Origin of the Early Cambrian Huayuan carbonate-hosted Zn-Pb orefield, South China: Constraints from sulfide trace elements and sulfur isotopes

The Huayuan Zn-Pb orefield in the southeastern Yangtze Craton contains over 20 Mt Zn + Pb metal resource @ 4%, including several large, Lower Cambrian carbonate-hosted deposits classified as Mississippi Valley-Type (MVT). The source of zinc and lead and the ore-forming processes involved are still unclear. Trace elements and sulfur isotope compositions of sulfides from the large-scale Limei and Danaopo Zn-Pb deposits in the orefield favor a fluid-mixing metallogenic hypothesis. Alteration/mineralization occurred in three stages (S1-S3), forming pale-yellow sphalerite (S1: Sp1), yellow-brown sphalerite and pyrite (S2: Sp2-Py2), and galena-pyrite (S3: Gn-Py3). Syn-sedimentary fine-grained pyrite (Py1) in the Lower Cambrian Qingxudong Formation (Fm.) algal lime-stone has high Co, Ni, and As concentrations, whereas hydrothermal Py2 and Py3 in sulfide-calcite veins are rich in Zn and Pb, respectively. All the pyrites have low Co/Ni ratios (<2), indicating that the Fe, Co, and Ni are from a sedimentary source. Sphalerites from Limei and Danaopo are rich in Cd, Cl, and Ge but poor in Fe, Mn, and Co. They were formed under low-medium temperature (134-236 degrees C; determined by the concentrations of Mn, Fe, Ga, Ge, In, and Sb) and in an acidic and low-fO(2) hydrothermal system, which are consistent with typical MVT ores. Lower Cambrian algal limestone (delta S-34(Sigma S) = 34.3-34.4 parts per thousand) contributed most sulfur to the ore sulfides (delta S-34 = 29.3-37.2 parts per thousand). Meanwhile, the Lower Cambrian Niutitang Fm. Pb-Zn-rich black shale is likely the main ore metal source, but its limited sulfate content may not be able to provide sufficient sulfur. We propose a mixing between metal-rich brines leached from the basal Niutitang Fm. black shale with H2S-rich fluid resulting from thermo-chemical sulfate reduction (TSR) of the Qingxudong algal limestone formation, thus destabilizing the base metal chloride complexes and precipitating the Zn-Pb ores.

Automated summary

The Huayuan Zn-Pb orefield in the southeastern Yangtze Craton is an important mineral resource area, containing several large Lower Cambrian carbonate-hosted deposits. The ore-forming processes in this area are complex and mainly resulted from fluid mixing. The mineralization occurred in three stages, forming different types of ores. The Limei and Danaopo deposits in the Huayuan orefield exhibit similar characteristics to typical MVT ores.