Mineralogy and trace element geochemistry of the early Cambrian black shale-hosted Zhongcun vanadium deposit, southern Qinling, China
Xuerui Fu, Lingang Xu, Hao Yan, Huishou Ye, Jianhua Ding
Abstract
The formation of black shale-hosted vanadium deposits is controlled by several factors including metal supply, biological processes, and marine sedimentary environments. Vanadium-bearing minerals have a complex mineralogy in black shale-hosted vanadium deposits, and the implications for the mineralization processes are still poorly understood. This study investigated the mineralogy, rare earth, and trace element geochemistry of the early Cambrian black shale-hosted Zhongcun vanadium deposit in the southern Qinling area. Analytical results from the scanning electron microscope, laser Raman probe, and electron probe microanalysis reveal five V-bearing minerals in members I and II of the Zhongcun deposit, of which roscoelite is the dominant V-bearing mineral. Ore samples from members I and II of the Shuigoukou Formation have remarkably negative Ce anomalies, indicating that the surface water was oxic in the early Cambrian paleo-ocean in the southern Qingling area. Ni/Co, V/Cr, and Th/U ratios suggest that the vanadium ores formed in an anoxic environment, and Member II was more sulfidic than Member I. These features suggest a redox-stratified basin with a redox-variable sedimentary environment in the early Cambrian. The absence of Eu/Eu*-V correlation, and Y/Ho and Sm/Yb ratios indicate that vanadium was dominantly precipitated from ambient seawater and hydrothermal input was negligible. V-bearing minerals, including roscoelite, mannardite, and anatase, formed during late diagenesis. Subsequently, oxidized hydrothermal fluids overprinted previously formed V-bearing minerals and formed V-bearing goethite and sulvanite.