Rare earth and major element geochemistry of Eocene fine-grained sediments in oil shale- and coal-bearing layers of the Meihe Basin, Northeast China

Abstract

The Meihe Basin is a Paleogene pull-apart basin. Long-flame coal, lignite and oil shale are coexisting energy resources deposited in this basin. Ninety-seven samples, including oil shales, coals, brown to gray silt and mudstone, have been collected from the oil shale- and coal-bearing layers to discover the rare earth element geochemistry. The total REE contents of oil shales and coals are 137–256μg/g and 64–152μg/g respectively. The chondrite-normalized patterns of oil shales and coals show LREE enrichments, HREE deficits, negative Eu anomalies and negligible Ce anomalies.The chemical index of alteration (CIA) as well as some trace elements is often used to reflect the paleoenvironment at the time of deposition. The results show that fine-grained sediments in both layers were deposited in dysoxic to oxic conditions and in a warm and humid climate, and coals were deposited in a warmer and more humid climate than oil shales. Oil shales and coals are both in the early stage of diagenesis and of terrigenous origin. Besides, diagrams of some major, trace and rare earth elements show that the fine-grained sediments of both layers in the Meihe Basin are mainly from the felsic volcanic rocks and granite, and that their source rocks are mostly deposited in the continental inland arc setting.The analysis of major elements shows that Si, Al, K and Ti, in both layers, are found mainly in a mixed clay mineral assemblage and that Si is also found in quartz. Sodium occurs primarily in clay minerals, whereas Ca is found mainly in the organic matter. In the coal-bearing layer, iron is mainly controlled by organic matter rather than detrital minerals. In contrast, in the oil shale-bearing layer, neither detrital minerals nor organic matter exert a control on the iron content. Analyzing the relationship between rare earth elements and major elements shows that REEs in the oil shales and the coals are both of terrigenous origin and are mainly controlled by detrital minerals rather than by organic matter. In both layers, REEs have no relationship with fine-grained phosphates, and during the weathering process, the REEs were not very mobile and were resistant to fractionation.