Geochemical constraints on the origin and environment of Lower Cambrian,selenium-rich siliceous sedimentary rocks in the Ziyang area,Daba region,central China

The Ziyang area is one of the two major regions of central China subjected to selenium (Se) poisoning. Systematic studies of Se contents of different lithologies from this area indicate that Lower Cambrian, carbonaceous, and siliceous strata host the highest Se contents (with Se contents of up to 278 ppm). We have investigated their geochemical characteristics (major and trace elements, and Si and O isotopes), origin, and sedimentary environment of formation. The siliceous rocks are characterized by a wide range in major elements, and are enriched in Se, Ba, Cu, Ni, V, As, Sb, and U relative to average continental crust. They are also enriched in light rare earth elements relative to heavy rare earth elements (La_N/Yb_N?=?1.64–35.7) and show weak or moderate negative Ce anomalies and strong positive Eu anomalies. δ³⁰Si_NBS-28 and δ¹⁸O_V-SMOW values range from –0.3‰ to 0.6‰ and 16.1‰ to 21.7‰, respectively. The homogenization temperatures of inclusions within the studied samples range from 113°C to 319°C, and their salinities from 1.2 to 13.7 wt.% NaCl equivalent. Our results suggest that the studied siliceous rocks resulted from hydrothermal sedimentation in a relatively anoxic semi-deep sea sedimentary environment. The hydrothermal fluid responsible for Se-mineralization involved the mixing of low-temperature high-salinity fluid, low-temperature low-salinity fluid, and a high-temperature low-salinity basinal fluid in the NaCl–(KCl)–H₂O system.