New constraints for crustal sulfur contamination of gold source: Evidence from complex δ34S of pyrite in the northwestern Jiaodong gold province, China

Abstract

The formation processes of the complex δ34S variations for the Jiaodong-type gold deposits are still equivocal, which cannot give a robust identification for the sulfur sources, by inference, Au sources. The δ34S and trace elements of ore-related pyrite collected from the northwestern Jiaodong gold province (Jiaojia and Sanshandao-Xinli goldfields) are analyzed by in-situ methods, to reveal more clues for understanding the complex δ34S variations and ore material sources. Gold deposits in the northwestern Jiaodong contain quartz-Au-pyrite (Py-a) stage (III) and quartz-Au-pyrite (Py-b)-polymetallic stage (IV). In the Jiaojia goldfield, Py-a has a δ34S range of 9.9–24.4‰, whereas Py-b has a δ34S range of 7.3–9.3‰. In the Sanshandao-Xinli goldfield, Py-a has a δ34S range of 6.9–34.7‰, and δ34S of Py-b ranges in 0.2–19.2‰. Single-point and mapping analyses show that As-Au-Co-Ni contents have considerable variations in Py-a and Py-b. The δ34S and As-Au-Co-Ni contents in Py-a and Py-b show diverse correlations. Equilibrium and Rayleigh (or sulfidation) fractionations cannot account for the large δ34S variation due to the pyrite-chalcopyrite-pyrrhotite buffer. The rapid decompression with oxidization of ore-forming fluids by immiscibility can only result in the δ34S decrease, rather than increase to 14.0–34.7‰ from the concentrated δ34S range of 9.6–13.4‰ in Py-a and Py-b. Therefore, the analyzed δ34S values in the ore-related pyrites should represent those of ore-forming fluids. Considering the simple physicochemical processes cannot reconcile the complex matches between large δ34S variations and As-Au-Co-Ni contents, it is suggested the As-rich and Au-As-rich ore-forming fluids with large δ34S range and variable Co-Ni concentrations should flow into mineralization sites during stages III and IV in the Sanshandao-Xinli goldfield. The sulfur with large δ34S range and Au-As-Co-Ni elements with variable contents in the ore-related pyrites of the Jiaojia and Sanshandao-Xinli goldfields were most likely inherited from the sedimentary sulfides that were reduced from seawater sulfates, by comparisons with those in the typical sedimentary, metamorphic hydrothermal and magmatic hydrothermal pyrites. Combining with previous researches for the ore-forming materials sources of the Jiaodong-type gold deposits, the metamorphic Precambrian rocks, especially those metasedimentary rocks containing sulfides reduced from seawater sulfates, can be considered as the feasible ore-forming materials sources for the large δ34S range and variable trace elements in the Jiaojia and Sanshandao-Xinli goldfields. Through the δ34S comparison for the Jiaodong-type gold deposits, we propose an important process that the single-sourced ore-forming fluids, which were initially derived from the metasomatized lithospheric mantle, have extracted the ore materials (including sulfur-Au-As elements) from the metamorphic Precambrian rocks (possible Precambrian metamorphic basement in the Jiaodong gold province or the subducted Late Neoproterozoic rocks of the YC), and mixed heterogeneously to precipitate pyrites with complex δ34S and trace elements throughout the Jiaodong-type gold deposits.