and food web of Clear Lake. Previous studies have described the vein material mineralogy and wall-rock alteration at Sulphur Bank as well as the composition of water in the flooded mine pit (Herman Impoundment). This study expands on those studies by investigating the stable isotopes of alunite, kaolinite, cinnabar, amorphous silica, and native sulfur. It is hypothesized that stable isotope measurements, previous field observations, and new observations are consistent with the shallow steam-heated zone of an active geothermal system capable of precipitating precious metals within a low-sulfidation epithermal ore forming system. The research objectives of this study are: i) to better understand the steam-heated environment at Sulphur Bank by studying the stable isotope systematics of alteration minerals, and ii) to assess the gold-silver mineralization potential of the Sulphur Bank geothermal system by comparison with epithermal deposits in the Geysers-Clear Lake area of northern California. For this study, a series of vein and wall-rock samples from the Sulphur Bank deposit 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. Stable isotope analyses have yielded δ34S values showing small differences between sulfate minerals (averaging +1.0 ±1.3 permil on the Vienna-Canyon Diablo troilite, or VCDT, scale) compared with native sulfur and cinnabar (averaging 0.0 ±1.2 permil VCDT); these values are consistent with S-isotopes being derived from a mafic magmatic source. Standard fractionation of measured δD, and δ18O values from alunite, kaolinite, and silica samples suggest parental fluids that are primarily meteoric in origin with calculated temperatures of 20 to 127 °C, indicating a range of temperatures consistent with the supergene and steam-heated environments of acid-sulfate alteration. Differences between δ18OOH and δ18OSO4 within alunite-group minerals show both isotopic equilibrium and disequilibrium suggesting low temperature re-equilibration of the sulfate hydroxide site to a range of temperatures as low as 20 °C after initial deposition at typical steamheated environment temperature of 110-130 °C. Data collected for this study is consistent with previous stable isotope studies of fluids at Sulphur Bank and confirm a primarily meteoric source of fluids. Other studies of springs and deposits in the Clear Lake region show similar fluid sources although notably gold-bearing systems display more isotopically enriched fluid values. This work was done in parallel with ongoing studies that are investigating active weathering processes and current groundwater and surface-water geochemistry of mercury and sulfur at Sulphur Bank. This study provides insights into the past hydrothermal activity that formed the alteration assemblages present at Sulphur Bank and help to provide more comprehensive geochemical characterization of the site and its evolution.
Muller, David E, "Field Observations and Stable Isotope Analysis of Alunite at the Sulphur Bank Mercury Mine, Clear Lake, California" (2023). ETD Collection for University of Texas, El Paso. AAI30523258.