A review of anomalous rare earth elements and yttrium in coal

Coal deposits have attracted much attention in recent years as promising alternative raw sources for rare earth elements and yttrium (REY), not only because the REY concentrations in many coals or coal ashes are equal to or higher than those found in conventional types of REY ores but also because of the world-wide demand for REY in recent years has been greater than supply. In addition to anomalies of enrichment or depletion of light-, medium-, and heavy-REY in coal deposits (normalized to Upper Continental Crust, Post-Archean Australian Shale, or North American Shale Composite), anomalies of redox-sensitive Ce and Eu, and, in some cases, of non-redox-sensitive La, Gd, and Y, could be used as geochemical indicators of the sediment-source region, sedimentary environment, tectonic evolution, and post-depositional history of coal deposits. Factors controlling REY anomalies in coal deposits include the geochemistry of terrigenous source rocks, ingress of hydrothermal fluids, influence of marine environments, percolating natural waters, volcanic ashes, and sedimentary environments of peat formation. Additionally, the smoothness of a normalized REY distribution pattern provides a simple but reliable basis for testing the quality of REY chemical analyses for coal and other sedimentary rocks.