秦岭山阳水沟口组黑色岩系微量元素地球化学及其沉积成矿背景的指示意义

Trace element geochemical characteristics of the Shuigoukou Formation black rock series in Shanyang area of the Qinling Mountains and their indication significance for sedimentation-mineralization

通过野外调研发现,秦岭造山带山阳地区水沟口组黑色岩系主要由黑灰色微晶灰岩、炭质泥板岩、灰白色粘土岩、黑灰色含炭-钙质硅板岩、灰黑色含炭泥质硅板岩及浅红色硅质板岩组成;岩系中磷结核及重晶石纹层常见。采用等离子质谱仪(ICP-MS)方法对岩系的稀土元素及微量元素测定,结果显示岩系中成矿物质含量丰富(如V、Mo、Ni、Ba、Pb、Zn、U、Ag、Cu、Cd、Tl、Bi和Cr等元素的平均含量是地壳丰度的几倍到几十倍),稀土元素和微量元素的含量与磷质、炭质以及钡关系密切;稀土模式曲线基本平行显示其成因相同;泥质和硅质岩类是主要的含矿岩性;不同程度的δEu正异常、中到弱的δCe负异常及相应的稀土、微量元素比值如Ce/La、V/(V+Ni)等指示岩石形成于干燥缺氧环境并有热水物质加入,La/Yb-Ce/La图解及La/Yb-REE图解表明岩系与海相沉积及基性岩成因关系密切。结合岩系的地质特征认为:山阳地区水沟口组黑色岩系形成于被动陆缘的一种干燥缺氧的深水-半深水滞留断陷局限海盆,期间热液(水)活动频繁、低等浮游生物发育,对岩系中矿物质的富集具有非常重要的意义。

According to field investigation,it is found that the black rock series of Shuigoukou Formation in Shanyang area of the Qinling Mountains is made up of micritic limestone,carbonaceous argillite,claystone,carbonaceous siliceous argillite,carbonaceous limestone and silicalite.Phosphatic nodule and barite are commonly seen in these rocks.The abundances of rare earth elements and some trace elements such as V,Ni,Co,U,Th in the black rock series were measured using ICP-MS method.Some conclusions have been reached： 1） The average concentrations of ore-forming elements（such as V,Mo,Ni,Ba,Pb,Zn,U,Ag,Cu,Cd,Tl,Bi and Cr） are several times to dozen times higher than their crustal abundances）,showing that they are rich in the rock series,especially in silicolites and argilloid.Higher content of trace elements and rare earth elements is closely related to phosphorus,TOC and barium.2）Different concentrations of rare earth elements are connected with different types of rock.The black rock series is usually of the LREE-enriched type.3） The samples are characterized by slight or intermediate negative Ce anomalies（δCe =0.40 ～1.00） and distinct positive Eu anomalies,exclusively with high δEu（2.1）.4）The ratios of U/Th display a large variation from 0.31 to 28.7,the ratios of w（V） /w（V ＋Ni）（0.51～1） and w（Ni） /w（Co）（4.8～49） are relatively high,the ratios of w（Ce） / w（La）（0.68～1.89） are normal,and the average rate of w（Co） /w（Zn） is 0.21.According to a comparison of these related parameters of REE and trace elements with the available data in the references and the La/Yb-Ce/ La and La/Yb-REE diagrams,it is suggested that the black rock series of Shuigoukou Formation was formed in the environment of a dry,anoxic and profundal to semi-profundal faulted depression sea basin belonging to a passive continental margin,with the action of hydrothermal fluids.