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

Abstract

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 ‰ δ65Cu) beyond the lithogenic background. Heavy zinc and cadmium isotope enrichment in the sulfidic samples (1.11 ± 0.18 ‰ δ66Zn, 0.31 ± 0.10 ‰ δ114Cd) is controlled by sulfide fractionation and contrasts with V-rich and barren shale (0.60 ± 0.18 ‰ δ66Zn, 0.00 ± 0.14 ‰ δ114Cd). The distinctly negative Ni isotope composition of the metalliferous unit (−0.84 ± 0.05 ‰ δ60Ni) 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.

Calcite U-Pb geochronology of Huayuan carbonate-hosted Zn-Pb mineralization in Central Yangtze Craton, South China

“…These ores are layered/laminated in nature and of low-to medium grade (~ 4 wt% Pb+Zn), with multiple generations of sulfides, carbonates, and barites, as well as minor fluorites 34 . The Lower Cambrian Niutitang Formation contains a significant amount of bitumen and was interpreted to be a source of hydrocarbons and other metals in the Huayuan district 33,69 .…”

New Potential Sulfide Reference Materials for Microbeam S‐Fe‐Cu Isotope Measurements

“…Sulfur (S), iron (Fe) and copper (Cu) are ubiquitous elements with different oxidation states that are heterogeneously distributed throughout reservoirs (Sawaki et al 2018, Paiste et al 2022, Smith et al 2022, McLoughlin et al 2023. The S-Fe-Cu isotope systems in sulfides have been well proven to be excellent geochemical tracers in diverse fields of geoscience, such as research on planetary evolution events, mineralisation mechanisms, hydrothermal activity and bioenvironmental processes (Li et al 2010, Marin-Carbonne et al 2014, Mount et al 2022, Hiebert et al 2016, Brzozowski et al 2021, Yu et al 2021, Lehmann et al 2022, Zhao et al 2022.…”