DISTRIBUTION OF SELENIUM IN COAL WASTE ROCK, ELK VALLEY, BRITISH COLUMBIA, CANADA

Waste-rock spoils created by mining of metallurgical-grade coal contain Se-bearing pyrite/selenides. Exposure of these previously buried sediments to oxygen can result in the oxidation of these minerals to more soluble selenite (Se⁴⁺) and selenate (Se⁶⁺) resulting in leaching and movement into surface- and/or ground-waters. In this study, we defined the Se-bearing mineral phases and complexes in samples from coal waste-rock spoils from the Elk Valley of southeastern British Columbia, Canada. Analyses included inductively coupled plasma mass spectrometry (ICPMS) of acid digested waste rock samples (n = 289), electron probe micro-analyses (EMPA) (n = 12) and X-ray absorption near–edge spectroscopy (XANES) (n = 8). The ICPMS analyses showed that Se is present throughout the waste rock spoil depth. The Se concentrations showed great variability within each waste rock dump and between spoils. This variability was attributed to the heterogeneity of the waste rock. Comparison of the Se K-edge XANES spectra of the waste rock samples to those of standard reference compounds (FeSe, BaSeO₃, and Na₂SeO₄) of known oxidation states (-2, +4, and +6, respectively) indicated that Se occurs predominantly as Se⁴⁺ (selenite) and Se^2- (selenide) in the waste rock samples. Linear combination fit analysis of Se K-edge XANES spectra of the waste rock samples to that of standard reference compounds indicated that 5.7 – 35.7 % of Se in the waste rock occurs as pyritic Se; 28.7 – 48.1 % as organoselenium; 2.3 – 37.6 % as Se⁴⁺ incorporated in minerals; and 1.2 – 51.5 % as exchangeable, oxyhydroxide and clay adsorbed forms of Se⁴⁺. EMPA analysis (elemental X-ray mapping) confirmed that the Se in the waste rock is associated with pyrite, chalcopyrite, sphalerite, barite and iron oxyhydroxide forms. These analyses are being used to estimate the rates of release of dissolved Se from the waste rock, define possible internal geochemical controls on Se, and thereby help to inform options for waste rock management.