Abstract:

Perpetua Resources Inc. (Perpetua) proposes to mine gold and antimony from three open pits at the Stibnite Gold Project, located in the historical Stibnite-Yellow Pine District in central Idaho, USA. Historical mining activities previously occurred within the project lease boundaries which have resulted in elevated antimony and arsenic concentrations in surface water and groundwater. The proposed mine plan includes the production of 115 million tonnes of ore and 280 million tonnes of development rock over a 20 to 25 year mine life. Geochemical characterization studies indicate that antimony and arsenic are elevated relative to average crustal concentrations and may be leached from development rock, which has the potential to contribute additional arsenic and antimony to downstream tributaries.

Perpetua has developed a mine plan incorporating reclamation of historical mine features in parallel with mine construction to reduce existing loadings and mitigate future increases in concentrations of antimony and arsenic. The mine plan has also been staged to sequence mining from the three open pits so mined out pits can be backfilled and capped and covered at closure. A detailed predictive water quality model was developed to evaluate changes to constituent concentrations in surface water during operations, closure and post-closure phases of the project. The model predictions indicate that mining of the Stibnite project will result in lower antimony and arsenic concentrations in comparison to pre-mining conditions. This paper presents a case study of integrating historic and future mine site remediation in mine planning activities to reduce constituent concentrations in downstream watercourses.

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Alan Haslam