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 mas-sive sulfide (VMS) deposits, Cu-Ni deposits of the mafic magmatic complexes, and ores of submarine sul-fide 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 degrees C in Au-saturated system by means of salt flux method. The bnss contained similar to 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 degrees C. An increase of temperature resulted in the sharp increase of Au concentration in both phases,similar to 1 log unit per 100 degrees C at f(S-2) close to S(l) saturation. Analysis of Au L-3-edge spectra recorded at 25-675 degrees C revealed that at 25 degrees C Au exists mainly in the metallic state. At t > 500 degrees C the spectral features of Au degrees 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 Cu2-xS and bnss contain Au in a mixture of stromeyerite-like and petrovskaite-like (Au0.8Ag1.2S) clusters. The chemical composition of both forms is close to CuAuS, where the nearest Au neighbors are two S atoms at RAu-S = 2.34-2.36 angstrom. 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 clus-ters stability with increasing temperature, in high-temperature ores formed at t > 350 degrees 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 - chal-copyrite, bornite, digenite, chalcocite. (c) 2022 China University of Geosciences (Beijing) and Peking University. Published by Elsevier B.V. on behalf of China University of Geosciences (Beijing). This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).