Origin of the Ediacaran Weng'an and Kaiyang phosphorite deposits in the Nanhua basin, SW China

摘要

The Weng'an and Kaiyang phosphorite deposits are located in Central Guizhou, southwestern China, and formed within the Sinian (Ediacaran) Doushantuo Formation. A systematic investigation of the geology, mineralogy, and geochemistry of these phosphorites was conducted to constrain the redox environment, origin, and genetic mechanisms of Ediacaran phosphorite. In the lower ore layer (layer A), the phosphate minerals are mainly globular phosphate intraclasts, which are characterized by special sedimentary and reworking textures, including microgranular sedimentary, isopachous cement, shrinkage crack, and spongy texture. The phosphate minerals from the upper layer (layer B) have microbial phosphate components, namely embryo- and algae-like microfossils. These phosphatic microfossils have biological structures, and both consist of interior structure and isopachous wall. The P2O5 concentration of phosphorites in layer A reach 32.31%, somewhat higher than that in layer B (28.7%), whereas the P content in globular phosphate intraclasts (40.05%) resemble that of the microbial phosphate component (41.19%). Rocks from layer A have "left-inclining" post-Archean Australian shale (PAAS)-normalized rare earth element (REE) distributions, with higher Ce anomalies (Ce-anom) ranging from -0.12 to -0.01 (average -0.07). In contrast, rocks from layer B have "hat-shaped" PAAS-normalized REE distributions, with lower Ce-anom ranging from -0.32 to -0.23 (average -0.28). These geochemical characteristics suggest that hydrothermal fluids mixed with normal seawater might have contributed to metallogenesis, and the redox environment transitioned from anoxic to oxic from layer A to B. We conclude that the lower Doushantuo globular intraclasts formed by the mechanical reworking of previous phosphatic sediments, which were dominated by the strong mechanical power of seawater. The upper Doushantuo microbial phosphorite, on the other hand, formed by microbially mediated accretionary growth.