Environmental Geochemistry of the Round Mountain Gold Mine, Nevada, and Mineralogical Controls on Acid Generation

Abstract

Abstract The Round Mountain mine in Nevada is one of the world’s largest volcanic-hosted precious metal deposits and has been commercially operated since 1976. The deposit is hosted by a thick sequence of Oligocene ash-flow tuffs and volcaniclastic rocks overlying a pre-Tertiary basement. Gold mineralization largely occurs as disseminated electrum in association with quartz, adularia, illite, smectite, pyrite, silver sulfosalts, manganese oxide, and iron oxides, with minor alunite, fluorite, and realgar in the upper portion of the deposit. The deposit was extensively oxidized during a period of Basin and Range faulting approximately 9 to 16 Ma. Multiple phases of geochemical characterization have been undertaken as part of project development, including over 740 acid-base accounting analyses and 38 humidity-cell tests operated for up to 193 weeks. These tests demonstrate that the Round Mountain host rock contains very little sulfide mineralization and show a low potential for acid generation and metal(loid) release. Where sulfide minerals are present, they are frequently partially or entirely encapsulated in nonreactive silicates or are present as coarse euhedral to subhedral pyrite grains that require more energy to oxidize compared to finer grained anhedral crystals. As such, there is likely to be a significant lag time to acid generation, and it is possible that acidic conditions may never develop under field conditions due to the combined effects of stable pyrite petrology and encapsulation, plus the lower leaching rates that would occur in the arid desert conditions in the field versus the laboratory. This prediction is supported by empirical field observations that indicate during the 40-yr period of operation there has been no acid generation associated with the waste-rock facilities. The multiple phases of geochemical characterization have been used to define procedures for operational waste-rock classification, segregation, and management at the Round Mountain mine based on lithology and oxidation.