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The Neoproterozoic granitic rare-metal pegmatites of the Gatumba mining district in western Rwanda have very low base-metal and sulfide content, and are very low in arsenic or other hazardous metals. However, their hydrothermal halos display slight enrichment in thallium at a meter scale, while arsenic may be enriched in associated tin-bearing hydrothermal systems in distant country rock. Columbite–cassiterite concentrates have a few hundred ppm of As, Bi, U and Th which, when calculated back to the pegmatite bulk rock, correspond to a composition near average continental crust. Pegmatite tailings material can provide a fertile substratum for agriculture because these rocks have no deleterious elements, buffer soil pH near neutral conditions, and also are slightly enriched in potassium and phosphorus compared to most soil cover in Rwanda. |
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