Acta Petrologica et Mineralogica

The Yangchang phosphate deposit is located within the shallow marine strata of the Meishucun Formation (∈₁ms) in the southwestern Yangtze block. Investigating the sedimentary environment evolution of the Meishucun Formation in this mining area holds significant implications for regional prospecting. This study conducts geochemical analyses of major and trace elements from the Meishucun Formation in the Yangchang phosphate mining area to explore the sedimentary evolution process and phosphate enrichment sites. The results indicate that the Meishucun Formation contains relatively high concentrations of CaO and MgO, with elevated loss on ignition. Notably, P₂O₅ is enriched in the upper part, while SiO₂ is enriched in the lower part. The first member of the Meishucun Formation (∈₁ms¹), formed during the early to mid-Cambrian transgression, exhibits multi-element enrichment characteristics. Specifically, the enrichment coefficients of U and Mo in the lower submember (∈₁ms^1-1) exceed 60 times the average crustal value, whereas those in the upper submember (∈₁ms^1-2) exceed 10 times the average crustal value. Significant differences in peak values of U and Mo enrichment are observed between the two submembers. The lower submember (∈₁ms^1-1) shows no Ce anomalies and weak negative Eu anomalies, while the upper submember (∈₁ms^1-2) displays weak negative anomalies for both Ce and Eu. These findings suggest that the first member of the Meishucun Formation was deposited in a subtidal low-energy environment, transitioning from a relatively anoxic environment in the lower submember (∈₁ms^1-1) to a relatively oxidized environment in the upper submember (∈₁ms^1-2). The second member of the Meishucun Formation (∈₁ms²), composed of endogenous sediments hosting phosphate ores, was formed in an intertidal or subtidal high-energy environment. Trace element characteristics suggest an anoxic environment, while negative Ce anomalies indicate an oxidized environment. This implies that the collophanite in the phosphorite of the second member crystallized and precipitated from supersaturated phosphorus-rich seawater above the subtidal oxidation-reduction interface under arid and hot conditions, subsequently depositing in a closed or semi-closed anoxic environment on the seafloor. The third member of the Meishucun Formation (∈₁ms³), consisting of endogenous sediments formed at the upper part of the supratidal or intertidal zone, inherits trace and rare earth element characteristics from the second member, indicating an oxidizing environment under dry and hot climatic conditions. In summary, the sedimentary environmental evolution of the Meishucun Formation can be divided into four stages: early transgression, middle transgression, late transgression, and regression, corresponding to climatic changes from arid or semi-arid and hot to warm and humid, then back to arid and hot. The seafloor also underwent four stages of sedimentary processes: reduction, oxidation, mainly focused on reduction, reduction and oxidation alternate with each other, and oxidation. We conclude that the Yangchang phosphate mining area was situated in the underwater intertidal or subtidal high-energy zones within the shallow shoals of the eastern Yunnan epicontinental sea, Yangtze Craton, where the formation and enrichment of collophanite are more favorable in relatively closed or semi-closed reducing environments or semi-reducing environments of seawater.

Yangchang phosphate mining area; Meishucun Formation; petrogeochemistry; collophanite; phosphorite; environmental evolution