Formation of As(II)-pyrite during experimental replacement of magnetite under hydrothermal conditions

The coupled geochemistry of Au and As in pyrite from hydrothermal ore deposits

“…% when pyrite was formed by replacement of pyrrhotite. In contrast, As 2+ incorporation is favoured when arsenian pyrite is formed via replacement of magnetite, under temperature conditions between 125 and 220 o C (Qian et al, 2012). Arsenic solubility in pyrite decreases with increasing temperature, and pyrite hosts only ~20-190 ppm As at ~500 o C and Au < 3 ppm (Tauson, 1999).…”

“…% in pyrite hosting As 3+ (Deditius et al, 2008). Recently, Qian et al (2012) synthesized arsenian pyrite that contains As 2+ species at concentrations as high as 24 wt. %.…”

“…For example, sorption of the dominant dissolved As-species As 3+ (OH) 3 (Pokrovski et al, 1996) under ambient conditions results in reduction of As 3+ to As 1-in the pyrite structure with local formation of a FeAsS-like precipitate (Bostick and Fendorf, 2003). Also, experimental data show that reaction paths involved in the formation of arsenian pyrite and speciation of As in its structure depend on the type or nature of the initial mineral nucleus (Fleet and Mumin, 1997;Qian et al, 2012).…”

“…At Au/As ratios above the solubility limit (Au:As = 1:200), Au is present as nanoparticles of native metal (Au 0 ), and at ratios below the solubility line, it is present as cationic Au in solid solution (Simon et al, 1999a;Chouinard et al, 2005a;Reich et al, 2005). Arsenic, on the other hand, usually occurs in its anionic As 1-state in pyrite (Simon et al, 1999a;Savage et al, 2000), although recent studies have reported cationic As 2+,3+ -species under oxidizing conditions typical of highsulfidation epithermal and porphyry deposits (Deditius et al, 2008;Qian et al, 2012) (Fig. 2).…”

“…In addition, sorption of Au may be enhanced by changes in the semiconducting properties of pyrite (n-type to p-type) due to the incorporation of As (e.g., Maddox et al, 1998;Rickard and Luther, 2007). Pokrovski et al (2002a) proposed that the surface of arsenian pyrite is locally reducing so that it may be responsible for the destabilization of Au-sulfide complexes and Au deposition, a process that would favor rapid growth of Au-bearing arsenian pyrite under non-equilibrium conditions (Reich et al, 2005;Deditius et al, 2008;Qian et al, 2012 and references therein).…”