The state of gold in phases of the Cu-Fe-S system:In situ X-ray absorption spectroscopy study

Chalcopyrite and bornite are the main Au-bearing minerals at Cu porphyry deposits,volcanogenic massive sulfide(VMS)deposits,Cu-Ni deposits of the mafic magmatic complexes,and ores of submarine sulfide edifices.Bornite and intermediate solid solutions with wide compositional variations(bnss and iss–high-temperature chalcopyrite,correspondingly),which can scavenge economic concentrations of Au,appear in the Cu-Fe-S system at ore-forming conditions.However,the state of Au in bnss and iss is yet unknown.To solve this conundrum,we synthesized samples with net chemical composition of bnss and iss,studied them by in situ X–ray absorption spectroscopy(XAS),and used the experimental data to explain the Au distribution among natural ore-forming minerals.The sulfide samples were obtained at 495–700℃ in Au-saturated system by means of salt flux method.The bnss contained1.2–1.6 log units more Au than iss:up to 18 wt.%Au in bnss vs 0.4 wt.%Au in iss at 700C.An increase of temperature resulted in the sharp increase of Au concentration in both phases,1 log unit per 100℃ at f(S2)close to S_((l)) saturation.Analysis of Au L_(3)-edge spectra recorded at 25–675℃ revealed that at 25℃ Au exists mainly in the metallic state.At t>500℃ the spectral features of Audisappear,and “chemically bound”Au predominates.The Au form of occurrence in the iss field is interpreted as Au-bearing clusters with a stromeyerite-like(CuAgS)structure.Digenite Cu_(2–x)S and bnss contain Au in a mixture of stromeyeritelike and petrovskaite-like(Au_(0.8)Ag_(1.2)S)clusters.The chemical composition of both forms is close to CuAuS,where the nearest Au neighbors are two S atoms at R_(Au-S)=2.34–2.36Å.Results of the present study allow to determine the state of Au and its concentration in the main Cu-bearing minerals of sulfide ores as a function of the T-f(S_(2))-compositional parameters.Due to the sharp increase of the CuAuS clusters stability with increasing temperature,in high-temperature ores formed at t>350℃ Au enriches Cubearing minerals in comparison with Cu-free or Cu-deficient ones.As a result,in these ores native gold,being a product of decomposition of the Au-bearing clusters,is associated with Cu-rich minerals–chalcopyrite,bornite,digenite,chalcocite.

嫦娥五号月壤中首次发现蒸发沉积成因的蓝辉铜矿

由于缺乏磁场和大气的保护,月球表面持续受到陨石和微陨石的轰击.撞击引起的气化沉积作用是月表典型的改造过程,该过程往往伴随独特矿物相的产生(如纳米金属铁以及铁硅化合物).研究团队通过对嫦娥五号月壤进行细致的扫描电子显微镜观察,在纯铁颗粒以及玻璃质表面发现了具有典型气相沉积特征的含铜组分,进一步通过透射电子显微镜以及电子能量损失谱确认了这些含铜沉积物为蓝辉铜矿这是首次在月壤中发现此类矿物.本研究提供了月表硫化物发生气化沉积的直接证据,揭示了月表撞击过程引起的气相组分迁移及其对月表物质的显著改造效应.同时,嫦娥五号月壤中蓝辉铜矿的发现进一步拓宽了人们对月表复杂矿物组成的认识.