Element geochemical characteristics of the Middle—Late Jurassic microclastic rock in the Easthern Junggar Basin: implications for tracing sediment sources and paleoenvironment estoration

The sand body at the bottom of the Middle Jurassic Toutunhe Formation from the Kamusite area in the eastern margin of the Junggar Basin is an important sandstone-type uranium ore bearing horizon. In this paper, thirty-three microclastic rock samples collected from the Middle-Late Jurassic Xishanyao, Toutunhe, Qigu and Kalazha Formations in the eastern margin of Junggar Basin were analyzed for REEs and trace elements by means of ICP-MS. The oxidation-reduction conditions of Xishanyao, Toutunhe, Qigu and Kalazha Formations and tectonic setting of their provenances and paleoclimate change are discussed to understand their implication for uranium mineralization. It is found that the relative B contents, Sr/Ba, B/Ga, U/Th, V/(V+Ni), V/Cr, Ni/Co ratios and other geochemical indices suggest that the the Xishanyao, Toutunhe, Qigu and Kalazha Formations were deposited in the fresh oxygenated water. The Fe²⁺/Fe³⁺ ratios indicate that the Xishanyao Formation characterized by the appearance of coal seam represent reduced environment, while the red bed in the upper Toutunhe Formation and Qigu Formation indicate a strong oxidation environment. From Xishanyao to Kalazar period, the Sr/Cu, FeO/MnO, Al₂O₃/MgO ratios indicate that the paleoclimate and paleoenvironment experienced variations from warm-humid to alternated dry-wet, semi-arid and arid.The top of the Xishanyao Formation and the bottom of the Toutunhe Formation are the key periods of dry-wet transformation. The CIA and ICV indexes show that the upper Toutunhe Formation, Qigu Formation and Kalazha Formation have relatively stable provenance. During the Xishanyao and early Toutunhe period, the tectonic activity around the Junggar Basin increased. The discrimination diagram of sedimentary tectonic environment shows that the provenances of the Xishanyao, Toutunhe, Qigu and Kalazar Formations in this study area were dominantly composed by felsic igneous rocks in the upper crust, with certain amount of continental tholeiite. The tectonic background was continental island arc, with part of the provenance hinted the tectonic background of continental margin. In summary, the paleoclimate and oxygenated paleowater transformation from Xishanyao period to Toutunhe period was crucial for large-scale sandstone-type uranium mineralization in this area.