Sulfide oxidation observed using micro-Raman spectroscopy and micro-X-ray diffraction: The importance of water/rock ratios and pH conditions

Oxidative dissolution of arsenopyrite and pyrite from two former mining sites located in the French Massif Central has been studied to determine some of the critical parameters controlling the formation of secondary phases. Micro-Raman spectroscopy (mu RS) and micro-scanning X-ray diffraction (mu SXRD) were used for mineralogical identification and for mapping the distribution of these alteration products. The two milling sites are characterized by different pH conditions and sedimentary environments. Enguiales exhibits acidic conditions (pH similar to 3) and consists of coarse-grained tailings on a steep slope, which have been leached by meteoric waters and represent an oxidizing environment with a high solid/solution ratio. In contrast, the Cheni site shows near neutral conditions (pH similar to 6.3) and consists of muddy tailings deposited in a settling basin, which exemplify an oxidizing environment with a low solid/solution ratio. Amorphous or poorly crystalline iron arsenate (e.g., amorphous scorodite or parasymplesite-like phase) are among the first products precipitated from oxidized arsenopyrites. These iron arsenates are highly mobile and are frequently observed in association with non-arsenian pyrites, where in some instances they have matured into more crystalline forms. Arsenic may also be trapped by amorphous or poorly crystalline iron (oxy)hydroxides, as has been observed for goethite or jarosite in the latter stages of sulfide oxidation. This study also shows that rings of elemental sulfur are formed around altered sulfides, but only when conditions are near neutral.