RARE EARTH ELEMENT ANOMALIES IN COAL FROM THE MIDDLE JURASSIC YAN’AN FORMATION IN THE ORDOS BASIN

The Ordos Basin is an important oil and gas producing basin in China. Particularly, the No. 2 coal seam of the Middle Jurassic Yan’an Formation in the Ordos Basin has yielded significant economic resources from mining activities, although some aspects of its geologic evolution are still unknown. For example, the rare earth elements (REE) concentrations in the No. 2 coal seam display anomalies that previous studies have difficulty interpreting or attributing to any single depositional mechanism.

In this study, we performed 1) coal geochemical analysis to determine the patterns of REE enrichment/depletion across stratigraphic intervals and 2) mineralogy of the No. 2 coal seam in the Jinjitan coal mine in northern Ordos Basin. The normalized REE plots show four types of variations: L-type (La_N/Lu_N>1), M-type (La_N/Sm_N<1, Gd_N/Lu_N>1), H-type (La_N/Lu_N<1), and N-type (slight difference between LREE and HREE). Three types (L, M and H) mainly appeared in the coal seam near the gangue, suggesting there has been volcanism during the coal formation. The N-type suggests some coal seam formed under stable basin conditions with little input of debris. The minerology shows pervasive halites and sylvites, which indicates that there was an abundance of saltwater during the coal formation. The presence of fused quartz grains along with blurred boundaries between the altered minerals and unaltered tuff signify high temperature events, and provide additional evidence of volcanic activity during the coal formation.

The REE analyses of the No. 2 coal seam shows that the volcanic materials bear felsic provenance, likely from the contemporaneous active continental margin of the continental island arc. The observed REE anomalies were most likely caused by volcanism, and thus presents new insights into the geologic history of the Middle Jurassic Ordos Basin.