The state of Au and As in pyrite studied by X-ray absorption spectroscopy of natural minerals and synthetic phases

Abstract

In many hydrothermal deposits arsenian pyrite contains economic concentrations of Au in an “invisible” form, which is structurally bound in pyrite or has grain size <0.1 μm. Here we use X-ray absorption spectroscopy (XAS) of natural minerals and synthetic phases to reveal the forms of Au occurrence in pyrite and determine the effect of physicochemical-compositional parameters on the concentration and state of “invisible” Au. A sample of natural arsenian pyrite with 300 ppm Au and 0.34 wt% As (average value) from the Samolazovskoe Au-sulfide deposit (Yakutia, Russia) was used to study the states of Au and As, and two arsenian pyrite samples from the Vorontsovka Carlin-type deposit (North Urals, Russia) were used to characterize the state of As. The synthesis experiments were performed employing the hydrothermal method at 300 °C/Psat and 450 °C/1000 bar, at contrasting redox states – in oxidized (H2S/H2SO4 redox buffer) and reduced (H2S predominates) fluids, in As-free and As-bearing systems. As a result, a series of samples of As-free and arsenian pyrites was obtained with Au concentrations from ca. 100 to 300 ppm. The concentration of “invisible” Au, which was homogeneously distributed within the samples, was independent of As concentration but decreased with increasing synthesis temperature. The concentration of Au dissolved in acidic sulfide hydrothermal fluid was not affected by the presence of As. The X-ray absorption XANES/EXAFS spectra were recorded simultaneously at Au L3-edge and As K-edge. The use of the high energy resolution fluorescence detection (HERFD-XAS) mode made possible to acquire data for the trace amounts of Au in As-rich samples. According to XANES spectroscopy Au occurs in pyrite in the 1 + oxidation state. Two forms of Au were detected in the pyrite samples: the solid solution Au and Au2S-like clusters/inclusions. In the solid solution state Au replaces Fe in the structure of pyrite and is coordinated only with sulfur atoms (NS = 6, RAu-S = 2.41 ± 0.01 Å) in all pyrite samples independently of the conditions of their formation/synthesis and As content, no As atoms were detected in the local atomic environment of Au. Thus, the Au-S distance in the 1st coordination shell increased by 0.15 Å relatively to the Fe-S distance in pure pyrite. The distant coordination shells of the solid solution Au correspond to the pyrite crystal structure. The presence of the Au2S-like clusters leads to a significant decrease of the calculated values of coordination number of Au and Au-S interatomic distance. In natural pyrite samples from the Vorontsovka deposit As1− replaces S in the anionic sites, increasing the 1st shell As-S and As-Fe distances by 0.07–0.1 Å relatively to the pure pyrite structure. A contribution of As3+ oxide was detected in pyrite from the Vorontsovka deposit. The synthetic pyrite samples contain As in the form of As1− solid solution and As3+ and As5+ oxides. Results of our study show that neither the concentration and speciation of As nor the redox state of the system affect the state of “invisible” Au in the studied pyrites.