Geological Characteristics and Ore-Forming Conditions of the Mesoproterozoic Qingshagou Mn Deposit on the Southern Margin of the Dunhuang Block, NW China

Sedimentary manganese deposits are an important mineral resource and also play a significant role in restoring the ancient marine environment. The redox environment of the ocean during the Mesoproterozoic era has long been a subject of controversy. This paper takes a large-scale sedimentary manganese deposit in northwest China as the research object, attempting to analyze the changes in the marine environment during the Mesoproterozoic era and their impact on the Mn mineralization processes. In recent years, an exploration breakthrough has been made in the manganese deposits in the Annanba region on the southern margin of the Dunhuang block, NW China. The Qingshagou Mn deposit is the largest and most representative in the region. Geochemical characteristics show that the ores have low average ratios of Fe/Mn (<0.19), V/Cr (<1.20), V/(V+Ni) (<0.54), and Al/(Al+Fe+Mn) (<0.30) but high average ratios of U/Th (~0.95), and Y/Ho (~28). The Post-Archean Australian Shale (PAAS)-normalized rare earth element (REE) patterns exhibit that middle rare earth elements are slightly enriched with weak positive anomalies for both Y (0.93~1.01) and Ce (0.97~1.15) and weak negative anomalies for Eu (0.87~0.99). The δ13CPDB value of the ore ranges from −13.4‰ to −23.3‰, which is consistent with the δ13CPDB value range of organic matter. Synthesizing these findings, it is concluded that the Qingshagou Mn deposit formed in a suboxic to oxic environment, the ore-forming materials were primarily derived from a submarine hydrothermal system, and the changes in redox conditions promoted the Mn precipitation.