Discussion On Forming Conditions And Process Of The Barite-Witherite Deposits In Lower Cambrian, Northern Daba Mountains, China

Many stratiform witherite deposits, located in the black rock series of Lower Cambrian on the border land of Shaanxi, Sichuan, Chongqing and Hubei provinces, make up a unique large barium mineralization belt of the world with stratiform barite deposits. Some of them are of superlarge scale. Most of the witherite deposits are hosted in the carbonaceous siliceous rocks of Lujiaping Formation with obvious sedimentary characteristics observed macroscopically. In order to provide scientific foundation for mineral predication and promote the rational exploitation and utilization of the ore belt, the paper analyzed the ore-forming conditions and processes of the barite-witherite deposits such as Huangbaishuwan, Miaozi, Bashan, and Wenyuhe.Data of major elements, trace elements, REEs and relative isotopes suggest that these deposits formed in the epicontinental hydrothermal environment. The accurate places were in a sag on the outer shelf probably. Semi-closed backwater had an important impact on the forming of deposits. Some data on organic geochemical indicate that the organic matters with high productivity are of sapropelic type. They derived from bacteria and algae in the sea and evolved into the phase of high maturation. Including CH₄, a great deal of hydrocarbons were pyrolyzed out.Strontium isotopic values of the ores show that barium was derived from the mantle. δ¹³C_pdb values of witherite are abnormal negatively, reflecting the participation of organic carbon. Values of sulfur isotopes suggest that seawater had provided sulfur for the formation of barite. Data of hydrogen and oxygen isotopes show that organic water was included in the ore-forming fluid.Hydrothermal water accelerated the migration and precipitation of barium, affected the characteristics of the ore-forming fluids as well as the redox conditions of the environment. Different types of paleo-hydrothermal water determined the diversities of these barite-witherite deposits. There was carbonate hydrothermal water field near the Daba Mountain Arc Fault. Plentiful anions of CO₃^2- got together in the backwater environment. At Wenyuhe and its neighborhood sulfate hydrothermal water fields have been detected. The upward removal of hydrocarbons (including CH₄) had a great impact on the forming of witherite.Factors that led to the paragenesia and separation of witherite and barite included the differences of their solubilities, chemical coprecipitation of several kinds of minerals and the redox reaction between CH₄ and SO₄^2- in the hydrothermal background. Under the circumstances of tectonic movement in the later, stratigraphic overturning led to the changing of orebody occurrences and the appearance of several alternations. Meanwhile the components, textures, structures and other aspects of the consolidated rocks were also changed.