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

• Published in: Geochimica et cosmochimica acta Vol. 75; no. 6; pp. 1411 - 1427
• Main Authors: Wen, Hanjie, Carignan, Jean
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15.03.2011
• Subjects: Alterations; China; chromium; Deposition; Deposits; Fractionation; isotope fractionation; isotopes; molybdenum; Nickel; seawater; selenium; Shale; Sulfides; China
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2010.12.021

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 δ 82Se values (−1.6‰ to 2.4‰ with a mean of 0.6‰) suggesting a major hydrothermal origin where aqueous Se was probably transported as H 2Se, along with H 2S, and precipitated directly as selenides or in sulfides. Se at the La’erma Se–Au deposit covers a larger range in δ 82Se values (−3.8‰ to 5.4‰ with a mean of 0.3‰), 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 δ 82Se values varying from −12.77‰ to 4.93‰. 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.