Concentration and mode of occurrence of trace elements in marine oil shale from the Bilong Co area, northern Tibet, China

Abstract

Abstract The Bilong Co oil shale zone is located in the South Qiangtang depression. This zone, together with the Shengli River-Changshe Mountain oil shale zone in the North Qiangtang depression, northern Tibet plateau, represents a potentially large marine oil shale resource in China. With the aim of better understanding geochemistry of marine oil shale, 18 samples from the Bilong Co area are studied and 56 elements in them are determined. The contents of Mo, Se, Cd, Cs, As, Bi, U Rb, Pb, Th, Li, Sr, and Zn are enriched from 1.3 to 35.0 times as compared with the average concentration in the crust (Clarke values), whereas the other elements are slightly higher/lower than the respective Clarke values. Compared to common Australian oil shales, the contents of Ce, Co, Cs, F, Ga, Hf, Nb, Nd, Pb, Pr, Rb, Sc, Sn, Th, and Zr in the Bilong Co oil shale samples exhibit relatively high levels, whereas As, Br, Cd, Cr, Cu, Dy, Er, Eu, Ho, Lu, Mo, Ni, Se, Tb, Tm, U, V, Y, Yb, and Zn are relatively depleted. However, the distribution patterns of trace elements are quite similar indicating that trace-element speciation in oil shale is governed by general processes rather than by individual geochemical mechanisms within one particular sample. The elements in the Bilong Co oil shale may be classified into three groups of association according to their modes of occurrence, i.e. Groups A, B, and C. Group A (Si–Al–K–Na–Ti–Zr–Sc–Th–Sn–Nb–Ga–Ta–Be–Hf–Cs–Li–Rb–ash–REEs) is strongly correlated with ash yield and mainly has an inorganic affinity. Group B (Cr–W–Mg–Mn–Br–F–Sr–Ba) has weakly or negatively positive correlation coefficients (with an exception of F) with ash yield. Group C (Zn–Bi–Cd–Ca–Fe–Pb–Hg–Co–Cu–Ni–P–Mo–Se–U–As–V) shows negative correlation coefficients with ash yield (with an exception of Bi) and possibly has an organic affinity.