Concentrations and modes of occurrence of platinum group elements in the Shengli River oil shale, northern Tibet, China

Abstract

The Shengli River–Changshe Mountain oil shale zone, located in the North Qiangtang depression, northern Tibet plateau, represents a potentially large marine oil shale resource in China. Twenty-one samples including oil shale, micritic limestone and marl were collected from the Shengli River area to determine the contents, distribution patterns and modes of occurrence of platinum group elements (PGEs) in marine oil shale. Oil shale samples from the Shengli River area have high ash yield (61.86–67.48%) and total organic carbon content (8.02–13.67%) with low total sulfur content (0.76–1.39%) and intermediate shale oil content (3.60–16.30%). The total PGE content in oil shale samples is very low (average 1.749 ng/g), approximately half the mean value of Chinese coals, but higher than that of USA coals. PGEs in oil shale samples from the Shengli River area are characterized by high Pd (average 0.82 ng/g), Os (average 0.15 ng/g) and Pt (average 0.66 ng/g) contents compared with Ru (average 0.05 ng/g), Rh (average 0.05 ng/g) and Ir (average 0.02 ng/g). Distribution patterns of individual PGEs are not uniform in the Shengli River oil shale section. The lower marl layer is characterized by the highest Ru and Rh concentrations, whereas the upper marl layer is characterized by the highest Os and Ir concentrations. The highest Pt and Pd concentrations are found in the lower and upper parts of oil shale seams, respectively. The individual PGEs in the oil shale samples from the Shengli River area exhibit various modes of occurrence. Ru is mainly present in P-bearing minerals and partly associated with the clay and Ca-bearing minerals. Rh is mainly controlled by clay minerals and partly present in the form of apatite. Os occurs mainly in clay minerals and, to a lesser extent, in pyrite and apatite. Pd and Ir are mainly associated with Ca-bearing minerals and apatite. Pt is mainly present in Ca-bearing minerals and partly related to organic matter and apatite.