Geochemistry of terrestrial oil shale from the Lunpola area, northern Tibet, China

Abstract

Abstract The Lunpola oil shale is the largest oil shale resource in Tibet. Twenty-four samples of oil shale, mudstone, and dolomite were collected from the Lunpola oil shale for mineralogical and trace element analyses. The vertical distribution and mode of occurrence of trace elements in this terrestrial oil shale were also discussed. Oil shale samples from the Lunpola area have high TOC contents (5.1–10.6%), high ash yields (74.8–84.7%), low total sulfur contents (0.19–1.03%), and high shale oil contents (5.0–10.3%). The major minerals in the oil shale samples are clays, quartz, calcite and dolomite, with minor quantities of feldspar, siderite, hematite, magnesite and pyrite. Other minerals such as halite, zeolite, barite, anhydrite, and mirabilite were detected in some oil shale samples. The contents of As, Ba, Be, Bi, Cd, Ce, Cs, Hg, La, Li, Mo, Nd, Pb, Pr, Rb, Se, Sm, Th, U, W, and Zr are enriched from 1.30 to 125 times as compared to their average crust abundance (Clarke values), whereas the other elements are slightly higher or lower than their respective Clarke values. Compared to common Australian oil shale, the Lunpola oil shales contain higher As, Ba, Be, Ce, Co, Cs, Ga, Gd, Hf, Nb, Nd, Pb, Pr, Rb, Sc, Sm, Sn, Th, and Zr, but lower Cd, Cu, Lu, Mo, Ni, Se, Sr, U, V, and Zn. Elements associated with organic matter and/or Fe-sulfides are highly enriched in all oil shale deposits, suggesting that their speciation in oil shale is governed by general processes (e.g., high organic matter content and anoxic environment), whereas elements associated with clay minerals exhibit various enrichments indicating that their speciation in oil shale is governed by individual geochemical mechanisms (e.g., source rocks). The elements in the Lunpola oil shale may be classified into three groups of associations according to their modes of occurrence. Group A elements (TiO 2 , Sc, Hf, Zr, Be, Li, W, V, Zn, SiO 2 , Al 2 O 3 , Rb, Ga, Sn, Cr, Ta, Cs, K 2 O, Th, Na 2 O, REEs, and ash yield) have high positive correlations with ash yield and a mainly inorganic affinity. Group B elements (As, TOC, MnO, Ni, Fe 2 O 3 , Cd, Co, Cu, Mo, Se, MgO, Bi, Pb, and Hg) show a negative correlation with ash yield, and most elements possibly have a likely organic affinity and/or Fe-sulfide affinity. Group C elements (Sr, CaO, U, and P 2 O 5 ) show a weak or negligible correlation with ash yield (with the exception of CaO). Elements of environmental concern (As, Bi, Hg, Mo, Pb, Se, and U) are present in high contents, and are clustered in the second association, i.e. they are hosted mainly in Fe-sulfides and/or organic matter.