GENESIS OF CAMBRIAN-HOSTED MVT DEPOSITS OF HUNAN (SOUTH CHINA): GEOLOGICAL AND GEOCHEMICAL CONSTRAINTS

The Zn-Pb-Ba ore deposits of the Huayuan mining field (Hunan, Southern China) are hosted by Lower Cambrian platform carbonates. The ores were studied by means of cement stratigraphy, microthermometry and isotope geochemistry. Fluid inclusion Th have been determined for two sphalerite generations, averaging 110 and 180°C, respectively, and for sparry calcites (90 to 180°C). Tm (ice) range for sphalerites between -19 and -11, whereas the calcites show again a spread between -23 and -12°C. The sulfide Pb-isotopes point to crustal metal sources. Most ores define a linear trend in Pb-Pb diagrams, indicating binary mixing possibly between ancient source rocks dominated by moderately high ²⁰⁷Pb/²⁰⁴Pb ratios and lower crustal reservoirs with elevated Th/U ratios. Based on ore Pb isotopes and petrographic observations, an early and a subsequent main stage of sulfide precipitation can be distinguished, the first involving fluids partly derived from sources with ²⁰⁶Pb/²⁰⁴Pb ratios > 18.30. Comparative U-Pb data on potential metal source rocks reveal that the Lower Cambrian sequence, even the silicoclastic lithotypes underlying the ore-bearing strata, can be excluded as primary metal sources. However, Rb-Sr data show that these rocks may have been potential sources for the Sr found in hydrothermal carbonates about 550 Ma ago. Sulfur isotopes on barites (38 -> 34‰ CDT) show values slightly higher than the d³⁴S of Cambrian sea water sulfate. Sulfur isotopic disequilibrium among the sulfides (sphalerites 33.8 -> 29.4‰ CDT; galena 29.6 -> 26.6‰ CDT), points to TSR processes, which took place at temperatures well above 100°C. Carbon isotope ratios are almost identical between Cambrian limestones and different generations of carbonate cements, while d¹⁸O ratios are strongly dependent from the hydrothermal gradients of the fluids. In conclusion, all the parameters of the Huayuan ore deposits are well consistent with typical MVT characteristics. In the absence of robust geochronological data, however, the mechanism responsible for the massive fluid flow that caused the mineralization remains unclear.