Mineral/water interactions in tailings from a tungsten mine, Mount Pleasant, New Brunswick |
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| Authors: | BM Petrunic TA Al |
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| Institution: | Department of Geology, P.O. Box 4400, University of New Brunswick, Fredericton, NB E3B 5A3, Canada |
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| Abstract: | The pore-water geochemistry and mineralogy of tailings derived from a granitic tungsten deposit were characterized by collecting pore-water samples at discrete depth intervals throughout the tailings for the analysis of major and minor element concentrations. Mineralogical samples from the oxidation zone were analyzed by X-ray diffraction, scanning electron microscopy combined with energy dispersive X-ray spectroscopy (SEM/EDS), electron microprobe (EMP) combined with wavelength dispersive X-ray spectroscopy (WDS), and transmission electron microscopy (TEM). The oxidation of sulfide minerals in the near-surface tailings leads to a decrease in pore-water pH and elevated SO4, As, and metal concentrations. The unusual mineralogy of this deposit, compared with that of commonly studied base-metal and gold deposits, results in several unique geochemical characteristics. The dissolution of fluorite releases F into the pore water; the F forms strong complexes with Al and enhances the dissolution of aluminosilicate minerals within the oxidation zone. As a result, high Al concentrations (up to 151.7 mg/L) are detected in the near-neutral pore water in the oxidation zone. The combined dissolution of aluminosilicates and carbonate minerals maintains the pH near 10 in the pore water at depth. Elevated concentrations of W (up to 7.1 mg/L) are detected in the pore water throughout the tailings, likely as a result of the dissolution of wolframite. Consistent with geochemical model calculations, results from SEM/EDS, EMP/WDS and TEM/EDS analyses indicate that secondary minerals, which occur as orange-brown coatings on grains of primary-minerals, are Fe oxyhydroxides. Examples of these secondary minerals display a fibrous habit at high resolution in the TEM. One of these minerals, which contains substantial amounts of Al, As, and Si as impurities, was identified by selected-area electron diffraction (SAED) analyses to be goethite. Another mineral contains relatively high amounts of Si, Pb, Bi, and As, and SAED analyses suggest that the mineral is two-line ferrihydrite. |
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