Variability of the copper isotopic composition in soil and grass affected by mining and smelting in Tsumeb, Namibia

The Tsumeb mining District (Namibia) is affected by prolonged exposure to multisource contamination related to the mining and smelting of copper ores. This study was performed to assess the extent of contamination of topsoil and grass shoots on the basis of the copper isotopic composition. Compared to an uncontaminated area (δ65Cusoil = −0.01 to +0.14‰; δ65Cugrass = −0.17 to +0.03‰), the soil and grass in the contaminated area are enriched in the heavy isotope (δ65Cusoil = +0.13 to +0.76‰ and δ65Cugrass = +0.15 to +0.64‰). The Δ65Cugrass-soil value exhibits small variations (+0.06 to −0.17) due to the variable degree of grass surface contamination, depending on the grass washing procedure used. The enrichment of soil and grass in the contaminated area in 65Cu reflects the Cu isotopic composition of old flotation tailings (δ65Cu = +0.95 to +1.1‰), slag (+0.11 to +0.55‰) and particles which originate from the smelting and flue gas cleaning processes (+0.15 to +0.49‰). The long history of mining and processing of copper ores in the Tsumeb area and the different isotopic composition of ore concentrates smelted in the local metallurgical plant do not allow assessment of whether the smelting process leads to isotope fractionation of copper. However, the isotopic composition of Cu in the sulfidic feed, slag and dust particles sampled over several hours showed that the dust collected from the dusthouse of the local smelter is isotopically lighter (δ65Cu = +0.15‰) relative to the isotopic composition of the charge (δ65Cu = +0.28 and +0.44‰). The results of this investigation demonstrate that the isotopic composition of Cu in soils and grasses can be used to assess the intensity of contamination in areas where Cu ores are mined and processed provided that the isotopic compositions of the ore processing products are different from the isotopic compositions of the uncontaminated soils and vegetation.