Crystallization of columbite-tantalite in pegmatites: Experimental constraints and implication for natural systems

• Applicant: Professor Dr. Francois Holtz
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: from 2008 to 2011
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 63444183

Tantalum (Ta) is a key element of the high-tech industry and economic concentrations of Tabearing minerals occur exclusively in granitic pegmatites, mainly as minerals from the columbitetantalite group [(Mn,Fe)(Nb,Ta)206]- Our understanding of the formation of economic Ta ore deposits requires information on the crystallization conditions of Ta-bearing minerals, such as crystallization temperature and Ta solubility in pegmatitic melts, which is not available up to now. Crystallization processes ih pegmatitic systems can occur over a wide temperature range from 900 to 400°C and constraints on crystallization temperatures are crucial to model the reactions leading to the formation of Ta-bearing minerals in economic concentrations. The goal of an ongoing proposal was to get experimental data on the crystallization conditions of columbite-group minerals. The project was built around two points: i) solubility measurements of the manganese end members of the columbite-group minerals as a function of temperature in highly-fluxed pegmatitic melts and ii) the calibration of thermometers to constrain the temperature range of crystallization of Tabearing minerals (Ta in zircon and Zr in tantalite; stability field of manganotantalite and ferrotapiolite pairs). The experiments were conducted in parallel with investigations of the geochemistry and textural features of zircon associated with Ta mineralization in natural pegmatites. Different types of experiments have been conducted with success in the last 14 months. The available results are very promising and should have a strong impact on our knowledge of processes leading to Ta mineralizations. To complement with success the tasks (i) and (ii) listed above, complementary studies are necessary and justify the prolongation of the project for a few months. During this prolongation, we will focus on 3 points: I) we will determine in more detail the solubility dependence of manganotantalite and zircon on the ASI [Al/Na+K+Li)] of the melt, in order to characterize the role of excess Al and excess alkalis on the complexing of rare metal cations in silicate melts; 2) we will calibrate the Zr-in-tantalite thermometer down lo low temperatures; and 3) we will investigate the incorporation mechanisms of hafnium and tantalum in zircon because natural zircon associated with tantalite always coniain significant amounts of Hf.

• DFG Programme: Research Grants