A KEY GEOCHEMICAL VARIABLE APPEARS TO CONTROL WASTE ROCK ACID GENERATION

Potentially acid-generating (PAG) waste rock is conservatively defined as material with an acid-base accounting (ABA) ratio <1.2. At the Robinson Mine, Ely NV, 177 kinetic humidity cell tests (HCTs) run for up to 108 weeks generated pH, Fe and SO₄ data that support a mine-specific PAG cut-off of 0.3, independent of lithology or pit location. This resulted in re-classification of 125 million tons of former PAG waste rock as non-acid generating (NAG) waste rock, allowing shorter haul distances and improved mine economics. The similarity in results between the field samples from Brazil and the Robinson laboratory samples demonstrates that the cutoff is not simply an artifact of the lab testing regimen. At the Turquoise Ridge Joint Venture, NV the cutoff for 98 HCTs representing 10 separate lithologies ranges from 0.5 to 1.1. At Mine B in Brazil, 343 waste rock profile and 33 surficial waste rock grab samples that had been exposed to the elements for >8 years evinced a similar phenomenon, with a PAG cut-off of 0.4. However, at Mine D, the ABA cutoff for 63 HCTs was 1.6, further demonstrating the need for mine specific ABA ratios. The data for 28 Hycroft HCTs also results in a 0.3 cutoff, however the data set is heavily weighted to PAG, although not all PAG material generated acid, while at the Marigold Mine, NV, 574 waste rock and 31 HCT pH measurements with an ABA ratio >0.3 failed to generate sub pH-6 leachate. Post-mortem HCT microprobe analysis demonstrated that pyrite in waste rock with sub-1.2 ABA ratios was frequently encapsulated by quartz or calcite, while the perimeter surfaces of “nano-pyrite” particles exhibited secondary rinding by amorphous iron oxide which may also thwart the onset of acidification.