Crystallization of columbite-tantalite in pegmatites: Experimental constraints and implication for natural systems
Final Report Abstract
The columbite-group minerals (Fe,Mn)(Nb,Ta)2O6 are common Nb-Ta minerals in granites and pegmatites. Although ore deposits for Tantalum with economic relevance are almost exclusively found in pegmatites, the formation conditions of columbo-tantalite and associated minerals in pegmatitic systems remain poorly understood. This petrological project consisted in several experimental and analytical tasks which should all contribute (1) to interpret the composition of Ta-bearing and associated minerals (zircon) in natural systems and (2) to constrain the conditions at which Ta-bearing minerals may crystallize from pegmatitic melts. The characterization of natural ore deposits shows that Ta-bearing minerals and zircon are frequently associated and cogenetic. The Ta content in zircons can be as high as 2 wt% Ta2O5 and the substitution mechanisms for the incorporation of Ta in zircon have been investigated. As a result of the complexity of the substitutions, a simple thermometer for the crystallization of zircon associated to Ta-bearing minerals could not be established. However, the substitutions mechanisms could be worked out. In the presence of high Al concentrations in the pegmatitic melts, we demonstrate the existence of the following exchange reaction Al3+ + Ta5+ = 2 (Si4+, Zr4+). However, in the presence of Phosphorus, much smaller amounts of Ta are incorporated into zircon, suggesting that P5+ competes with Ta5+ in tetrahedral sites. Manganese is incorporated only in the presence of P and has no interaction with Ta. The conditions at which columbo-tantalite crystallizes in natural systems are strongly dependent on the solubility of this mineral in highly-fluxed pegmatitic melts. It is demonstrated that the solubility of manganotantalite (MnTa2O6) and manganocolumbite (MnNb2O6) is higher in water-rich flux-rich pegmatitic melts (containing, F, B, Li, and P) when compared to granitic melts. However, in contrast to previous ideas, the addition of fluorine alone has no effect on the solubility of these minerals. Structural parameters of the silicate melt such as the role of alkalis (including Li) or of excess aluminum seem to be crucial. Finally it is demonstrated that the two phase domain in the system FeTa2O6 - MnTa2O6 (compositional gap between co-existing manganotantalite and ferrotapiolite) exists at temperature as high as 1000°C. The results will be combined with ongoing viscosity determinations of highly fluxed silicate melts to work out a general model explaining the local accumulation of columbotantalite in pegmatites.
Publications
- (2009) Magmatic vs. hydrothermal origins for zircon associated with tantalum mineralization in the Tanco pegmatite, Manitoba, Canada. American Mineralogist, 94, 439-450
Van Lichterveld, M., Melcher F., Wirth R.
- (2010) Solubility of manganotantalite and manganocolumbite in pegmatitic melts. American Mineralogist, 95, 537-544
Bartels A., Holtz F., Linnen R.L.
- (2010) Solubility of manganotantalite, zircon and hafnon in highly-fluxed peralkaline to peraluminous pegmatitic melts. Contrib. Mineral. Petrol., 160, 17-32
Van Lichtervelde M., Holtz F., Hanchar J.
- (2011) Incorporation mechanisms of Ta and Nb into zircon and implications for pegmatitic systems. American Mineralogist, 96, 1079-1089
Van Lichtervelde M., Holtz F., Dziony W., Ludwig T., Meyer, H.-P.
- (2011) Influence of fluorine on the solubility of manganotantalite (MnTa2O5) and manganocolumbite (MnNb2O5) in granitic melts - an experimental study. Lithos, 122, 165-174
Fiege A., Kirchner C., Holtz F., Linnen R.L., Dziony W.
