Acta Petrologica et Mineralogica

The Cambrian life explosion is closely related to the redox environment of plaeoocean, and the Lower Cambrian phosphorite in the Upper Yangtze Block has recorded a relatively complete information of the Early Cambrian. The Yangchang phosphate deposit in western margin of the Yangtze Block is a super-large phosphate deposit occurring in the Meishucun Formation of the Lower Cambrian, which can reconstruct the paleomarine environment of this period. In this paper, ICP-MS and other techniques were used to analyze the elemental geochemical composition characteristics of the phosphorite and its surrounding rocks and explore the geological environment of the Yangchang phosphate deposit. The results show that the CIA value of phosphorite is 46.40~65.60, with an average of 57.43, indicating that the weathering degree is low, the total amount of rare earth element (ΣREE) in phosphorites in this deposit is relatively low (140.0×l0^-6~237.4×l0^-6), but yttrium is relatively rich (87.0×l0^-6~183.2×l0^-6). The North American standardized rare earth distribution pattern is relatively flat, the LREE/HREE is of 4.22~7.50, shows that the diffrernce of light and heavy rare earth elements is not obvious, with obvious Ce negative anomaly (0.22~0.42), Eu anomaly is not obvious with δEu of 0.90~1.21, and U/ Th ratio of 2.55~27.68, which reflect that the phosphorite was affected by a certain influx of hydrothermal fluid during the sedimentary of phosphorites. Combined with the analysis of δCe, V/Cr, Y/Ho, Sr/Cu and other elements, it is indicate that from the bottom of Dengying Formation in the Sinian Period, to the phosphate-bearing layer Cambrian Meishucun Formation, to the upper cover Niutitang Formation, the ancient ocean redox environment changes from the relative reduction environment at the bottom to the oxidation environment upward until the top becomes the relative reduction environment. The paleoclimatic environment is cold and dry-warm and moist alternatively appear. The salinity of ancient seawater is the evolution process of salt water to semi-salt water to salt water to fresh water. The above analysis shows that the phosphorite deposit in a relatively oxidized saline water and arid climatic environment, and the climatic environment is relatively arid during sedimentation, and may be affected by the influx of hydrothermal fluid.

phosphorite; Meishucun Formation; sedimentary environment; Yangchang phosphate deposit; western margin of the Yangtze Block