RARE EARTH ELEMENTS AND YTTRIUM (REY) IN COAL MINE DRAINAGE FROM THE ILLINOIS BASIN, USA
Our results show that the CMD in the Illinois Basin exhibit larger heterogeneities of REY patterns than those described in other USA coal-mining districts. Significantly, the Illinois CMD have higher contents of REY and critical-REY, with Y and Nd being the main critical-REY. Furthermore, CMD in the Illinois Basin were found to contain small amounts of economically valuable metals, including Li, Zn, Ni, Co, and V, that could potentially be co-extracted with REY to enhance the economic values of Illinois CMD.
Synthesis of geochemical data suggests that Mississippi Valley-Type (MVT) hydrothermal activity that affected coal-bearing strata probably played a key role in producing CMD enriched in economically valuable metals. Specifically, our data points toward two sources of REY enrichment in Illinois CMD, namely (1) coal mine wastes containing solid matrixes with high Al and Si contents (i.e., clay minerals and silicates), and (2) the extent to which coal-bearing strata were affected by hydrothermal solutions enriched in metals (i.e, Zn, Ni, Co, and REY). The sites with the highest contents of REY and critical-REY were predominantly located along the western and northern margins of the Illinois Basin. Sites with the highest hydrothermal input as approximated by Mississippi-Valley-type mineralization are prime candidates for REY and other metals (i.e., Zn, Ni, Co, Cu), thus making Illinois CMD a potentially attractive alternative domestic source of extractable REY, especially critical-REY, as well as economically valuable metals.