Extreme variation of sulfur isotopic compositions in pyrite from the Qiuling sediment-hosted gold deposit, West Qinling orogen, central China: an in situ SIMS study with implications for the source of sulfur

Abstract

High spatial resolution textural (scanning electron microscope (SEM)), chemical (electron microprobe (EMP)) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS)), and sulfur isotopic (secondary ion mass spectrometry (SIMS)) analyses of pyrite from the Qiuling sediment-hosted gold deposit (232±4 Ma) in the West Qinling orogen, central China were conducted to distinguish pyrite types and gain insights into the source and evolution of sulfur in hydrothermal fluids. The results reveal an enormous variation (−27.1 to +69.6‰) in sulfur isotopic composition of pyrite deposited during three paragenetic stages. Pre-ore framboidal pyrite, which is characterized by low concentrations of As, Au, Cu, Co, and Ni, has negative δ 34 S values of −27.1 to −7.6‰ that are interpreted in terms of bacterial reduction of marine sulfate during sedimentation and diagenesis of the Paleozoic carbonate and clastic sequences, the predominant lithologies in the deposit area, and the most important hosts of many sediment-hosted gold deposits throughout the West Qinling orogen. The ore-stage hydrothermal pyrite contains high concentrations of Au, As, Cu, Sb, Tl, and Bi and has a relatively narrow range of positive δ 34 S values ranging from +8.1 to +15.2‰. The sulfur isotope data are comparable to those of ore pyrite from many Triassic orogenic gold deposits and Paleozoic sedimentary exhalative (SEDEX) Pb-Zn deposits in the West Qinling orogen, both being hosted mainly in the Devonian sequence. This similarity indicates that sulfur, responsible for the auriferous pyrite at Qiuling, was largely derived from the metamorphic devolatization of Paleozoic marine sedimentary rocks. Post-ore-stage pyrite, which is significantly enriched in Co and Ni but depleted in Au and As, has unusually high δ 34 S values ranging from +37.4 to +69.6 ‰, that are interpreted to result from thermochemical reduction of evaporite sulfates in underlying Cambrian sedimentary rocks with very high δ 34 S values. The variations in Au content and sulfur isotopic compositions across a single ore-stage pyrite grain may reflect displacement of indigenous groundwater with low δ 34 S values by auriferous metamorphic fluids with high δ 34 S values. The very low-grade metamorphism of the host rocks and the metamorphic derivation of sulfur for the ore pyrite indicate that the Qiuling sediment-hosted gold deposit is an epizonal manifestation of an orogenic gold system in the West Qinling orogen.

Metal remobilization and ore-fluid perturbation during episodic replacement of auriferous pyrite from an epizonal orogenic gold deposit

“…Samples containing iron sulfides, and typical host rocks, were prepared as polished thick and thin sections. The thick sections were examined under optical microscopy to characterize the multiple generations of sulfides, and areas of interest (3-5 mm in diameter) were drilled out and mounted in 25-mm diameter epoxy discs together with pyrite standards for SIMS sulfur isotope analysis (Chen et al, 2015).…”

“…High-precision secondary ion mass spectrometry (SIMS) S-isotope analysis was carried out using the Cameca IMS-1280 at the Institute of Geology and Geophysics (Beijing), Chinese Academy of Sciences. Analytical conditions and procedures for high-precision S-isotope analyses with this IMS-1280 are described in detail elsewhere (Chen et al, 2015). We used the same parameters as Chen et al (2015) and applied a Cs + primary ion beam, with a spot size of approximately ~10×15 μm, impact energy of 10 kV, and an intensity of ~2.5 nA.…”

“…Ion probe mapping using NanoSIMS (Nanoscale secondary ion mass spectrometry) has the advantage of combined sub-50 nm lateral resolution and low detection limits and provides an attractive option for qualitative mapping of low concentrations of trace elements at high spatial resolution (McLoughlin et al, 2011). Furthermore, SIMS, with a spot size as small as 10 μm, provides an opportunity to study sulfur isotopic variation between the different generations of sulfides and their replacements (Chen et al, 2015).…”

Ore-Forming Processes of the Daqiao Epizonal Orogenic Gold Deposit, West Qinling Orogen, China: Constraints from Textures, Trace Elements, and Sulfur Isotopes of Pyrite and Marcasite, and Raman Spectroscopy of Carbonaceous Material

“…BSR commonly produces sulfur isotope fractionation between SO4 2and H2S of 4 to 46 per mil (mean 21 per mil;Canfield and Teske, 1996;Habicht et al, 1998). The lowest δ 34 S value observed for py1 (-30.9‰) suggests BSR may have occurred alongside a process associated with sulfur disproportionation of intermediate sulfur species (e.g., S 0 , S2O3 2or SO3 2-; fractionation of 7 to 11 per mil;Canfield and Thamdrup, 1994;Habicht et al, 1998;Cheshire and Bish, 2012;Chen et al, 2015).This large δ 34 S range of stage S2 pyrite and marcasite (-7.7 to +22.0‰) is comparable to diagenetic pyrite in the underlying Paleozoic sedimentary sequences of the WQO Devonian phyllite and metagreywacke[-29.0 to +17.5‰, mean = -0.9‰, s.d. = 17.6, n = 12;…”

A review of the Early Mesozoic granitoids in the Qinling Orogen: Implication for gold metallogeny

“…Detailed investigations of the individual intrusions have been carried out in geology, geochronology, and geochemistry since 1990s (e.g., Deng, Liu, Zhang, Hu, & Li, ; Jiang et al, ; Li et al, ; Li et al, ; Liu et al, ; Luo, Zhang, & Lü, ; Qin, Lai, & Li, ; Qin, Lai, Wang, & Li, ; Qin et al, ; Qin et al, ; Qin, Lai, Wang, & Li, ; Qin, Lai, Diwu, Ju, & Li, ; Sun, Li, Sun, Zhang, & Li, ; Wang et al, ; Wang, Wang et al, ; Wang, Wang, & Zhang, ; Yang et al, ; Yang, Liu et al, ; Zhang, Jin et al, ; Zhang, Xiao, Zhang, Yuan, & Jin, ; Zhu et al, ). In addition to the Early Mesozoic granitoids, the other attractive geological features in the Qinling Orogen is the extensive distribution of gold deposits (Figure ; Jin, Li, Hofstra, & Sui, ; Liu, Lui et al, ; Liu, Dai et al, ; Liu, Lui et al, ; Liu, Liao et al, ; Mao et al, ; Zeng et al, ; Yang, Deng et al, ; Yang, Liu, et al, ), which were classified as orogenic (Chen et al, ; Chen & Santosh, ; Mao et al, ), Carlin‐type (Chen, Zhang, Zhang, Pirajno, & Li, ; Yang, Zhu et al, ) and Carlin‐like (Liu, Dai et al, ; Liu, Tan, Wang, & Wei, ; Zhang, Du, Wang, & Qi, ) in view of mineralization style, alteration assemblages, and geochemical signatures. Although the Early Mesozoic granitic intrusions are widespread over the West Qinling Orogen, their petrogenesis and tectonic implications have not yet been addressed in detail, more importantly their role in gold mineralization is controversial and poorly understood.…”