A new palaeo-weathering and leaching model for the formation of the ultra-large high-grade Kaiyang phosphate deposit, Guizhou, China

The processes that formed the ultra-large and high-grade Kaiyang phosphate deposit, which is located in Guizhou Province, China, remain unclear. Here, we present the results of a petrological and mineralogical investigation of the compact and white lamination-bearing phosphorite within the deposit. These white lamination-bearing phosphorites contain many silicified collophane breccias that are surrounded by collophane cements. Electron probe microanalysis indicates that the phosphorite within the deposit is enriched in P 2 O 5 , SiO 2 , K 2 O, and Al 2 O 3 and is depleted in FeO, F, MgO, SO 3 , and Na 2 O compared with the more common clastic phosphorite rocks. In addition, the coated-grain phosphorite located beneath this layer contains a cement that was gravitationally transported and was formed by leaching. These new data indicate that the deposit represents a crust formed as a result of palaeo-weathering. The widespread presence of this type of phosphatic rock in the Kaiyang area indicates that palaeo-weathering played a key role in the formation of the phosphorite deposits in this region.