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Syenite, Fangcheng Nb deposit, Magmatic and hydrothermal titanite, Fluid metasomatism, Microanalysis
Abstract:
Titanite is presented here as a tracer for reconstructing the mineralization history of Nb during magmatic and hydrothermal processes in the Fangcheng Nb deposit, central China. Three types of titanites with magmatic and hydrothermal origins are distinguished. The magmatic titanite (Ttn I), is generally wedge-shaped with larger grain size (up to 1.5 mm). The hydrothermal titanite, either overgrows the magmatic titanite as a thin rim (Ttn IIA), or occurs as bead-like clusters coexisting with hydrothermal albite, fluorite and/or Nb-rich oxides (Ttn IIB; anhedral and lesser than 200 μm). The Nb contents in the magmatic titanite (mean of 1.2 wt%), tend to be lower than in the hydrothermal types (mean of 1.7 wt%). The hydrothermal titanites show enrichment in Al2O3, F, Ta, Y, HREEs and Sn and depletion in TiO2, Zr, Hf, Th and LREEs relative to the magmatic titanite. Using Zr-in-titanite as a geothermometer, the magmatic titanite yields higher crystallization temperatures (mean of 760 °C) than the hydrothermal titanites (mean of 610 °C). Titanite U-Pb dating results reveal that the Fangcheng magmatic and hydrothermal titanites were formed at 870 Ma and 406 Ma, respectively. These results are consistent with the zircon U-Pb ages of this study and previous publication as well as the regional tectono-magmatic activities, indicating two discrete episodes of Nb mineralization events at Fangcheng. Isotopic data show distinctly lower εNd (t) values (−4.8 to −10.3) in the hydrothermal titanites as compared to the magmatic titanite (−0.6 to −4.1), confirming that these two generations of titanite involve distinct sources.
Based on all results, we propose that the early magmatic titanite nucleated and crystallized in a high-T and Nb-rich alkaline magma during the Neoproterozoic. Early Paleozoic hydrothermal fluids partly replaced the magmatic titanite (Ttn I), forming the hydrothermal titanite rims (Ttn IIA). During this process, Nb and other HFSEs and REEs were re-activated and subsequently re-precipitated into secondary titanite (Ttn IIB), rutile, pyrochlore and euxenite.
