In situ multiple sulfur isotopes and chemistry of pyrite support a sedimentary source-rock model for the Linwang Carlin-type gold deposit in the Youjiang basin, southwest China

Abstract

The Linwang gold deposit is a representative example of Carlin-type gold deposits in Youjiang basin, southwest China. High-grade ores are hosted in silicified and brecciated siltstone, sandstone, and mudstone of the Middle Triassic and are controlled by high-angle reverse faults. Optical microscopy pyrite texture, in situ laser ablation-inductively coupled mass spectrometry (LA-ICP-MS) trace elements, and secondary ion mass spectrometry (SIMS) multiple sulfur isotopes analyses are used to examine the different types of pyrites to constrain the sources of reduced sulfur and metals for the Linwang deposit.Petrography and pyrite chemistry studies distinguished three generations of pyrite: Pre-ore pyrite (Py 1) and ore pyrites (Py 2 and Py3). Pre-ore pyrite is framboidal, anhedral to euhedral pyrite with highest concentrations of Co, Ni, and Pb and is interpreted to have a sedimentary (diagenetic) origin. Ore pyrites were formed by hydrothermal fluids via sulfidation of Fe-bearing minerals in the host rocks. They commonly rimed and partially replaced the pre-ore pyrite and have an As-rich inner rim (Py 2) and an Au-rich outer rim (Py 3) with lowest contents of Co, Ni, and Pb. All pyrites contain Au, As, Sb, Cu, Tl, and Se, with increasing concentrations from Py 1 to Py 3.In situ SIMS multiple S isotope analyses show that all pyrites in the Linwang deposit have narrow ranges of δ34S values, with average of +5.13‰ for Py 1, +4.84‰ for Py 2, and +4.53‰ for Py 3, and near zero of Δ33S values (−0.02‰ to +0.03‰), suggesting that the S within the Linwang deposit was most likely sourced from the Triassic sedimentary rocks that host the deposit. A near zero Δ33S signature in all pyrites indicates no record of the assimilation of Archean sulfur, implying the deep basement of Archean rocks beneath the basin were not involved the formation of the Linwang deposit.Oxygen and hydrogen isotope compositions of ore fluids (δ18OH2O = +11.7‰ to +16.5‰, δD = −71‰ to −57‰) indicate a metamorphic origin. Based on these data, we propose a sedimentary source-rock model for the Linwang deposit in which the ore fluids were generated by release of previous Au-bearing sedimentary pyrites during metamorphism of sedimentary rocks. The stress relaxation of Yanshanian orogeny resulted in the ore fluids flowed upward along the fault zones into structural highs where they reacted with Fe-bearing minerals in the host rocks to deposit Au-rich pyrite. The preconcentration of Au, As, and Sb in sedimentary pyrite and gold deposition process are key to the formation of the Linwang deposit and may apply to the genesis of other Carlin-type gold deposits in the Youjiang basin.