Research on the Metallogenic Enrichment Model of Poly-Metallic Black Shales and Their Geological Significance: A Case Study of the Cambrian Niutitang Formation

The Lower Cambrian Niutitang Formation was deposited precisely during the Cambrian Explosion period, a short-lived interval marked by drastic shifts in oceanic chemistry and climate. This stratigraphic sequence preserves a comprehensive record of early-ocean evolution and constitutes a world-class reservoir for rare and precious metals, widely termed the “poly-metallic enrichment layer”. Despite its metallogenic prominence, the genetic model for metal enrichment in the Niutitang Formation remains contentious. In this study, we employed inductively coupled plasma mass spectrometry (ICP-MS), carbon and sulfur analyzer, and X-ray fluorescence spectrometry (XRF) to quantify trace-metal abundances, redox-sensitive element distribution patterns, rare-earth element signatures, and total organic carbon contents. Results reveal that metal enrichment in the Niutitang Formation was governed by temporally distinct mechanisms. Member I records extreme enrichment in As, Ag, V, Re, Ba, Cr, Cs, Ga, Ge, Se and In. This anomaly was driven by the Great Oxidation Event and intensified upwelling that oxidized surface waters, elevated primary productivity and delivered abundant organic matter. Subsequent microbial sulfate reduction generated high H2S concentrations, converting the water column to euxinic conditions. Basin restriction imposed persistent anoxia in bottom waters, establishing a pronounced redox stratification. Concurrent vigorous hydrothermal activity injected large metal fluxes, leading to efficient scavenging of the above metals at the sulfidic–anoxic interface. In Members II and III, basin restriction waned, permitting enhanced water-mass exchange and a concomitant shift from euxinic to anoxic–suboxic conditions. Hydrothermal metal fluxes declined, yet elevated organic-matter fluxes continued to sequester Ag, Mo, Ni, Sb, Re, Th, Ga, and Tl via carboxyl- and thiol-complexation. Thus, “organic ligand shuttling” superseded “sulfide precipitation” as the dominant enrichment mechanism. Collectively, the polymetallic enrichment in the Niutitang Formation reflects a hybrid model controlled by seawater redox gradients, episodic hydrothermal metal supply, and organic-complexation processes. Consequently, metal enrichment in Member I was primarily governed by the interplay between vigorous hydrothermal flux and a persistently sulfidic water column, whereas enrichment in Members II and III was dominated by organic-ligand complexation and fluctuations in the marine redox interface. This study clarifies the stage-dependent metal enrichment model of the Niutitang Formation and provides a theoretical basis for accurate prediction and efficient exploration of polymetallic resources in the region.