Arsenic mineralogy of hydrothermal deposits

Arsenian pyrite is one of the more important host minerals for gold (Reich et al., 2005 Arehart, 1996). Gold probably exists as a solid solution (Au+) in arsenian pyrite or as Au(0) nanoscale inclusions within the minerals (Reich et al., 2005, 2781). Gold-bearing arsenian pyrite may form through the following reaction in hydrothermal fluids (Reich et al., 2005, 2790)  

Decreases in H2S concentrations through boiling and volatilization of hydrothermal fluids would tend to precipitate gold-bearing arsenian pyrite. 

Guerrot, 2000 Pokrovski, Kara and Roux, 2002, 2361). Arsenian pyrite is also common in hydrothermal deposits at temperatures as low as 150 °C and as high as 250-300 °C (Kesler, Riciputi and Ye, 2005, 132 Reich et al., 2005, 2788 Pokrovski, Kara and Roux, 2002, 2375, 2361). Most pyrites contain only 0.02-0.5 wt % arsenic (Welch et al., 2000, 597). However, arsenian pyrites may host up to 6 wt % arsenic as a solid solution with sulfur (Reich and Becker, 2006). Although pyrites from Nevada, USA, contain as much as 19.76 wt%, much of this arsenic exists as nanoscale arsenopyrite or other mineral inclusions rather than as a true solid solution (Reich et al., 2005 Reich and Becker, 2006, 2784-2786). 

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