Abstract
The widely varying sulfur isotopes in some Mississippi Valley-type (MVT) Pb-Zn deposits are often attributed to bacterial sulfate reduction (BSR), whether thermo-chemical sulfate reduction (TSR) also contributes is debatable. The Chipu Pb-Zn deposit in South China (0.7 Mt Pb + Zn reserves @ 10.4 wt%) is hosted by late Ediacaran organic-rich evaporitic dolostone and contains three assemblage types of hydrothermal minerals: pyrite (stage I), sphalerite-pyrite-galena (stage II; main ore stage), and galena-pyrite (stage III). High spatial resolution in situ S isotopic data of pyrite and sphalerite have a wide range of δ34S values from −31.1‰ to +33.8‰, which differ from previously reported data (δ34S = −5.6‰ to +21.9‰). Also, an important feature is that stage I pyrite has extremely low δ34S values (-31.1‰ to −17.3‰), while stage II (δ34S=+12.5‰-+16.2‰) and III (δ34S=+19.8‰-+33.8‰) sulfide minerals have heavy sulfur isotopes. Such isotope signatures indicate different stages of sulfides may have different reduction mechanisms. Together with in situ Pb isotopes, we propose that both BSR and TSR of local marine sulfate were involved during the formation of the Chipu deposit. Furthermore, the arrival of the hot, basement-derived fluids caused a change from BSR to TSR that occurred in sulfate-open to nearly sulfate-closed systems.
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