Abstract:Objectives:Zhuguangshan pluton, located in the middle of the Nanling large igneous province in Southern China, is an important ore concentration area of granite-type uranium in China. Achievevments were mainly on pluton, fluids, tectonic, alteration and metallogenic prognosis, but little study about minerals genesis and the interrelation between them. This study focused on the characteristics of arsenic-bearing pyrite and its relationship with uranium mineralization. Methods:Thin section preparation and single mineral separation of pyrite were completed by Langfang Integrity Geological Service Co. Ltd. X-ray powder diffraction, X-ray photoelectron spectroscopy, Back scattered electron imaging(BSE) and electronic probe microanalysis were completed at the Modern Analysis and Test Center of Central South University. He—Ar isotopic and cathodeluminescence analysis were completed at the Analysis and Test Center of Beijing Research Institute of Uranium Geology, CNNC. Xray powder diffraction samples are pure pyrite grains selected from uranium ore, the samples were crushed to 100% passing 200 mesh at least and assayed at 40 kV, 40 mA, 2°/min and 0.02°/step using the Brukerproduced D8 Advance diffractometer. Rietveld refinement was performed on diffraction patterns using Fullprof software, Rietveld method and Pseudo-Voigt model. Fe(S099As001)2 (ICSD:109376) structure was selected as the initial model. The refinement steps include scaling factor, zero correction, background, cell parameters, peak pattern parameters, temperature factor, atomic coordinate and site occupation, etc. DFT-based first-principle computation was performed by CASTEP software, with 2×2×2 super-cell applied. This energy convergence standard was 1.0×10-5 and the electron pole was small, so All bands/EDFT was used for the calculation. The ESCALAB 250Xi instrument was selected as a XPS model, the spot size was 200 μm, the analyser mode was CAE pass Energy, the voltage was 30.0 eV and the energy step length was 0.05 eV. The American FEI-produced Hellos Nanolab 600i instrument was selected as a BSE model, the samples used for this analysis were natural fracture surface samples, and the accelerating voltage was 5~20 kV. A JXA-8230 analyzer was selected as the electronic probe microanalysis instrument model, the accuracy analysis of the main elements(content>5%)≤1%, minor elements is less than or equal to 5%. The accelerating voltage was 15kV, the electronic current was 20.8 μA, the electronic beam diameter was 1 μm, and ZAF correction was adopted. Chinese Academy of Geological Science standard was adopted for calibration: As—InAs, Se—ZnSe, U—CalSTD, Fe—FeS2, S—FeS2, Ni—Ni, W—W, Zn——ZnS, Co—Co, Ti—Ti, Cu—CuFeS2, Cr—Cr, Sb—Sb2S3, Mo—Mo, Bi—Bi, Te—Te and Sn—Sn. The Helix SFT instrument was selected as a He—Ar isotope analyzer and He in air as the standard \[n(3He)/ n(4He)=1.343×10-6±0.013×10-6\]. This experiment adopted the step-by-step vacuum crushing method to extract He and Ar from fluid inclusions. Results: Microstructure determined the pyrite associated relations with pitchblende in space was fine-grained arsenic-bearing pyrite, which formed before the uranium mineralization, released S2-, Fe2+ during uranium metallogenesis and replaced by late-stage siliceous and uranium minerals. Main and trace elements analysis by electron microscope indicates that pyrite in uranium ore is depleted in sulfur and rich in As, Mo, Cu, Zn, Pb, Sb. He—Ar isotopic analytical results indicate that arsenicbearing pyrite has high 3He value and n(40Ar)/ n(36Ar) ratio, was the product of curst-mantle mixed fluid which crust fluid was dominant. Rietveld structure refinement and density functional theory calculation results show that the essence of uranium reduction is due to the addition of As, S atomic have produced a contribution during the valence band and conduction band near the Fermi level, and possess highly activity. Conclusions:Granular pyrite associated with pitchblende in uranium ore is early-formed and depleted in sulfur with in arsenic-rich, which were related to the curst-mantle mixed fluid where crust fluid was dominant. Arsenic-bearing pyrite played an important role in uranium mineralization, with the participation of As, 3s and 3p orbits in S atoms of the pyrite contributed to both the valence band and conduction band near Fermi Level and S atoms have large activity, which is a key factor for the reduction of uranium. The mineralization process of uranium has experienced three stage of the soft base—hard base—acid fluid, which finally resulted into the heterogeneity of uranium mineralization characteristics in fractal scaling.