Sedimentary facies characteristics and metallogenic model of the lower Cambrian Maidiping formation in the Huangjiaping phosphate deposit, Mabian County, southern Sichuan

Abstract

The lower Cambrian Maidiping formation in the Mabian area of southern Sichuan is a crucial ore-bearing horizon for Kunyang-style phosphate ores. The sedimentary paleoenvironment controls the distribution of phosphate ore. Based on the field section measurements and borehole core observations of the phosphorus-bearing strata of the Maidiping formation in the Huangjiaping area of Mabian, the characteristics of the sedimentary phases, phosphate massifs, and the mode of phosphate ore genesis of the Maidiping formation were studied in detail. The study shows that the Maidiping formation has developed carbonate tidal flat sedimentary facies, and six micro-phases can be identified in three subphases: supratidal flat, strand flat, and subtidal flat. The supratidal flat includes supratidal beach and supratidal dolomite flat; the strand flat includes tidal channel, intertidal beach, and intertidal limestone flat; the subtidal flat is only developed with the low-energy subtidal flat, showing a sedimentary evolution sequence of sea recession and sea erosion in the vertical direction. Based on the finding, we established the bay–tidal flat depositional model in the Maidiping formation. The sedimentary facies strictly controls the enrichment of phosphorite, and the intertidal beach and tidal channel with high energy hydrodynamics in the strand flat is the most favorable environment for phosphorite formation. The sand–gravel phosphorite is the most widely developed phosphorite type in the Mabian area. The mineralization mode of phosphate deposits is that the rising ocean current brings phosphorus-rich seawater into the tidal flat environment of the bay, and the phosphorus condenses and accumulates in the form of colloid-chemical, by biological and chemical interactions, forming semi-solidified and weakly-solidified phosphate sediments, which are then subjected to water scouring, crushing, transporting, and precipitation again, and then formed into high-grade phosphate masses after compaction and consolidation.

Keywords

• mabian area
• maidiping formation
• sedimentary facies
• phosphorite
• metallogenic model