The Depositional Enviroment And C-O-Sr Isotopic Characters Of Witherite Deposits In Qingling-Daba Mountain Areas, SW, China

The Witherite deposits occur in the Daba Mountains and Southern Qinling region measures more than 300 km in length, extending from Sichuan Province along the border between Shanxi, Hubei and Henan Provinces on the north margin of the Yangtze Platform. At the present time more than 40 witherite deposits or witherite-barite deposits have been discovered in this ore zone. These witherite deposits constitute one of the largest barium mineralization zones in China, and are also among the world's largest witherite deposits. Ziyang Wamiao (WM) and Chengkou Bengkouwan (BK) Witherite deposits occur in the Lower Cambrian Lujianpiing Formation of black shale sequence, and Ankang areas's witherite deposits occur in the Lower Silurian Daguiping Formation of siliceous rocks, are studied by means of petrography, element and isotope geochemistry analyses.Witherite ores from WM and BK are dark-gray. Main structures are massive, laminated, lentoid and network. The dominant textures include rose-petal seriate, grainy and microcrystalline textures. Witherite ores from Ankang area are white or gray. Main structures are massive, laminated, lentoid and network. The dominant textures include grainy and microcrystalline textures.The dark-gray witherite ores show higher vanadium, molybdenum, uranium concentrations, higher U/Th and V/(V+Cr) ratios, lower (U+V)/(U+V+Mo) ratios than those in the white ores. suggesting a more strongly anoxic environment (possibly euxinic) in the Lower Cambrian age for the dark-gray ores from WM and BK, while the white ores from the Lower Silurian strata in Ankang areas may have been deposited under suboxic-anoxic conditions in absence of free H2S.The dark-gray Witherite ores from Lower Cambrian strata have significantly lower 87Sr/86Sr ratio (0.708278～0.708714) than the Cambrian seawater's 87Sr/86Sr ratio (0.709), indicating the Sr contained in Witherite was mainly from the volcanic materials and sediments in underlying Pre-Cambrian strata, contained Lower 87Sr/86Sr. While, the white Witherite ores from Lower Silurian strata have significantly higher 87Sr/86Sr ratio (0.708240～0.708981) than the Silurian seawater's 87Sr/86Sr ratio (0.707), indicating the Sr was mainly from the volcanic materials and sediments in underlying Cambrian-Ordovician stratum which contained relatively higer 87Sr/86Sr ratio.The carbon isotopes of the Witherite ores are-11.8‰～-19.8‰, indicating the CO32- in Witherite, was derived largely from degradation of organic carbon resultant, polycondensation and dehydroxylation of bio-organic matter in sediments at the early stage of diagenesis.We propose the "hydrothermal mineralization" metallogenic model for Barium mineralization (in general):at early diagenesis, heated sea water made Ba2+ concentrated from underlying strata during the leaching extraction process, upwelling through the channel of breaking structure, then form barite. With the precipitation and further burial of barite in sediments, form a relatively closed diagentic environment. Meanwhile, the organic carbon released HCO3- or CO32- to the pore water due to organic matter's sulfate-reducing, degradation, polycondensation and dehydroxylation process, resulting in an obvious increase in HCO3- concentrations of pore water, and then witherite precipitated, forming Witherite ore bodies.