Transportation and evolution of trace element bearing phases in stream sediments in a mining – Influenced basin (Upper Isle River,France) |
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| Authors: | C Grosbois A Courtin-NomadeF Martin H Bril |
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| Institution: | Université de Limoges, Laboratoire HydrAsa, Equipe ETM, UMR 6532 CNRS, 123 av. Thomas, 87060 Limoges cedex, France |
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| Abstract: | Arsenic-bearing stream sediments enter the Upper Isle River, an Au mining-influenced basin (France), by the discharge of mining sites, tailings runoff and weathering of mineralized veins in granites and gneiss. Some fresh ochreous As-rich deposits on the river banks and in floodplains are identified as additional As-rich point sources (As between 0.07 and 6.5 wt.%). The <63 μm fraction of stream sediments contains elevated As bulk concentrations, ranging from 160 to 890 mg/kg, compared to the geochemical background (70 mg/kg on average). It is also enriched in Cd, Hg and W. Spatial variations of these trace elements show 3 significant increases corresponding to the 3 drained mining districts. They decrease down river but are still enriched 30 km downstream of the mining districts due to downstream transportation. Three types of trace element-bearing phases have been identified as: (i) detrital primary sulfides, with high in situ As percentages (up to 43.7 wt.%). They also carry significant amounts of W according to the differences in chemical compositions of the total and light <63 μm fractions. These sulfide particles do not show any sign of alteration in the oxygenated stream sediments; (ii) Secondary Fe–Mn oxyhydroxides,some with very high in situ As2O5 concentrations (up to 59.8 wt.%) and with about 40% of the total Cd composition. They occur as fresh precipitates in the river banks and floodplains and as discrete particles in stream sediments and (iii) Al–Si fine-grained phases. Their major element composition is highly variable with in situ As2O5 concentrations ranging between 430 and 5020 mg/kg. This type of solid phase is also the major carrier of Hg. |
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