Geology and Geochemistry of Se-Bearing Formations in Central China

摘要

It is widely believed that selenium, a typical dispersed element, does not form independent deposits. However, this has been challenged recently by discoveries and investigations of Se-bearing formations in China. In this paper we present our findings with a systematic discussion on the genesis of these Se-bearing formations. The Se-bearing formations typically are developed in tensile tectonic settings, mainly along the margins of the Yangtze and be Tarim cratons, or within the Yangtze craton. They are mostly Cambrian, but a few are Permian in age. Lithologically, they largely consist of carbonaceous cherts and carbonaceous shales (slates) that contain abundant organic mater. In general, the Se-bearing formations are characterized by exceptional associations of elements. In particular, they are characterized by elements with multiple oxidation states, such as V, U, P, Ni, Mo, etc., which are typical for the black shale series, and y elements like As, Ba, and Sb, which are considered as indicative elements for hydrothermal sedimentation. In some cases, they also contain Au and platinum-group elements (PGE), which are quite unusual for ordinary depositions. Organic geochemical studies and biomarker investigations revealed that he C_(org) in rocks are relatively high in these Se-bearing formations, ranging from 1.3 percent to 17.8 percent. The fraction of soluble organic matter extracted from the samples is commonly less than 50 ppm. Gas chromatographic (GC) analyses suggest that saturated hydrocarbons in these formations consist mainly of low-carbon-number alkane, with the highest peak of C_(15)-C_(18) and with low ratios of Pr/Ph (0.62-1.03). Using gas chromatographic-mass spectrometric (GC-MS) analyses, many biomarkers have been identified, including the relatively complete series of long-chain tricyclic terpanes and hopanes in m/z 191 mass chromatogram and the steroids in m/z 217 mass chromatogram. Evidence suggests that the source or precursors of the organic matter might have been some sort of bacteria and/or algae. The reducing sedimentary environments that hosted these bacteria and algae favored the growth and preservation of such organisms. It was these organisms that acted as the carrier of Se, leading to Se enrichment and deposition. Finally, some evidence also indicates that the source of the Se was also closely related to the formation of hydrothermal sedimentation of cherts.