The marine redox evolution and the formation model for the early Cambrian Gongxi-Tianzhu barite deposits in the South China Block

Abstract

Late Ediacaran to early Cambrian is marked as a vital ore-formation interval of the Earth’s history, particularly for the bedded barite deposits hosted in cherts and shales. Although plenty of work has been carried out, controversies still exist, such as how the marine paleoenvironment changed in this critical interval, and where the origin of ore-formation materials (i.e., barium ions) was. To address these questions, here, we conducted systematic geochemical research on Gongxi-Tianzhu barite deposits in the South China Block, using redox proxies (MoEF-UEF, Mo-TOC, U-TOC patterns, and elemental ratios), REY patterns, and Sr isotopes. Redox proxies were used to reconstruct the paleoenvironment evolution in the Gongxi-Tianzhu region during the early Cambrian interval. Redox proxy results revealed that the bottom water was oxygen-depleted during most of the depositional time of the Niutitang Formation. The bedded and nodular barites in the Niutitang Formation were formed in the Sulfate-Methane Transition Zone (SMTZ) during the synsedimentary and early diagenetic processes. Formation of the barite ores depended greatly on the redox state variation of the bottom water and the barium remobilization in the sediments. The ore-formation event ceased when the supply of oxygen and sulfate from the upper water stopped, and the pre-stored Ba ions in the anoxic bottom seawater were exhausted and the SMTZ rose back into the water again. REY patterns and 87Sr/86Sr values recorded coeval seawater signals without hydrothermal activities during the studied interval. Based on the elemental and isotopic evidence, a three-stage redox-diagenetic model was established for the formation of the barite deposits in South China.