Selenium isotopes trace the source and redox processes in the black shale-hosted Se-rich deposits in China

We analyzed the Se isotopic composition of black shales and related kerogen and sulfide fractions from the Zunyi Ni-Mo-Se deposit, the La'erma Se-Au deposit and the Yutangba Se deposit in southern China to constrain metal sources and accumulation processes, both subjects of disagreement in the scientific community. Se at the Zunyi Ni-Mo-Se polymetallic deposit displayed a restricted range of delta Se-82 values (-1.6 parts per thousand to 2.4 parts per thousand with a mean of 0.6 parts per thousand) suggesting a major hydrothermal origin where aqueous Se was probably transported as H2Se, along with H2S, and precipitated directly as selenides or in sulfides. Se at the La'erma Se-Au deposit covers a larger range in delta Se-82 values (-3.8 parts per thousand to 5.4 parts per thousand with a mean of 0.3 parts per thousand), suggesting Se redistribution following redox transformations, leading to kinetic isotopic fractionation. The largest Se isotopic variation so far in natural terrestrial samples was found in the Yutangba Se deposit, with delta Se-82 values varying from -12.77 parts per thousand to 4.93 parts per thousand. On the basis of variations in Se isotopes in the deposit, along with other geological and geochemical evidence, the redox model (supergene alteration) explains the occurrence of native Se in the deposit. Overall, hydrothermal systems may be a potentially important Se source to form economic deposits in comparison to seawater sources. Significantly, our study indicates that either secondary hydrothermal or supergene alteration is a key factor in Se enrichment in black shales. Redistribution of Se, and probably other redox-sensitive metals like Mo, Cr and V, leads to isotopic fractionation which may be used to fingerprint such alteration/precipitation processes. (C) 2010 Elsevier Ltd. All rights reserved.