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 °C in Au-saturated system by means of salt flux method. The bnss contained ~1.2-1.6 log units more Au than iss: up to 18 wt.% Au in bnss vs 0.4 wt.% Au in iss at 700 °C. An increase of temperature resulted in the sharp increase of Au concentration in both phases, ~1 log unit per 100 °C at f(S₂) close to S_(l) saturation. Analysis of Au L₃-edge spectra recorded at 25-675 °C revealed that at 25 °C Au exists mainly in the metallic state. At t > 500 °C the spectral features of Au° disappear, 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-xS and bnss contain Au in a mixture of stromeyerite-like and petrovskaite-like (Au_0.8Ag_1.2S) clusters. The chemical composition of both forms is close to CuAuS, where the nearest Au neighbors are two S atoms at R_Au-S = 2.34-2.36 Å. 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(S₂)-compositional parameters. Due to the sharp increase of the CuAuS clusters stability with increasing temperature, in high-temperature ores formed at t > 350 °C Au enriches Cu-bearing 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.