X-ray spectroscopic study of the chemical state of “invisible” Au in synthetic minerals in the Fe-As-S system


X-ray spectroscopic study of the chemical state of “invisible” Au in synthetic minerals in the Fe-As-S system

Trigub, A. L.; Tagirov, B. R.; Kvashnina, K. O.; Chareev, D. A.; Nickolsky, M. S.; Shiryaev, A. A.; Baranova, N. N.; Mokhov, E. V. Kovalchuk A. V.

Minerals of the Fe-As-S system are ubiquitous components of Au ores in many deposits of hydrothermal origin, including world-class volcanogenic massive sulfide, low-temperature epithermal and mesothermal ones. The “invisible” (or refractory) form of Au is present in all types of hydrothermal ores and in many cases predominates. Knowledge of the “invisible” Au chemical state (local atomic environment/structural position, electronic structure and oxidation state) is a fundamental problem crucial for understanding conditions of the ore formation, is necessary for physical-chemical modeling of hydrothermal Au mineralization, and will help to create more efficient technologies of ore concentration and Au extraction. We report investigation of the “invisible” Au chemical state in synthetic analogues of natural minerals: pyrite (FeS2), arsenopyrite (FeAsS), and löllingite (FeAs2). The compounds were synthesized using hydrothermal (pyrite) and salt flux techniques (all the minerals) and studied using X-ray absorption fine structure spectroscopy (XAFS) in high energy resolution fluorescence detection (HERFD) mode in combination with first-principles quantum chemical calculations. The concentration of the “invisible” Au in the synthesized löllingite (800±300 ppm) was much higher compared to arsenopyrite (23±14 ppm), whereas the lowest Au content with zonal Au concentration profiles was observed in crystals of salt flux pyrite. The concentration of Au in the hydrothermal pyrite (40-90 ppm) is independent on sulfur fugacity and probably reflects the maximum Au solubility at the experimental P/T parameters (450 °C, 1 kbar). It is shown that Au replaces Fe in structure of löllingite, arsenopyrite, and hydrothermal pyrite. The nearest-neighbors Au-ligand distance increases by 0.14 Å (pyrite), 0.16 Å (löllingite), and 0.23 (As), 0.13 (S) Å for arsenopyrite relative to Fe-ligand distance in pure compounds. Distortion of the local atomic structure around the Au is negatively correlated with the distance and disappears at r > ~4 Å. The chemically bound Au is stable only in hydrothermal pyrite, whereas the pyrite synthesized in the absence of hydrothermal fluid contains only Au°. Heating (metamorphism) of hydrothermal pyrite results in decomposition of the chemically bound Au with formation of Au° nuggets which coarsen with temperature. Depending on chemical composition of the host mineral Au can play a role either of cation or anion: the Bader atomic partial charges of Au decrease in the order of pyrite (+0.4 e) > arsenopyrite (0) > löllingite (-0.4 e). Our results suggest that other noble metals (platinum group elements, Ag) can form the chemically bound refractory admixture in base metal sulfides/chalcogenides. The chemical state, as well as the concentration, of this form of noble metals can differ depending on the composition of the host mineral and the ore history.

Keywords: invisible gold; pyrite; arsenopyrite; löllingite; synthetic minerals; X-ray absorption spectroscopy; atomic charges

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Publ.-Id: 23610