紫阳黄柏树湾和竹山文峪河毒重石矿床锶同位素及碳氧同位素研究

Carbon, oxygen and strontium isotopic studies of Huangboshuwan witherite depo sit at Ziyang and Wenyuhe witherite deposit at Zhushan, China

紫阳黄柏树湾毒重石矿床和竹山文峪河毒重石-重晶石矿床呈层状或似层状产于下寒武统下部或其相当层位的硅质岩中,矿体受岩性和岩相控制作用明显。对矿床中毒重石、钡解石和方解石的锶同位素及碳氧同位素的研究结果表明,形成这些矿物的碳主要来自沉积物中的生物有机质在早期成岩阶段经降解、缩合及脱羧基作用所形成的烃类物质或生物气;而锶主要为沉积物孔隙水中海水锶与沉积物中火山碎屑物质蚀变过程中所释放的锶的混合。毒重石形成于早期成岩阶段沉积物的孔隙水介质中,形成毒重石的成矿流体主要为早期成岩阶段沉积物中由海水、有机质组分和火山物质组分相互叠加和混合而组成的孔隙水有机成矿流体。毒重石矿石中广泛发育的生物碎屑及粒屑结构说明生物作用通过生物成因重晶石(bio-barite)的形式将海水中的Ba2+浓集并沉降于海底,形成钡矿床的初始富集体。因而,海水中生物作用和沉积物的早期成岩作用是形成本区毒重石矿床的主要机制。

Being stratiform or stratoid, witherite-barite deposit at Zhushan occur in the lower Lower Cambrian deposit at Ziyang and the Wenyuhe siliceous rocks and the orebodies are remarkably controlled by lithological characteristics and petrography. Strontium, carbon and oxygen isotopic studies of witherite, barytocalcite and calcite have shown that the carbon, involved in the formation of these minerals, was derived largely from hydrocarbons and biogenetic gases resultant from degradation, condensation and dehydroxylation of bio-organic matter in sediments at the early stage of diagenesis; the strontium was a mixture of strontium in pore water and that released from alteration of volcanic mater in the sediment. Strontium, carbon and oxygen isotopic studies unanimously demonstrate that witherite was precipitated in organic carbon-rich pore water medium during the early stage of diagenesis. Extensive occurrence of biodetritus and clastic texture in witherite ores strongly evidences that Ba^2＋ was concentrated and settled down in the form of bio-barite on the seafloor as a result of biological processes, thereafter forming the initially enriched orebodies of barium deposits. Biological processes in seawater and early diagenesis in sediments are the major ore-forming mechanisms of witherite deposits in the region studied.