Sciacca, JE, Wielinga, BW, Johnson, BS, Riese, AC & Andrews, CB 2023, 'Attenuation of acid rock drainage from buffering by naturally occurring calcite at the Leviathan Sulfur Mine, California', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, https://doi.org/10.36487/ACG_repo/2315_053
(https://papers.acg.uwa.edu.au/p/2315_053_Sciacca/)
Abstract: The former Leviathan Sulfur Mine is located within the East Fork Carson River drainage of California and Nevada. The sulfur was deposited as part of a high sulfidation epithermal system common to the Walker Lane structural belt. Acid rock drainage (ARD) occurs at the site and has impacted groundwater and surface water. The oxidation of pyrite and, to a lesser extent arsenopyrite and sulfur, within hydrothermally altered ore rock and mine spoil (waste), is the source of ARD. The ARD is characterized by low pH, and high sulfate and metals concentrations. 
Calcite is present within unaltered country rock, comprised of volcanic and volcaniclastic sedimentary units, outside of the hydrothermally altered core and ore deposit area. The calcite has been mapped as occurring as fracture and vug-filling cement within lahars and sandstones, and filling fractures and vugs in basalt and andesite. In addition, a downslope sandstone outcrop containing fracture-filling calcite has been observed and documented. The presence of calcite was further confirmed by x-ray diffraction analysis of rock samples from boreholes within sedimentary units downslope (downgradient) of the mine; and was additionally confirmed in unaltered rock in 25 boreholes surrounding the altered area. 
A literature review showed calcite is commonly found in such epithermal deposits throughout the region. The calcite precipitates as platy crystals filling voids or crusts filling fractures on the periphery of shallow alteration zones at temperatures generally less than 1800 C. The precipitation occurs through exsolution of CO2 in boiling fluids from the volcanic-hydrothermal system.
Sampling and analysis of groundwater from monitoring wells shows evidence of buffering of ARD by calcite vertically and laterally downgradient of the mine. This included an increase in bicarbonate and pH coupled with decreasing metals and metalloid concentrations in wells that had calcite confirmed in the boreholes, compared to well boreholes without calcite. The mapped extent of ARD also shows the vertical and lateral extent is limited by the pervasive occurrence and buffering by calcite in bedrock underlying the shallower unconsolidated unit.

Keywords: acid drainage, calcite, pyrite, natural attenuation