ARE TAILINGS FROM VOLCANOGENIC MASSIVE SULFIDE AND SEDIMENTARY-EXHALATIVE ZINC-LEAD DEPOSITS A POTENTIAL SOURCE OF CRITICAL MINERAL RESOURCES?

Waste from ore processing such as mill tailings, particularly those that contain high concentrations of sulfide minerals such as pyrite and pyrrhotite, can contain critical mineral (CM) commodities. Sulfide-rich tailings can also contain potentially toxic elements and acid-generating potential and may represent a significant environmental burden. Reprocessing tailings can serve the dual purpose of providing a novel source of CMs while also mitigating potential environmental risks. However, the concentrations and mineralogical hosts of the CMs within the tailings will impact the economic viability of tailings reprocessing.

Mill tailings samples from volcanogenic massive sulfide (VMS) and sedimentary-exhalative (SEDEX) deposits from the United States, Canada, and Finland were analyzed to determine the bulk CM content and the CM content of the sulfide minerals. Samples containing >2% sulfide minerals were selected for analysis by SEM-EDS, EMPA, and LA-ICP-MS. Chemical characterization reveals that CM concentrations vary both between deposit types and among sulfide grains within a sample.

Pyrrhotite grains from ultramafic VMS deposits contained the most consistent Co concentrations of ~200-600 ppm (mean ~415 ppm), while pyrrhotite grains from siliciclastic mafic VMS deposits contained a wider range of Co values (from 160-1200 ppm) with a higher mean (~750 ppm) concentration. Pyrrhotite grains from both ultramafic and siliciclastic mafic VMS deposits contained similar mean Ni concentrations (~500 ppm), but there are rare grains in the siliciclastic mafic VMS sample with significantly elevated Ni concentrations (up to 4200 ppm) that may represent inclusions or zoning within the pyrrhotite. Similarly, most siliciclastic mafic pyrite grains contained Co and Ni concentrations of ~500 ppm, although some rare pyrite grains likely contained Co-sulfide inclusions. Unlike in the other two VMS deposit types examined, pyrite grains from bimodal-felsic VMS and SEDEX deposits are only enriched in As (~750-1100 ppm).

CM concentrations in sulfide minerals are consistent with the bulk CM concentrations in the tailings. However, differences in CM content between individual sulfide mineral grains indicate that understanding this variability is key to understanding the resource potential of mine waste materials.