GEOCHEMISTRY OF RARE EARTH ELEMENTS IN MARINE OIL SHALE – A CASE STUDY FROM THE BILONG CO AREA, NORTHERN TIBET, CHINA

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. The content and modes of occurrence of rare earth elements (REEs) in selected oil shale samples from the Bilong Co area were studied by inductively-coupled plasma mass spectrometer (ICP-MS) and statistical methods, respectively. The total organic carbon (TOC) content (6.75–19.20%) of oil shale samples is high with low or moderate total sulfur (St, d) content (1.05–2.00%) and intermediate shale oil content. The total rare earth element (∑REE) content of oil shale samples ranges from 63.69 to 117.85 µg/g. The average REE content of 13 oil shale samples from the Bilong Co area is slightly higher than that of USA coals and micritic limestone samples from the Bilong Co area, but lower than that of worldwide black shales. The oil shale samples from the Bilong Co area show shale-like chondrite or NASC-normalized REE patterns similar to those of micritic limestone samples from this area, indicating that REEs of these different lithological samples may have been derived from a similar terrigenous source. REE content of oil shale samples is highly positively correlated with ash yield and shows a positive correlation with Fe and a negative correlation with organic sulfur, and the vertical variations of REEs mainly follow those of Si, Al, K, Na and Ti. All these facts indicate that the REE content in oil shale seams is mainly controlled by clay minerals and, to a less extent, by pyrite, as well as partly associated with oil shale organic constituents.