Abstract
Ore minerals of niobium and tantalum are typically associated with pegmatites and rare metal granites; these include columbite, tantalite, wodginite, titanowodginite, microlite and pyrochlore. Solubility and crystallization mechanisms for columbite-(Mn) and tantalite-(Mn) have been extensively studied in haplogranitic melts, with little research into other ore minerals. In this study the solubilities of wodginite, titanowodginite, microlite and pyrochlore are compared to those for columbite-(Mn) and tantalite-(Mn) in a flux-rich haplogranitic melt of alumina saturation index (ASI) 1.0 (ASILi of 0.83) at 700–850 °C and 200 MPa. The effect of melt composition on the solubilities of wodginite, titanowodginite, and microlite compared to tantalite-(Mn) is also investigated in highly fluxed haplogranitic melts of ASI 1.0, 1.10, and 1.24 (ASILi of 0.83, 0.90, 1.02), at 700 °C and 800 °C and 200 MPa. The log solubility product (logKsp) at 750 °C and melt ASI of 1.0 is highest for tantalite-(Mn) (−2.32 mol2/kg2) followed by columbite-(Mn) (−2.68 mol2/kg2), and pyrochlore (−3.71 mol3/kg3) titanowodginite (−3.73 mol3/kg3), wodginite (−3.77 mol3/kg3), and microlite (−3.78 mol3/kg3), the latter four of which are almost identical within error. However, solubility can also be expressed as a mineral-melt partition coefficient, with higher partition coefficients reflecting lower solubilities. The tantalum mineral-melt partition coefficients of wodginite (27.0), titanowodginite (30.2), and tantalite-(Mn) (29.6) are identical within error; microlite is higher (80.3, reflected by lower concentrations of tantalum in the melt for stoichiometric saturation) because it contains a major melt cation, sodium. The niobium mineral-melt partition coefficient for columbite-(Mn) (66.1) is higher than for pyrochlore (50.1). Wodginite, titanowodginite, pyrochlore, columbite-(Mn) and tantalite-(Mn) have similar temperature and melt composition dependences, but conversely, microlite solubility increases with ASI. As all studied minerals have been shown to have similar relative solubilities at stoichiometric saturation, it can be concluded that, in general, the presence of bivalent (Ca, Mn, Fe) and some tetravalent cations (Sn, Ti) control which tantalum and niobium phases crystallize in rare metal deposits.
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