Abstract
The Kangxiwa–Dahongliutan pegmatite field in the Western Kunlun Orogen, China contains numerous granitic pegmatites around a large granitic pluton (the Dahongliutan Granite with an age of ca. 220 to 217 Ma), mainly including barren garnet-, tourmaline-bearing pegmatites, Be-rich beryl-muscovite pegmatites, and Li-, P-rich albite-spodumene pegmatites. The textures, major element contents, and trace element concentrations of columbite-group minerals (CGM) and cassiterite from three albite-spodumene pegmatites in the region were investigated using a combination of optical microscopy, SEM, EPMA and LA-ICP-MS. The CGM can be broadly classified into four types: (1) inclusions in cassiterite; (2) euhedral to subhedral crystals (commonly exhibiting oscillatory and/or sector zoning and coexisting with magmatic cassiterite); (3) anhedral aggregates; (4) tantalite-(Fe)-ferrowodginite (FeSnTa2O8) intergrowths. The compositional variations of CGM and cassiterite are investigated on the mineral scale, in individual pegmatites and within the pegmatite group. The evolution of the pegmatites is also discussed. The variation of Nb/Ta and Zr/Hf ratios of the cassiterite mimics the Nb-Ta and Zr-Hf fractionation trends in many LCT pegmatites, indicating that these two ratios of cassiterite may bear meanings regarding the pegmatite evolution.
Highlights
Niobium and tantalum, considered as strategic metals, are widely used for manufacturing high-technology products [1,2] and columbite-group minerals (CGM) with a chemical formula (Fe,Mn)(Nb,Ta)2 O6 are the most important minerals in which Nb and Ta are major components.CGM commonly occurs in granitic pegmatites and highly evolved granites [3,4,5,6]
We focus on the NW-SE trending Tianshuihai terrane (TST) where the Kangxiwa–Dahongliutan pegmatite field is situated
From Type 3 CGM to Type 4 CGM + ferrowodginite, the compositional trend is characterized by significant increase in Ti, Sc, Zr, Hf, and Sn and slight decrease in W, accompanied by decreased Mn/Fe (Figure 10)
Summary
Niobium and tantalum, considered as strategic metals, are widely used for manufacturing high-technology products [1,2] and columbite-group minerals (CGM) with a chemical formula (Fe,Mn)(Nb,Ta) O6 are the most important minerals in which Nb and Ta are major components.CGM commonly occurs in granitic pegmatites and highly evolved granites [3,4,5,6]. Recent studies show that chemistry of CGM possibly reflects the chemistry of magma sources and pegmatite melts [5,6,7]. That compositional variations of CGM can potentially reveal fractionation of pegmatite melts as well as hydrothermal evolution of pegmatites [8]. Most studies on CGM from granitic pegmatites focus on major and trace elements such as Nb, Ta, Ti, Sn, Fe, and Mn [9,10,11,12,13,14], partly due to the limitations of analytical methods. Few studies paid attention to variation of trace element (including Li, Zr, Hf, and REE) concentrations in CGM and its relationship with pegmatite evolution [5,7,8]. More research needs to be carried out in order to understand the behavior of trace elements in CGM as well as in other Nb-Ta oxides
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