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

Abstract：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.