Organic Geochemistry And Ore-forming Environment Of The Large Witherite Deposits In Northern Daba Mountains, China

More than 40 witherite and barite deposits (or points) locate in northern Daba Mountains, which is a large and unique witherite metallogenic belt in the world. The ore bodies are hosted in the lower Cambrian Lujiaping silicolite formation. Previous studies show that there are close relations between organic matter and ore formation. However, the origin, type, and evolution of the organic matter and its significance in mineralization are still unclear. Moreover, using rock organic geochemistry to trace the ore-forming structural-sedimentary environment is lack of an essential research. Based on the traditional ore geochemistry studies, in this paper, we use modern analysis technologies (GC-MS, ESEM) to explore the organic geochemistry characters and ore-forming environment of the witherite deposits.Major elements, trace elements, REE and stable isotopes analysis of ores and ore-hosting silicolite formation show that ore-forming materials are marine origins. Barium in ores is originated mainly from basic volcanic rocks in the basement or deep hydrothermal waters. While, ore-related carbon is derived from organic matters in marine sediments. These organic matters are experienced an evolution by two mechanisms: decarboxylation and direct oxidation. Metallogenesis take place in the restricted basins of continent margin and the formation of witherite deposits are resulted from the mixing of hydrothermal fluids and sea waters.Comprehensive studies on Rock-Eval, TOC, chloroform bitumen"A" and biomarker indicate that: (1) parent organic matters are sapropel kerogen that transformed from marine plankton, algae and bacterium. The parent matters are most bluegreen algae and with the adding of thimbleful land organic compounds. (2) organic matters are in the high and ultra mature stage, which is thought to be influenced by both heating and biodegradation. (3) barium and organic carbon in ores shows the tendency of negative correlation, this suggests that, to some extent, barium accumulation is consistent with the evolution of organic matter maturity. (4) sedimentation and mineralization occurs in the water body environment characterized by deoxidize-weak oxidation and closure-openness.According to the regional tectonic-sedimentary evolution, the formation of witherite deposits was controlled by the evolution of regional growth faults. The growth faults not only supplied the transport channels for deep hydrothermal fluids which containing the ore-forming materials, but also produced a local restricted sedimentary environment that accumulate organic matters and benefit the sedimentation-mineralization to form. Moreover, episodic activities of the growth faults changed the sedimentary environment and resulted in the interbedding of witherite and barite ores. Furthermore, growth faults of different orders controlled the water body changing from closure to openness, and finally formed the zoning of witherite and barite deposits from south to north of the Daba Mountains.