Geochronology, geochemistry and genesis of the Early-Middle Triassic Dongping-Zurong large manganese ore deposit, southwestern Guangxi

The Dongping-Zurong large manganese ore deposit is located in the southwest of Guangxi manganese ore belt which is an important enrichment region of manganese ore in Guangxi Province. The Beisi Formation of Lower Triassic which is deposited in a trench is the mineral hosted strata, and it is mainly composed of a set of siliceous and argillaceous rocks, together with limestones. Calcium rhodochrosite, calcimangite and mangandolomite are the main ore minerals. In this paper, the manganese mineralization age, genesis, sedimentary environment, tuffite source area and tectonic backgrounds of the deposit are studied through analyses of petrology, zircon U-Pb geochronology and geochemistry of the manganese ore and tuffites at the top of the formation. Zircon SHRIMP U-Pb dating results show that the zircons of three tuffites are in the shape of an autochthonous to semi-autochthonous plate-column, with well developed magmatic zircon oscillating belt structure. Three weighted mean Pb-206/U-238 ages of 250.8 +/- 2.1Ma(MSWD = 0.98), 250.6 +/- 2.2Ma(MSWD = 0.49) and 243.6 +/- 2.3Ma(MSWD = 0.44) have been obtained for the tuffites on top of manganese ore, which indicates that the "Dongping" type manganese ore was formed in Early to Middle Triassic (no later than 241 similar to 246Ma). The characteristics of trace and rare earth elements and multiple diagrams for discrimination of structural environment on tuffites display geochemical characteristics of typical subduction-related arc volcanic rocks, which indicated that mineralization may be affected by volcanic action from Pingxiang-Dongxing in Guangxi. Such ratios as U/Th, V/Cr, V/(V + Ni) and Ni/Co of redox sensitive trace elements from manganese ores indicate that the manganese ore deposits were formed in sub -oxidation to oxidation water conditions in Dongping-Zurong manganese ore deposit, southwestern Guangxi. The diagrams of logU-logTh, Fe/Ti-Al/(Al + Fe + Mn), the trace elements content of Ba, the ratios of Ba/Sr also show hydrothermal sedimentary characteristics, implying that hydrothermal activities played a vital role in the enrichment of manganese and the formation of the deposit. By comprehensive analyses on the ore deposit geology research results, Dongping-Zurong manganese ore is formed in the back-arc basin, and the hydrothermal activities played a vital role in the enrichment of manganese and the formation of the deposit. In addition, the manganese ore deposits were formed in sub oxidation to oxidation water conditions.