The Hycroft Mine is an open pit gold mine situated along the eastern edge of the Black Rock Desert in NW Nevada, a semi-arid region where annual evaporation significantly exceeds precipitation. The Hycroft deposit is hosted in volcanic breccias and conglomerates of the Tertiary Kamma Mountain volcanic group that have been faulted by north-trending structures that acted as conduits for hydrothermal fluids. An acid-leaching event resulted in the development of vuggy, quartz alunite-altered zones produced by the removal of most minerals, including those capable of buffering acid. Accessory minerals associated with the acid leach zones are rare, except native sulphur which is locally abundant. The complex sulphur mineralogy of the Hycroft deposit, coupled with the deficiency of acid-neutralizing minerals, limits the application of traditional Sobek-style Acid-Base Accounting (ABA) methods that rely upon sulphur speciation data to predict acid-generation potential. A comparison of results from geochemical predictive tests including the modified Sobek ABA method, Net Acid Generation (NAG) test and kinetic humidity cell test (HCT) illustrates the complexity of interpreting ABA data for material containing native sulphur and low solubility sulphate minerals such as alunite.
Comparing ABA data to HCT results shows the predictions of acid generation from the ABA test are not consistent with those from the HCT, which is considered a more definitive test for the prediction of acid and metals mobility. The results of this study indicate the ABA test does not provide a reliable prediction of acid generation for samples containing native sulphur and low solubility sulphate minerals. A reasonably good correlation is, however, observed between the HCT and NAG results, suggesting the NAG test is a more effective tool for predicting the geochemical behavior of the Hycroft deposit, and potentially also other deposits with complex sulphur mineralogy. The presence of native sulphur in the acid leach material presents additional challenges with respect to acid-generation prediction due to the unique properties of this mineral. Although acid generation is predicted from the NAG test for samples containing native sulphur, the actual amount of acid and metal release generated by this mechanism in the HCT is limited in comparison to that associated with natural pyrite oxidation, which is the main contributor to acid rock drainage. The potential for acid generation from this mechanism is further limited by the semi-arid site conditions that are unlikely to produce conditions similar to the humidity cell tests that are designed to accelerate the weathering process and enhance mineral-water reaction rates. This study demonstrates the importance of considering the site-specific mineralogical characteristics of the deposit in the selection of the most appropriate geochemical test methods to use in the prediction of potential acid generation.