Origin of the Mesoproterozoic Jingtieshan bedded barite deposit, North Qilian Mountains, NW China: Geochemical and isotope (O, S, Sr) evidence

摘要

The bedded Jingtieshan barite deposit, which occurs in the western part of North Qilian, NW China, is intimately associated with banded iron formation (BIF) and is hosted in weakly metamorphosed Mesoproterozoic clastic-carbonate rocks. Although numerous studies have focused on the BIF, the origin of the barite mineralization is poorly understood. The barite orebody occurs as a lenticular/stratiform unit overlying and locally interlayered with the BIF. In this contribution, we present a comprehensive and integrated study of the geochemistry and S-O-Sr-isotope characteristics of the Jingtieshan barite deposit, thereby constraining its origin and the depositional conditions in the Mesoproterozoic ocean. The ore is composed of barite, minor carbonate minerals, specularite, and other minerals and is characterized by low Sr concentrations (711-1469ppm), total rare earth element concentrations (7-28ppm), and La/Ce values, relative enrichment in light rare earth element, positive Gd anomalies, and elevated La-n/Ce-n, which indicate that mineralizing fluids were derived from seawater and submarine hydrothermal solutions. Barite separates yield S-34=19.5-29.1 parts per thousand and O-18=12.7-23.6 parts per thousand, which are consistent with a coeval seawater origin for SO42- modified bacterial sulfate reduction. Moreover, the Sr-87/Sr-86 values suggest that Sr, as well as Ba, was derived from contemporaneous seawater and hydrothermal solutions, with additional radiogenic Sr derived from late-stage hydrothermal alteration. The redox-sensitive trace element ratios of V/(V+Ni), V/(V+Cr), and V/Cr, combined with negative Ce anomalies (0.11-0.82), indicate that the barite formed under oxic marine conditions in a redox stratified ocean. Geological, geochemical, and isotope data support a sedimentary-exhalative origin for the Jingtieshan barite deposit, where barite was deposited under oxic conditions in a shallow marine environment when ascending Ba-bearing submarine hydrothermal