POTENTIAL CRITICAL MINERAL AND BASE METAL RESOURCES IN NON-FUEL MINE WASTE FEATURES IN THE FOUR CORNERS REGION, USA

Significant economic and technological changes have occurred in parallel with domestic mining production; however, these changes may not have paralleled rapid increases in demand for critical minerals associated with the green energy transition and other needs of humankind. As a result, there may be significant metals in mine waste, requiring new characterization of those features. The U.S. Geological Survey’s (USGS) Earth Mapping Resources Initiative (Earth MRI) project is funding State partners and the USGS’ USMIN project to build a national inventory of non-fuel mine waste features. USMIN has focused on mine waste features greater than 200,000 m², and we recently completed a dataset for the Four Corners region (AZ, CO, NM, UT). Combined, these large features cover ~420 Mm² and we estimate their total volume as ~20,000 Mm³. Approximately 85% of that total volume is derived from calc-alkaline porphyry Cu deposits that were historically exploited for Cu, Au, and Ag, and these waste features may contain further metals.

We use approximate values to estimate the endowment of commodities in these combined porphyry Cu waste features. Assuming an average waste density of 1.8 t/m³ yields approximately 31 Gt of porphyry Cu mine waste, approximately 60% of which are stockpiles and tailings. Compiling grades in the region suggests that ore may have initially contained approximately 0.56 ± 0.16% Cu, 0.02 ± 0.01% Mo, 0.1 ± 0.05 g/t In and 1.01 ± 0.62 g/t Re, based on variations in available assay data. We estimate initial extraction efficiencies of 85 ± 10% for Cu and 30 ± 5% for other metals of interest for the materials from stockpiles and tailings and estimate the remaining waste rock nominally contains 5% of these materials. Therefore, we estimate the combined porphyry Cu waste features may contain approximately 19 ± 5.9 Mt Cu, 2.7 ± 1.4 Mt Mo, 1.3 ± 0.66 kt In and 13 ± 8.1 kt Re. This endowment is hypothetical, underscoring the need for detailed characterization of mine waste. However, the Cu endowment is approximately equivalent to the endowment of a giant to supergiant Cu deposit, which are the largest 1-10% of Cu deposits on Earth. These data suggest that reprocessing mine waste may help provide base and critical minerals to support the renewable energy transition.