SYSTEMATIC EVALUATION OF MINE WASTE SITES FOR CRITICAL MINERALS RESOURCES – A CASE STUDY OF THE CALVERT TUNGSTEN MINE

At present, many legacy mine sites are seen only as liabilities: deposits mined out and left to produce environmental pollution to be cleaned up at public expense. Subsequent advances in mineral processing and increasing demand for specialty elements have encouraged a second look to these ‘exhausted’ mine sites. In addition to the potential for unconventional critical mineral resources, re-handling of these mined materials provides the opportunity for their remediation.

The Calvert Mine in Beaverhead County, Montana, was a producing open-pit tungsten mine until 1962. It is located on a classic calc-silicate skarn deposit. The mine is un-remediated with adjacent mine dumps comprising boulder-to sand-size material. Subdivision and composite sampling of these dumps provides an estimate of remaining mineral endowment at the mine site. Grades are highest in the fine (<3.175 mm) size fraction, up to 0.15 wt.% W (0.19 wt.% WO₃). Many tonnes of mineralized waste remain. Total area of the dumps is 29,966 m². Specific tonnage estimations depend on accurate LiDAR surveys and the degree of compaction in the waste piles. Optical petrography and automated mineralogical analysis demonstrate moderate scheelite abundances in the waste along with gangue beryl, calcite, chlorite, diopside, garnet, hematite, pyrite, and quartz. As the waste is already at surface and partially comminuted, quantitative mineralogy is the remaining hurdle to develop optimal processing flowsheets for critical mineral concentration and extraction.