Acid-Base Accounting Tests in Combination with Humidity Cells Help to Predict Waste Rock Behavior

Potentially acid-generating (PAG) waste rock is defined in Nevada, USA as material with an acid-base accounting (ABA) ratio < 1.2, based on the acid neutralizing potential (ANP) to acid generating potential (AGP) ratio. However, waste rock humidity cell tests (HCTs) used to define PAG often fail to release the anticipated acidity. For example, the pH, Fe, and SO4 data from 150 lithology-specific HCTs run on Robinson Mine material for up to 108 weeks supported an empirical 0.3 ABA ratio PAG cut-off. At Turquoise Ridge, the cutoff for 49 HCTs representing current and prospective waste rock was 0.5; however, at Mine C, the ABA ratio cutoff for 63 HCTs was 1.6, demonstrating the need for a mine-specific analysis. The ABA ratio cutoff works at these three mines due to a wide range of ANP (e.g. 0.3-963 t of CaCO3 per Kt of waste rock) and AGP (0.3-495 t/Kt waste rock at Robinson). At the Hycroft mine, the 28 HCTs had a much smaller range of ANP (0.3-71 t/Kt waste rock) and AGP (3-147 t/Kt waste rock), so no ABA ratio cutoff can be defined. At the Marigold Mine, 31 HCT and 605 paste pH measurements with an ABA ratio > 0.3 failed to generate sub-pH 6 leachate. In this case, while the ANP ranged from 0.3 to 230 t/Kt waste rock, the AGP was too low to develop a plausible ABA ratio cutoff. Post-mortem HCT mineralogy demonstrated pyrite encapsulation in quartz or calcite in waste rock with sub-1.2 ABA ratios, while the surfaces of the micro-pyrite particles exhibited secondary rinds of amorphous iron oxide, which may also thwart acidification.