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Four main and three minor rare-earth-element (REE)-bearing minerals were identified and quantified in the Kiirunavaara apatite iron ore tailings using optical microscopy, an electron probe microanalyzer (EPMA) and a mineral liberation analyzer, and their chemical compositions were analyzed by the EPMA and laser ablation inductively coupled plasma-mass spectrometry. REEs are shown to be contained in the minerals apatite, monazite, allanite, titanite, zircon, thorite and synchysite. In zircon, thorite and synchysite, they occurred in only trace amounts and contributed limited amounts to the total REE budget, and these are consequently of minor importance. Monazite occurred as inclusions in apatite and as free particles, 90 percent liberated. Allanite occurred to some degree in mixed grains with magnetite but also as free particles. Monazite mainly reported to the apatite concentrate, while allanite and titanite largely went to the tailings, the latter preferably to those fractions smaller than 38 µm. The amount of titanite in the finest tailings fraction was 2.3 weight percent, containing close to 1 percent REEs, with heavy rare earth elements (HREEs) making up 28 percent of the total REEs. However, a texturally distinct group of titanite grains showed an HREE/REE ratio of up to 67 percent. Furthermore, titanum dioxide analyses indicate that titanite is preferentially released into the tailings from the secondary magnetic separation step in the concentrator. Our data therefore suggest that titanite, occasionally enriched in HREEs, can be extracted from the processing stream and might thus be considered a new source for REEs at Kiirunavaara and similar deposits.

Abstract: The partitioning of a number of trace elements (Ba, Nb, Zr, Y, REE, etc.) between orthopyroxene, garnet, and carbonate-silicate melt was experimentally studied using a belt apparatus at pressures of 3.5–4.2 GPa and temperatures of 1300–1500°C. The experimental products were investigated by electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The experimental melts varied from carbonatitic (∼5 wt % SiO2) at low temperatures (1300–1350°C) to kimberlitic compositions (30 wt % SiO2) at high temperatures (1500°C). The partition coefficients of most elements between orthopyroxene and melt (D i Opx/L ) and garnet and melt (D i Grt/L ) were almost independent of melt composition (temperature). The D i Opx/L values ranged from <0.01 for the most incompatible Ba and light REE to 0.02–0.08 for moderately incompatible Zr, Y, and heavy REE. The D i Grt/L values were approximately an order of magnitude higher, ∼0.07 for light REE, 0.7 for Y, and 1.5 for Yb. The character of D i Grt/L variations in the systems studied is in general similar to that established for silicate melts without volatile components. However, the differences in the behavior of moderately incompatible and compatible elements (e.g., light and heavy REE) in the experimental systems are less pronounced compared with CO2-free systems. Considering carbonate-silicate and silicate melts as possible agents of mantle metasomatism, it can be concluded that the former can efficiently transport heavy REE, and the latter have a greater affinity for Nb, Ba, and light REE. A characteristic feature of mantle rocks enriched by carbonate-silicate melts is high Ba/La ratio coupled with relatively weakly fractionated REE distribution patterns. It was shown that the high degrees of enrichment observed in natural kimberlites can be explained by a two-stage scenario, including a preliminary invasion of carbonate-silicate melt into depleted harzburgites in the lower parts of the lithosphere and subsequent very low degree melting.

AbstractIn situlaser ablation high resolution ICP-MS analyses of scheelite from hydrothermal veins at the Archaean Mt. Charlotte gold deposit (Western Australia) show inhomogeneous REEdistribution at small scale (<100 µm). In a limited number of samples, variations of the cathodoluminescence (CL) colours from blue to yellow are linked to the REEcontent of scheelite, and reveal oscillatory zoning of the REEwith zone widths between 1 µm and 100 µm. However, CL failed to reveal the zoning in most inhomogeneous scheelite samples. A nuclear microprobe has been used to characterize the distribution of REEin these samples. No reasonable map for the distribution of REEcould be obtained by particle induced X-ray emission, because of interferences with W-Llines. However, monochromatic ionoluminescence (IL) maps collected at the wavelength of the main REE3+luminescence peaks revealed oscillatory zoning. Therefore, IL is a powerful tool for mapping the distribution of REEin natural scheelite. Monochromatic IL maps allow us to determine the nature of the inhomogeneous distribution of REEin scheelite, fundamental information for using the REEin this mineral as a marker for the chemistry of ore-forming fluids, and for interpreting Sm-Nd isotopic data.