湖北恩施双河硒矿床地球化学特征及成因探讨

Geochemical and Genetic Characteristics of Selenium Ore Deposit in Shuanghe, Enshi, Hubei Province

湖北恩施双河硒矿床赋硒岩石的化学成分中SiO2含量不仅相对较低,而且变化范围较大;Al2O3,K2O,TiO2,MgO含量相对较高。主要氧化物比值与火山或热水成因硅质岩接近。在氧化物协变图解中样品主要投影在火山成因区内。Al/(A1+Fe+Mn)的平均值约为0.72,最大值为0.93,大于生物成因型硅质岩沉积物,说明有富Al的陆源物质加入。来自深源的微量元素Se,Te,Mo表现出超异常的富集;而V,Ag,Sb,Cr等元素也有较高程度的富集,这些特征与世界上火山成因硅质岩相似。更为重要的是在赋硒岩石夹层中首次发现了泥化的火山碎屑角砾,由此表明该矿床物质来源与火山碎屑沉积物之间有着密切的成因联系。同时研究区样品中的Co/Ni<1,U/Th>1;在P2O5-Y的相关图解中亦呈现出某些热水成因的特点。稀土元素δCe呈负异常,平均值为0.61;REE总量和同期、邻区的峨眉山玄武岩相近;HREE有富集趋势,北美页岩标准化稀土配分曲线以不同程度略向左倾斜,这些特征又表明该矿床在其成岩成矿过程中一定程度上受热水作用影响。

The relatively low SiO2 and high Al2O3, K2O, TiO2, MgO values and the ratios of SiO2/Al2O3, SiO2/ MgO, SiOJ（ K2O ＋ Na2O） of the seleniferous rocks in Shuanghe are similar to those of the volcano-sedimentary cherts. As seen in the TiO2-Al2O3 and （K2O ＋ Na2 O）-Al2O3 diagrams, the data points mainly fall within the field of volcano-sedimentary cherts. The ratios of Al/（Al ＋ Fe ＋ Mn） （0.72 in average, with a maximum of 0.93） are higher than the values of the biogenetic siliceous rocks. The extreme enrichment of Se, Te, Mo and high contents of V, Ag, Sb, Cr also show some igneous characteristics. It is more important for the first time to discover the argillized pyroclastic breccia in the interlayers of seleniferous rock in Yutangba digging. The data points of the seleniferous rocks in P2O5-Y correlation diagram along with the ratios of Co/Ni 〈 1 and U/Th 〉 1 all show some hydrothermal characteristics. REEs are characterized by total similarity in ∑REE concentration with Emeishan basalts, relative enrichment in HREE, negative Ce anomaly and gradually increasing NASC-normalized value with increasing atomic number of REEs. Generally, it can be judged from the analytical results of major elements, trace elements and REEs that the genesis of the seleniferous rocks in Shuanghe was closely related to the Permian volcanism synchronous with eruption of Emeishan basalts and belonged to the volcanic detrital sediment and accompanied by the hydrothermal activity.