Origin of the carbonate-hosted Danaopo Zn-Pb deposit in western Hunan Province, China: Geology and in-situ mineral S-Pb isotope constraints

Abstract

The western Hunan-eastern Guizhou (WHEG) Pb-Zn metallogenic belt hosts around 300 carbonate-hosted Pb-Zn deposits (total Pb-Zn metal reserve: >20 Mt), accounting for a major proportion of Pb-Zn resource in China. The origin and ore-forming process of these Pb-Zn deposits are still under debate. The newly-discovered Danaopo Zn-Pb deposit (~150 Mt ore @ 2.58% Zn and 0.48% Pb), which was formed during the Early Devonian orogenic event (~410 Ma), is one of the largest Zn-Pb deposits in the region. In this study, LA-MC-ICP-MS S-Pb isotope analyses were performed on sulfides (pyrite, sphalerite, and galena) and sulfate (barite) from the Danaopo Zn-Pb deposit. δ34SVCDT values of the diagenetic and late hydrothermal barites are of +36.5 to +36.9‰ and +27.1 to +28.1‰, respectively. The sulfides δ34SVCDT values (+23.5 to +35.5‰) imply a thermochemical sulfate reduction (TSR) origin from coeval seawater sulfates, as supported also by the dissolution cavities of diagenetic barite and the nodular and stripe-textured calcite. Furthermore, in-situ sulfide Pb isotopic ratios of different ore-forming stages and altitudes are highly similar: 206Pb/204Pb = 18.144–18.254, 207Pb/204Pb = 15.718–15.797, and 208Pb/204Pb = 38.378–38.605. Such narrow Pb isotopic ranges suggest well-mixing of multiple Pb sources, which likely include the Lower Cambrian Niutitang Fm. (LCNF) black shale, Proterozoic Banxi Group metamorphic basement rocks, and the Lower Cambrian Qingxudong Fm. (LCQF) reef limestone. Considering the deposit geological and S-Pb isotopic features, we suggest that the hydrothermal fluids may have circulated and extracted metals from various sequences in the basin. The fluids then ascended along the Huayuan-Zhangjiajie fault zone and mixed with reduced sulfur-bearing fluids from ore-bearing strata and precipitated the sulfide ores. Mineralization at the Danaopo deposit is thus best classified as Mississippi Valley Type (MVT), which is controlled by the ore-hosting reef limestone and various fault structures.