Phosphorus-bearing strata in Maidiping Formation of the Lower Cambrian in Leibo area of Sichuan Province and its potential of phosphate resources

This paper is the result of mineral exploration engineering.

Objective The phosphate deposit accumulation scope of Maidiping Formation of the Lower Cambrian covers a wide area from Leibo County (Sichuan) to Yongshan County (Yunnan). The study of phosphorus-bearing strata is not only helpful to the reconstruction of the paleogeographic environment and its varied characteristics, but also can obtain the formation and distribution of phosphate deposits in combination with the study of tectonic background, and provide a thedeposittical basis for the development of phosphate deposit.

Methods Based on the cdeposit record and section measurement, this paper studies the thickness, lithology, sedimentary facies, sedimentary model, tectonic background, basin properties, genetic mechanism of phosphate deposit, petrological features of phosphorite, and their changes in Maidiping Formation in Leibo area.

Results The significant difference of lithology and lithofacies of the Maidiping Formation in Leibo area are within 50 kilometers, and the characteristics of phosphorite are well explained. It is pointed out that the Leibo area was an uplift and depression environment during the sedimentary period of Maidiping Formation, and the shallow shelf facies was absent. Dolomite facies area belongs to tidal flat facies, which is a favorable area for the phosphate deposit accumulation. Siliceous rocks belong to deep-water shelf facies and are of hydrothermal origin formed by volcanic activity. The accumulation of phosphorite is the result of volcanic activity in siliceous lithofacies area, biological development, movement of high temperature ocean current from the bottom to the top, sedimentation of the seawater with high P₂O₅ carried by ocean current decomposing biological remains in dolomite facies area on both sides of the sag area and post-sedimentation transformation. Based on the tectonic background, basin nature, source of phosphorus, migration, sedimentation and enrichment of phosphorus, the deposit-forming mechanism of phosphate is described, and the formation of phosphate deposit in Leibo area is suggested by the interaction of many factors, and the corresponding deposit-forming model of phosphate deposit is established. The siliceous rocks in the depression are the result of the hot water activity under volcanism.

Conclusions The study revealed a new phosphorus mineralization style in the late stage of rift basin development from Late Neoproterozoic to Early Cambrian, i.e., the paleogeographic pattern of alternating uplift and depression with the weak volcanic activity. A new idea of predicting and evaluating the phosphate deposit in dolomite facies area on both sides of the depression is put forward. We suggest that the prospecting space of the Maidiping Formation phosphate deposit should be expanded from the depression area to the surrounding areas.