CRITICAL MINERALS IN MINE WASTE: ENRICHMENT AND DEPORTMENT OF CRITICAL MINERALS IN GANGUE PYRITE AND PYRRHOTITE FROM VOLCANOGENIC MASSIVE SULFIDES

Mine waste could be an untapped source of critical mineral (CM) resources, but evaluating their potential requires understanding the mineralogical deportment of potentially valuable commodities. Gangue minerals, such as pyrite (Py, FeS₂) and pyrrhotite (Po, Fe_1-xS), may host CMs but are typically viewed as waste and separated during ore processing. The goal of this project is to link the concentrations and mineralogical deportment of CMs hosted in gangue Py and Po. Determining a link between CM concentrations in Py and Po and deposit type will allow these relationships to be used as a screening tool to identify and prioritize prospective mine waste "deposits”.

Gangue sulfide minerals from volcanogenic massive sulfide (VMS) mine tailings were used as a test case due to their high sulfide mineral content and geologic diversity. VMS deposits are classified by the associated volcanic and sedimentary host rocks, which range from mafic to felsic. The primary commodities and ore minerals vary, but these deposits generally contain >40% sulfides by mass. Ore minerals make up between 10 to 20% of that total, with the remainder, primarily Py and Po, partitioning into mine wastes. We analyzed sixteen flotation mill tailings samples from VMS mines to determine bulk mineralogy and chemistry. Subsequently, Py and Po grains from the tailings samples and an additional 13 ore samples (5 from the same deposit as a tailings sample) were analyzed by SEM, EMPA, and LA-ICP-MS to determine the concentration and distribution of CMs.

CM concentrations vary by both deposit type and among grains within samples. Although the mean Py- and Po-associated CM concentrations are consistent with bulk chemistry CM concentrations of the tailings, there is variability in individual grain measurements, likely due to elements occurring in both solid solution and as discrete mineral inclusions, highlighting the diversity of potential enrichment mechanisms. Py and Po from tailings and ore samples from the same deposit express the similar enrichment in CMs, suggesting that CMs hosted in Py and Po consistently report to the tailings. Py from bimodal-mafic deposits have the highest concentrations of cobalt (Co), while most Po have Co concentrations below the limit of detection. Conversely, both Py and Po from mafic-ultramafic and siliciclastic-mafic deposits have approximately the same median Co concentration.