The biomarker signatures in the Niujiaotang sulfide ore field: Exploring the role of organic matter in ore formation

Abstract

The Niujiaotang Mississippi Valley-Type (MVT) Pb–Zn deposit (SW China) hosts 0.35 Mt Zn (5.85–24.48 wt%) and 5.3 kt Cd (0.04–1.43 wt%). Although previous inorganic geochemical studies have indicated a significant role of organic matter in mineralization, critical organic geochemical evidence is still lacking. This study presents the first integrated biomarker investigation of solid bitumen derived from the host rock of the deposit, clarifying the sources of organic matter, its thermal evolution, and metallogenic implications. Hydrothermal alteration assessments reveal that biomarker distributions have undergone only mild biodegradation and reliably preserved source and maturity information. Multiple maturity proxies indicate an over-mature stage (>150 °C), which meets the thermal requirement for thermochemical sulfate reduction (TSR). Saturated and aromatic hydrocarbon fingerprints, combined with carbon-isotope data, demonstrate derivation from Lower Cambrian source rocks dominated by marine algal/bacterial organic matter with minor terrestrial higher-plant input. The relative abundances of dibenzothiophene series compounds (5.84–9.54 %, mean 7.27 %) versus biphenyls (1.77–7.01 %, mean 4.62 %) provide robust molecular evidence for extensive TSR reactions. This study strengthens the interpretation that sulfur-rich hydrocarbons act as the principal reductant in TSR, facilitating the conversion of stratal sulfate to H₂S and thereby establishing the reduced sulfur reservoir essential for Pb–Zn precipitation. Macroscopic intergrowths of solid bitumen with sulfide and carbonate gangue, alongside microscopic hydrocarbon inclusions in sphalerite and dolomite, corroborate this model. These organic-geochemical constraints refine genetic concepts for MVT deposits and emphasize the value of integrating petroleum system analysis into exploration workflows for analogous Pb–Zn systems.