Isotopic footprints of the giant Precambrian Caixiashan Zn-Pb mineralization system

Abstract

Carbonate-replacement Zn-Pb deposits are formed through fluid circulation within carbonate rocks and are an important global source of Zn and Pb. The Caixiashan carbonate-replacement deposit is one of the largest Zn-Pb systems in China. The Mesoproterozoic syn-sedimentary stage (I) of the Caixiashan deposit is characterized by sulfides with negative δ34S values (−25.3 to −8.6‰) generated by organic sulfate reduction, which co-precipitated with the host rocks. The Neoproterozoic main sulfide precipitation stage (II) was caused by thermo sulfide reduction resulting in positive δ34S values (7.9–12.3‰). Carbonates from stage I have δ13CPDB of −1.2 to 2.8‰ and δ18OSMOW of 17.1–24.0‰, similar to those of marine carbonate rocks. Dolomite and calcite from stage II have δ13CPDB ranging from −5.1 to −0.2‰ and δ18OSMOW of 14.1–19.7‰, consistent with dissolution of carbonate rock. In contrast dolomite and calcite from stage III have mantle C-O signatures with the δ13CPDB ranging from −7.5 to 0.4‰ and δ18OSMOW from 5.8 to 13.2‰. C-O isotopic signatures suggest that the original carbonates were dissolved by the acid generated from sulfide precipitation consistent with the sulfide precipitation process. Subsequent precipitation of carbonates created large volumes of dolomite alteration that coated the sulfides and possibly contributed to the preservation of the ore, as dolomite would dissolve more slowly than calcite. Carboniferous magmatism (stage III) resulted in a magmatic overprint on the stage II sulfide. The metal sources of these sulfide ore packets are mainly derived from the Mesoproterozoic carbonate rocks as indicated by the lead isotopes of sulfides and also C-O isotopes of carbonates. The deposit is characterized by a complex paragenesis with multiple overprinting sources of hydrothermal fluids and sulfide over 700 m.y. The syn-sedimentary stage (∼1019 Ma) and main mineralization stage (859–837 Ma) at Caixiashan are chronologically and tectonically associated with the assembly and break-up of the Rodinia supercontinent, whereas the magmatic-hydrothermal modification stage (∼353 Ma) is associated with the closure of the Paleo Asian Ocean during the Early Carboniferous.