Fingerprinting of conflict minerals: the coltan case
Publikationen: Konferenzbeitrag › Abstract/Zusammenfassung › Forschung
- Bundesanstalt für Geowissenschaften und Rohstoffe
Since about the year 2000, the African continent has developed into a major exporter of coltan from rare-element pegmatites and their eluvial and alluvial weathering products. Coltan – an important tantalum source - is a central African trade name of mineral concentrates chiefly composed of members of the columbite-tantalite group [(Fe,Mn) (Nb,Ta)2O6], with additional tapiolite, wodginite, ixiolite, microlite, cassiterite, rutile and other heavy mineral phases. Tantalum is a rare metal whose strength, chemistry and electronic properties make it valuable in many high technology and medical applications. The use of tantalum has, for instance, been instrumental in reducing the size of mobile phones. Although the major resources of tantalum are located in Australia, Brazil and Canada, African countries account for more than 75 percent of the current world production (2013: 1,230 metric tons Ta2O5), and artisanal and small-scale mining of coltan is essential to many local economies. Probable links between raw material exports and continuing violent conflicts, especially in the Democratic Republic of the Congo (DRC), are widely discussed. On behalf of the German Federal Ministry for Economic Cooperation and Development (BMZ), the Federal Institute for Geosciences and Natural Resources (BGR) commenced a pilot project to test the general viability of a traceability system that would proof the origin of coltan. Such a system would allow ore produced within regions affected by civil war to be distinguished from other sources. Combined mineralogical, geochemical and geochronological signatures of columbite-tantalite ores are used to trace the origin of ore concentrates. The chemical composition of Nb-Ta oxides in pegmatites and rare-element granites is characterised by a strong variability. However, major as well as trace elements show regional characteristics that, in many cases, allow for the distinction of ore provinces based on the composition of a representative number of grains. The major element variations in columbite-tantalite, tapiolite, wodginite and ixiolite reflect differentiation of, and fractional crystallization from melts. The degree and type of the variation of trace element concentration in Nb-Ta oxides is complex and depends on multiple factors, including crystal-chemical parameters (e.g., ion radius, charge), melt chemistry (e.g., presence of fluxing elements), internal differentiation of the melt, reaction with host rocks, and melt source characteristics. All these factors are superimposed on each other and result in characteristic trace element signatures for Nb-Ta oxides from different ore provinces. About 15,000 electron microprobe and ca. 10,000 LA-ICP-MS datasets from rare element pegmatite and rare metal granite deposits ranging in age from the Archean to the Mesozoic and covering all continents except Antarctica were evaluated and demonstrate variations over two to four orders of magnitude for most trace elements. The most important minor elements in columbite-tantalite are Ti, W, Zr and, Sn (median for the dataset is > 1000 ppm), followed by U, Hf (100-500 ppm), Al, Mg, Pb (10-100 ppm), Sc, Li, Mo, Y, Th and, Tl (1-10 ppm). The median concentrations of the REE, As, Sb, Bi, and Sr are below 2 ppm, and concentrations of Be, Rb and Ba in CGM are usually below the detection limit of the LA-ICP-MS method. Considering the coltan “fingerprint” for ore concentrates originating from the African continent, the determination of mineral formation ages using the U-Pb isotope system traces the origin of a concentrate to one out of five “age provinces” known from African tantalum-bearing pegmatites, i.e. >2.5 Ga, 2.0 Ga, 0.9-1.0 Ga, 0.5 Ga and <0.3 Ga. Mineralogical and geochemical attributes of coltan concentrates and columbite-tantalite group minerals further allow distinction of single deposits within a given province. Using appropriate statistical tests, similarities and dissimilarities between coltan concentrates are calculated. The coltan fingerprinting method has become an integral part of a mineral certification scheme that is currently being installed in the Great Lakes Region of Central Africa.
|Status||Veröffentlicht - 12 Aug 2015|