Mineralogical and geochemical study of apatite and dolomite from the Bayan Obo giant Fe-REE-Nb deposit in Inner Mongolia: New evidences for genesis

Abstract

The Bayan Obo Fe-REE-Nb deposit is the world’s largest resource of REE and its origin still remains controversial. To decipher the genesis of this unique deposit, we conducted a detailed mineralogical observations using scanning electron microscope (SEM), cathodoluminescence (CL) and in-situ micro-analyses on chemical compositions of the dolomite and apatite by EPMA and LA-ICPMS techniques. A wide range in SrO contents in dolomite mineral grains from coarse-grained dolomite marble has been observed. The SrO contents in the margin of the dolomite mineral reach up to 1.04 wt%, while the SrO contents in the core are as low as 0.16 wt%. In contrast, the SrO contents of the dolomite minerals of fine-grained dolomite marble are basically invariable, mainly concentrating on the range of 0.1 wt%–0.2 wt%. The MnO and FeO concentrations of dolomite mineral grains from the coarse-grained dolomite marble (MnO, 0.24 wt%–0.45 wt%; FeO, 2.27 wt%–3.42 wt%) are significantly lower than those of dolomite from fine-grained dolomite marble (MnO, 0.84 wt%–3.32 wt%; FeO, 3.59 wt%–12.16 wt%). The big differences in major elements between two types of the H8 dolomite marble imply different forming processes. Three types of apatite show big differences in cathodoluminescent signatures, major and trace elements. Type I fluorapatite has few REE mineral inclusions and Type II fluorapatite hosts abundant tiny REE mineral (monazite) inclusions. Type III fluorapatite from strongly deformed coarse-grained dolomite marble forms abnormally large monocrystals with elongated orientation. Both Type I and Type II fluorapatite from fine-grained dolomite marble samples display the heterogeneous component regions under BSE, with obvious depletion in Na and REE in BSE-dark areas observed in the X-ray compositional maps. Unaltered (BSE-bright) regions in Type I and Type II fluorapatite have high REE (17470–77090 ppm), Y (714–8162 ppm), Na (2023–10258 ppm), Sr (2126–6422 ppm) concentrations, obviously different from Type III fluorapatite (REE, 2452–7496 ppm; Y, 157–285 ppm; Na, 501–1464 ppm; Sr, 6878–9949 ppm). The REE, Na, Th and Pb depletion in BSE-dark regions in Type I and Type II apatite correspond to two styles of REE remobilization during fluid-induced dissolution-reprecipitation processes. Late-stage REE, Na, Ba, CO32−, SO42−-rich mesothermal hydrothermal fluid evolved from carbonatite was responsible for REE leaching in Type I fluorapatite. The Sr-rich, Na-poor, REE-poor Type III fluorapatite and associated elevated SrO contents in the margin of dolomite imply that the deposit underwent subsequent fluid infiltration by Sr-rich, REE-poor, Na-poor metamorphic fluid, causing the REE leaching in Type II fluorapatite and the consequent formation of monazite inclusions. As a common REE-bearing mineral in Bayan Obo deposit, apatite displays a diversity of textures and occurrences, fingerprints multi-stage metasomatic fluids and provides new insights into the mechanism of REE enrichment and remobilization.