The Age and Sedimentary Environment of the Kunyang Phosphate Deposit, Central Yunnan: Constraints from Re-Os Isotopes

Abstract

BACKGROUND The Kunyang phosphate deposit in central Yunnan is the largest marine sedimentary phosphorite deposit in China. The deposit is hosted in black shale of the Lower Cambrian Meishucun Formation. However, there is still some controversy on the material source of the phosphorite and the deposition age of the black shale. OBJECTIVES To precisely date the black shale and provide direct constraint for the depositional age of the phosphorite and to better understand the successive depositional environment evolution from the phosphorite to black shale in vertical profiles. METHODS Negative Thermal Ionization Mass Spectrometry (NTIMS) Re-Os isotope tracer and dating studies were performed on phosphorite of Zhongyicun Member and black shale of Shiyantou Member in the Kunyang phosphate deposit. RESULTS The Re-Os isochron age of black shale in the Shiyantou Member is 521.9±5.4 Ma, which directly constrains the deposition age of black shale in the Kunyang Phosphate deposit. The phosphorite and black shale samples have initial 187Os/188Os values from 0.6576 to 0.7671 and of 0.887, respectively. The phosphorite and black shale samples have 187Re/188Os values of 2.582-240.5 and 96.24-341.8, respectively. The δEu and δCe values of trace elements in the phosphorite and black shale show changes from low to high in the vertical profile. This evidence indicates that the formation of the Kunyang phosphorus deposit occurred in the period of rapid erosion in the mainland and a large amount of terrigenous debris involvement in diagenesis-mineralization also occurred. CONCLUSIONS Based on previous studies, the atmospheric oxygen content increased rapidly, aggravating the weathering of the continental crust and the input of a large number of highly radioactive terrestrial Os caused the initial 187Os/188Os initial ratio to increase gradually during the Cambrian Period. In addition, due to the rise of the global sea level, the paleo-marine environment of the Lower Cambrian Meishucun gradually changed from shallow water oxidization to deep water reduction. The Kunyang phosphorus deposit formed during the early stages of the sea-inward-cycle sedimentation.

Keywords

• re-os isotope
• black shale
• phosphorite
• transgression
• meishucun formation