Hydrothermal alteration of pyrochlore and associated minerals of an alkali pegmatite from Zomba-Malosa (Malawi): A record f pyrochlore-fluid re-equilibration
Final Report Abstract
Detailed mineralogical, geochemical, and geochronological investigations of the alteration of zircon, pyrochlore, and fergusonite from an alkali pegmatite from the Zomba Malosa district in Malawi have been carried out using a wide range of analytical techniques, such as SEM, EMP, Raman spectroscopy. TEM, LA-ICPMS, and SHRIMP. Re-equilibration processes in these minerals have produced pseudomorphic replacement textures. Textural features of zircon and pyrochlore indicate at least two alteration events. In case of zircon, uneven reaction fronts penetrate into the crystals producing secondary zircon that is characterized by lower Th concentrations and the presence of micro-porosity and thorite inclusions. LA-ICPMS and SHRIMP U-Pb data of unaltered and altered domains in zircon yielded indistinguishable ages of ~119 Ma. The thorite inclusions formed contemporaneously with secondary zircon as demonstrated by Th-U-total Pb dating by electron microprobe. They also show alteration features such as the occurrence of non-formula elements Ca, Al, and Fe. These domains yielded Th-U-total Pb model ages as young as 5 Ma. The Investigated pyrochlore is severely radiationdamaged, as shown by TEM and Raman spectroscopy, and exhibits two texturally distinguishable zones that cross-cut each other and that are also compositionally different. The youngest alteration zones are comprised of concentric rings penetrating into the crystal, whereas the older zones are highly porous and are distinguishable from the primary pyrochlore by slightly lower U concentrations. LA-ICPMS U-Pb data also indicate that the older zones fonned soon after the crystallization of the pyrochlore whilst the alteration event that formed the younger alteration domains can be linked to the alteration of the thorite inclusions inside the secondary zirocn in more recent times. The porosity in the altered zircon along with the occurrence of thorite inclusions as well as the porosity in the older alteration domains in pyrochlore suggests that the replacement of zircon and pyrochlore occurred via an interface-coupled dissolution-reprecipitation process soon after emplacement of the pegmatite. Detailed TEM work shows that the younger alteration domains in the pyrochlore are partly recrystallized, i.e., they contain crystalline domains in an amorphous matrix. The micro-textural features indicate that the second alteration must have proceeded by a diffusion-reaction process similar to that described for the alteration of radiation-damaged zircon.
Publications
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(2008) Hydrothermal alteration of an alkali pegmatite from Zomba-Malosa, Malawi. 86th Ann. Meet German Mineral. Soc, Abstract Volume, 363
Soman A , Geisler T , Tomaschek F., Berndt J., and Putnis A.
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(2008) Multi-stage hydrothermal alteration of accessory minerals from an alkali pegmatite from Zomba-Malosa (Malawi). Geochim. Cosmochim. Acta, 72, A882
Soman A., Geisler T , Tomaschek F., Bemdt J., and Putnis A.
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(2009) TEM study of hydrothermally altered pyrochlore from Zomba-Malosa, Malawi. Annual Report, Bayerisches Forschungsinstitut für Experimentelle Geochemie und Geophysik, University of Bayreuth, 95-97
Soman A., Geisler T., and Pollok K.
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(2010) Alteration of crystalline zircon solid solutions: a case study on zircon from an alkali pegmatite from Zomba-Malosa, Malawi. Contrib. Mineral. Petrol
Soman A., Geisler T., Tomaschek F., Grange, M., and Berndt J.
