Phosphogenesis and mineralization backgrounds of South China during the Ediacaran Period

The phosphate deposits of the Ediacaran Doushantuo Formation of the South China are typical representatives of the late Neoproterozoic phosphogenesis and represent an important source of phosphate deposit resources in China. The mineralization backgrounds and formation mechanism of the Doushantuo phosphate deposits, however, are still controversial. The results of previous researches were systematically analyzed to reveal tectono-depositional pattern and ocean redox states of the South China during the Doushantuo Age, to clarify the distributional pattern of the Doushantuo phosphorites, and to construct their mineralization model. The results show that the South China exhibits overall depositional pattern of high in the west and north and low in the east and south, and the Yangtze Block displays an alternating platform (high) and basin (low) pattern. Meanwhile, the Ediacaran ocean was characterized by dynamic redox structure with a euxinic wedge. The Doushantuo phosphorites are mainly distributed on the margin of shallow-water Yangtze platform or shelf, which are subdivided into four phosphate ore concentration belts (i.e., western Hubei, northwestern Hunan, western Hunan and central Guizhou). Of these, the ore layers of western Hubei and central Guizhou phosphate ore concentration belts are mainly stratiform. The thickness of ore layers and ore grade of the Doushantuo phosphate deposits show apparent variations on a regional scale. In addition, ore and gangue minerals are represented by collophane, and dolomite and clay minerals, respectively; these phosphate deposits are dominated by colloidal and granular texture. The phosphorus element is primarily sourced from deep phosphorus-bearing seawater related to upwelling, continental weathering and hydrothermal fluids. In the oxic-anoxic transitional zones of shallow platform, the phosphates within sediment pore water are initially concentrated via organic and Fe-(oxyhydr)oxides pumping models. When the phosphate concentrations exceed saturation threshold, collophane begins to be precipitated, which would likely be broken, transported and redeposited to create phosphorites with a grain-supported texture. Moreover, the microbial phosphorites are mainly formed in a peritidal setting, as a result of initial phosphate enrichment similar to collophane and blocking effect of microbial mats on the diffusion of phosphate back into overlying seawater.