Seminar

The origin and formation process of orogenic gold deposits has long been an issue of debate. The ore-forming fluids of the giant Mesozoic orogenic gold deposits in eastern North China Craton (NCC) were traditionally thought to originate from metasomatized mantle or from subducted oceanic slab. Based on a combination of Fe isotope and in-situ S isotope study on the Mesozoic Baiyun gold deposit, we alternatively proposed that the ore-forming fluids could be derived from the metamorphic dehydration of the Paleoproterozoic basement rocks due to the Paleo-Pacific plate subduction beneath eastern NCC. The Baiyun gold deposit contains the quartz vein type and altered rock type mineralization that was developed in the Paleoproterozoic rocks. Our data revealed two distinct groups of S isotopic compositions (δ³⁴S) for the pyrites from the altered rock type ores, one is positive (+13.5‰ ~ +16.2‰) and the other negative (-10.6‰ ~ -3.0‰), while those from the quartz vein type ores show consistently negative values (-10.7‰ ~ -7.5‰). The positive δ³⁴S values of the pyrites from the altered rock type ores are comparable to those from the host rocks (+3.3‰ ~ +16.1‰), and thus should be the relict pyrite from the latter. This demonstrates that only the negative δ³⁴S values of pyrites in both types of ores (-10.7‰ ~ -3.0‰) and the δ⁵⁶Fe values of pyrites from the quartz vein type ores (+0.30‰ ~ +0.48‰) can represent the isotopic characteristic of ore-forming fluids at Baiyun. The negative δ³⁴S values of ore-forming fluids indicate reduced sedimentary sulfur sources, probably being released from dehydration of the Paleoproterozoic volcano-sedimentary sequences. We further calculated the Fe isotopic compositions of the released metamorphic fluids based on the mineral (Olivine/Pyrite/Magnatite)-fluid isotopic fractionation in the source within a metamorphic dehydration framework, and the results agree with that of the ore-forming fluids (-0.78‰ ~ -0.37‰) in equilibrium with the pyrites from the Mesozoic gold deposits in the NCC. Therefore, the S and Fe isotopes provide independent lines of evidence on the sulfur and iron sources, respectively, and both support the metamorphic dehydration model for the genesis of the Mesozoic gold deposits in the NCC. The intimate relationship between Fe and S isotopic systematics makes the combined application of them the built-in tracers for hydrothermal ore deposits.