Early Cambrian highly metalliferous black shale in South China: Cu and Zn isotopes and a short review of other non-traditional stable isotopes

• Published in: Mineralium deposita Vol. 57; no. 7; pp. 1167 - 1187
• Main Authors: Lehmann, Bernd, Pašava, Jan, Šebek, Ondřej, Andronikov, Alexandre, Frei, Robert, Xu, Lingang, Mao, Jingwen
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.10.2022; Springer Nature B.V
• Subjects: Biological activity; Bottom sediments; Brines; Cadmium; Cadmium isotopes; Cambrian; Chromium; Copper; Earth and Environmental Science; Earth Sciences; Enrichment; Euphotic zone; Fractionation; Geology; Gold; Heavy metals; Hydrothermal activity; Isotope composition; Isotope fractionation; Isotopes; Isotopic enrichment; Leaching; Mercury; Mercury (metal); Microbial activity; Microorganisms; Mineral Resources; Mineralogy; Molybdenum; Nickel; Nickel isotopes; Oxidation; Oxidoreductions; Passive margins; Platforms (geology); Redox properties; Sapropels; Sedimentary rocks; Selenium; Shale; Shales; Silver; Stable isotopes; Sulfate reduction; Sulfides; Sulphate reduction; Sulphides; Vanadium; Water circulation; Water column; Zinc; Geochemistry; Copper isotopes; Black shale; Zinc isotopes; China
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-022-01097-0

Early Cambrian black shales on the Yangtze Platform host a regionally distributed highly metalliferous sulfide-rich carbonaceous unit which has been the subject of extensive debate. This marker unit, with a few centimeters or tens of centimeters in thickness, displays extreme enrichment in Mo and Ni (wt% range), and in a broad spectrum of other metals such as As, Au, PGE, Re, Cu, Zn, Cd, Ag, Sb, Se, Tl, and Hg, and occurs discontinuously along the western passive margin of the Yangtze Platform. It grades laterally in stratigraphically equivalent meter-thick vanadium-rich shale and tens-of-meter-thick sapropelite (combustible shale). New Cu and Zn isotope data, combined with existing Cd, Cr, Ni, Mo, Hg, and Se isotope and other chemical data, allow to attempt an integrated view on the formation of this intriguing unit of hyper-enriched metalliferous black shale. The authigenic Cu enrichment in the 1000-ppm range has produced no or little Cu isotope fractionation (0.03 ± 0.26 ‰ δ 65 Cu) beyond the lithogenic background. Heavy zinc and cadmium isotope enrichment in the sulfidic samples (1.11 ± 0.18 ‰ δ 66 Zn, 0.31 ± 0.10 ‰ δ 114 Cd) is controlled by sulfide fractionation and contrasts with V-rich and barren shale (0.60 ± 0.18 ‰ δ 66 Zn, 0.00 ± 0.14 ‰ δ 114 Cd). The distinctly negative Ni isotope composition of the metalliferous unit (−0.84 ± 0.05 ‰ δ 60 Ni) with Ni in the percent range has been interpreted as due to hydrothermal activity related to the leaching of mafic rocks and their sulfides. Sorption processes (Fe-oxyhydroxides) and redox cycling in the water column and the bottom sediment with microbial activity could be an alternative interpretation. The extreme metal enrichment can be understood as due to a process chain, from high biological productivity in the oxic photic zone to sulfate reduction in the deeper sulfidic water column and upper sediment layer. Key to the metal enrichment seems to be extremely low clastic sedimentation and advanced carbon destruction by anaerobic oxidation. Hydrothermal input of basinal brines along the rifted margin of the Yangtze Platform was likely a part of this scenario.