Geochemical Constraints on the Manganese Mineralization during the Latest Maokou Epoch of Permian——Example from the Nancha manganese ore deposit in Zunyi city, northern Guizhou

Key words: 
• Nancha manganese ore deposit /
• geochemistry /
• oxic to dysoxic /
• mineralization

Abstract: The manganese ore deposit in Zunyi is one of the important types of manganese ore deposits in the Permian in China. The Nancha manganese ore deposit is a medium-scale subtle one found in the manganese ore zone of Zunyi area in recent years, and is the representative of the Zunyi-type manganese ore deposits. Trace element, rare earth element, and carbon isotope geochemical studies on manganese-bearing rock in Nancha manganese ore deposit show that samples predating Mn-ore deposition have V/(V+Ni) ratios between 0.6 and 0.65 (close to 0.60),and V/Cr ratios from 0.71 to 2.05 (avg. 1.34), as well as moderate Ce negative anomalies, indicating a dysoxic water condition under which they were deposited. The samples of Mn-ore deposits yield V/(V+Ni) ratios close to 0.45, and V/Cr ratios between 0.71 and 2.05, as well as weak Ce negative anomalies, pointing to an oxic to dysoxic condition. Those samples postdating the Mn-ore deposition have V/(V+Ni) ratios between 0.65 and 0.85 (avg. 0.76), and V/Cr ratios of > 4.18 mostly, as well as slightly Ce positive anomalies, suggesting an anoxic to dysoxic condition. The δEu values range mostly between 0.50 and 1.00, indicative of a weak influence of hydrothermal activity. Carbon isotope data reveal an obvious negative excursion across the Mn-bearing succession, reaching the nadir in the manganese ore horizon (-12.5‰ V-PDB), indicative of organic involvement likely associated with organic decaying. Therefore, the Nancha manganese ore deposit was initially precipitated as manganese (hydro-)oxides in the oxic water condition, which were reduced to Mn²⁺ and subsequently combined with CO₃^2- to form the carbonate manganese ore deposit (MnCO₃) below the sediment-water interface while the redox interface was gotten rising due to the sea-level rise (or transgression).