Coupled trace element and SIMS sulfur isotope geochemistry of sedimentary pyrite: Implications on pyrite growth of Caixiashan Pb-Zn deposit

Colloform pyrite with core-rim texture is commonly deposited in carbonate platforms associated with the sulfide ores such as the Caixiashan Pb-Zn deposit. However, the genesis of colloform pyrite in Pb-Zn deposits, its growth controls and their geological implication are insufficiently understood. Integration of in-situ trace element and SIMS sulfur isotopes has revealed geochemical variations among these pyrite layers. These colloform pyrite occur as residual phases of core-rim aggregates, the cores are made up of very fine-grained anhedral pyrite particles, with some rims being made up of fine-grained and poorly-crystallized pyrite, while the other rims were featured with euhedral cubic pyrite, which are cemented by fine-grained calcite and/or dolomite with minor quartz. Sulfur isotope analysis shows that some well-preserved rims have negative δ³⁴S values (-28.12‰ to -0.49‰), whereas most of the cores and rims have positive δ³⁴S values (>0 to +44.28‰; peak at +14.91‰). Integrating with the methane and sulfate were observed in previous fluid inclusion study, we suggest that the ³⁴S depleted rims were initially formed by bacteria sulfate reduction (BSR), whereas the positive δ³⁴S values were resulted from the sulfate reduction driven by anaerobic methane oxidation (AOM). The well-developed authigenic pyrite and calcite may also support the reaction of AOM. Combined with petrographic observations, trace element composition of the colloform pyrite reveals the incorporation and precipitation behavior of those high abundance elements in the pyrite:Pb and Zn were present as mineral inclusion and likely precipitated before Fe, as supported by the time-resolved Pb-Zn signal spikes in most of the analyzed pyrite grains. Other metals, such as Hg, Co and Ni, may have migrated as chloride complexes and entered the pyrite lattice. Arsenic and Sb, generally influenced by complex-forming reactions rather than substitution ones, could also enter the pyrite lattice, or slightly predate the precipitation of colloform pyrite as mineral inclusions, which are controlled by their hydrolysis constant in the ore fluids. The colloform pyrite may have grown inward from the rims. The successive BSR reaction process would enrich H₂³²S in the overlying water column but reduce the metal content, the nucleation of these pyrite rims was featured by strongly negative sulfur isotopes. The following AOM process should be activated by deformation like the turbidity sediment of the mudstone as the sulfide deposition are associated with fault activities that caused the emission of methane migration upward and simultaneously replenishing the metal in the column. The higher AOM reaction rate and the higher metal supply (not only Fe, but with minor other metals such as Pb and Zn) caused by sediment movement enhanced the metal concentration within the pyrite lattice.