Source of selenium in Handan geochemical anomaly belt: Evidences from petrology and geochemistry of Upper Paleozoic in western Handan, China

Abstract

Handan lies in a geochemical anomaly belt characterized by enrichment of selenium (Se) in rock, soil, and water, contrasted with deficiency of Se in the North China Plain. In this study, 27 environmental soil samples and 96 rock samples in the Upper Paleozoic were analyzed by inductively coupled plasma optical emission spectrometer (ICP-OES) and inductively coupled plasma mass spectrometry (ICP-MS) for major and trace element concentrations to determine the source of Se in the environment.Se concentrations in soil vary from 0.054 μg/g to 2.869 μg/g, with an average of 0.598 μg/g. Ignoring the anthropogenic contamination, Se concentrations in soils elevate with the decrease of distance from outcrops in Mountain Zishan (Mt. Zishan). There is a strong positive correlation between Se and Al2O3 (r = 0.679, P < 0.01, n = 27), and between Se and TiO2 (r = 0.638, P < 0.05, n = 27). Furthermore, Se is significantly negatively correlated with MgO, Na2O, K2O, and Sr (r = −0.674, −0.755, −0.716, −0.739, respectively, P < 0.01 for all, n = 27). It indicates that Se in the soil is derived from weathering products.Se is the most highly enriched trace element within Upper Paleozoic, which is composed of Benxi Formation (BX Fm.), Taiyuan Formation (TY Fm.), Shanxi Formation (SX Fm.), and Shihezi Formation (SHZ Fm.), in outcrops in Mt. Zishan area, western Handan. Se concentrations of rock samples vary from 0.021 μg/g to 4.971 μg/g, with an average of 0.401 μg/g. The maximum of Se concentration is 99.41 times higher than the Upper Continental Crust (UCC). The order of the rest trace elements enrichment degrees is S > Pb > As > Ga > Li > Bi > Th > Sc > In > U > Cd > Y > Rb > Ba > Cr > V > Be > Cs > Ni > Zn > Tl > Sr > Co > Mo > Cu. Se enrichment appears in two types of rocks, which are gray clay rocks and black mudstones and shales (including coal and gangue). Stratigraphically, Se is more highly enriched in TY Fm. and SX Fm., with average enrichment factor (EF) values of 14.64 and 5.59, respectively.Chemical index of alteration (CIA) and several environmentally sensitive elements, such as Th, U, Sr, Ba, V, Cr, and Ce, were employed for sedimentary environment analysis. The results revealed that Se-enriched rocks were deposited mainly in a weak, reducing freshwater environment, and their sediments were subject to an intermediate chemical weathering process. The discrimination diagrams of SiO2-TiO2, F1-F2, ∑REE-La/Yb, and La/Sc-Co/Th indicate that the terrestrial clastic sediments were dominated by multiple types of original rocks. Se-laden clastic materials derived from mafic and ultramafic rocks in Yinshan ancient land provided main source of Se in Upper Paleozoic in western Handan, and adsorption of clayey and organic materials and biological accumulation induced Se enrichment ultimately.