Identification and characterization of secondary minerals formed in tungsten mine tailings using transmission electron microscopy

Secondary minerals formed in tailings derived from a W-rich deposit were investigated in detail using transmission electron microscopy (TEM). The study focused on secondary minerals that formed in the vicinity of oxidized sphalerite [ZnS] and tennantite [Cu10(Fe,Zn)2As4S13] grains. Samples for TEM analysis were prepared directly from petrographic thin sections using a focused ion beam instrument. This method insured that spatial relationships among primary grains, secondary minerals and the pore spaces were maintained. The results from this study indicate that the secondary coatings associated with sphalerite and tennantite are composed of several discrete phases. The phases identified in this study include an Fe-Zn-As-O phase, secondary sulfides, native Cu, an Fe-Si-O phase, an In-O phase, and wulfenite [PbMoO4]. The Fe-Zn-As-O phase precipitates directly from the pore water and the nearby primary mineral grains act as a source for some of the elements (e.g., Zn from sphalerite, As from tennantite). Secondary Cu sulfides were found at the outer margins of sphalerite and roquesite [CuInS2] grains. It is likely that these Cu sulfides form as a result of interactions between the primary grain and aqueous Cu(II) present in the pore water, similar to what occurs in supergene environments. A secondary sulfide that was composed of variable amounts of Cu, Zn, As, Fe and S was also identified along the outer margins of tennantite. Native Cu was found in association with chalcopyrite [CuFeS2] inclusions that were present in one of the sphalerite grains and probably represents a low-temperature secondary phase. The oxidation of chalcopyrite in the presence of aqueous Si leads to the formation of a nanocrystalline or amorphous Fe-Si-O phase. Roquesite oxidation leads to the formation of a crystalline In-O phase, which is likely dzhalindite [In(OH)3]. Wulfenite was found in the interstitial voids present in the Fe-Zn-As-O phase suggesting that it forms by direct precipitation from the local pore water. The results from this study indicate that secondary coatings consist of complex secondary phases that may only be distinguished at the nanoscale. The TEM investigations reveal details regarding mineralogical sinks and sources for aqueous components that may otherwise be overlooked.