Metal enrichment in the Cambrian black shale: Evidence from pyrite overgrowth and NanoSIMS sulfur isotopes

The Lower Cambrian black shale has attracted much attention due to its role in elucidating the chemical evolution and sulfur cycles of the global ocean, as well as its economic significance as a source of phosphate, barite, and high-grade NiMo polymetallic sulfide ores. However, the occurrence of abnormally high metal accumulation within thin ore layers across extensive areas remains contentious. Pyrite, that occurs as framboids or fine- to coarse-grained euhedral-subhedral crystals, is a ubiquitous sulfide found in both host black shale and metalliferous ore horizons. This study delineates a three-stage growth of pyrite within the Chuanyanping and Sancha NiMo sulfide layers in western Hunan Province, South China, based on the micro-textures, chemical compositions, and sulfur isotopes. The earliest pyrite (PyI) appears as framboids (3–7 μm) or as small euhedral-subhedral grains (<20 μm). It exhibits a depletion in trace elements and negative δ34SV-CDT values spanning from −34.2 to −17.4 ‰, indicating the formation in a euxinic water column via microbial sulfate reduction (MSR) during syngenesis. The later pyrite (PyII), which is associated with gersdorffite, appears as thin overgrowth rings encircling PyI or euhedral to subhedral crystals displaying complex oscillatory and lacy zoning patterns. It is characterized by elevated Cu (up to 5.84 wt%), Ni (up to 3.56 wt%), and As (up to 1.70 wt%) concentrations and highly variable sulfur isotopic compositions. The overgrowth rings exhibit δ34SV-CDT values spanning from −9.6 to 12.4 ‰, while the euhedral to subhedral crystals have inhomogeneous sulfur isotopic signatures, with the cores enriched in heavy sulfur (31.7 to 33.7 ‰ δ34SV-CDT) and the rims marked by comparatively light sulfur (−7.1 to 7.2 ‰ δ34SV-CDT). This isotopic variation reflects the joint effect of thermochemical sulfate reduction (TSR) and hydrothermal-derived sulfur. Specifically, the initial influx of high-temperature hydrothermal fluids produced minimal isotopic fractionation between sulfide and seawater sulfate via TSR, facilitating the formation of heavy‑sulfur PyII. Subsequently, hydrothermal sulfur became the dominant source for the deposition of light‑sulfur PyII, with a minor contribution from dissolution of PyI. Given the close association of organic matter with ores, hydrocarbons may significantly facilitate metal transport via organometallic complexing, with Ni primarily derived from hydrothermal fluids and Mo from seawater as indicated by previous Ni and Mo isotopic analyses. The barren pyrite (PyIII), which presents as overgrowth matrix cementing early PyI and PyII, has positive δ34SV-CDT values (15.5 to 34.6 ‰), suggesting formation via MSR in closed, sulfate-limited environment, most likely following deep burial. Collectively, the chemical and sulfur isotopic evidence, coupled with petrographic observations, highlights the crucial role of hydrothermal impulse in the formation of hyper-enriched black shale.