Geochemical evolution of Nb-Ta-Sn oxides from pegmatites of the Cape Cross–Uis pegmatite belt, Namibia

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Marco E. Ciriotti
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Geochemical evolution of Nb-Ta-Sn oxides from pegmatites of the Cape Cross–Uis pegmatite belt, Namibia

Messaggio da Marco E. Ciriotti » mer 03 ott, 2018 11:17

Prossima pubblicazione.

Referenza:
▪ Fuchsloch, W.C., Nex, P.A.M., Kinnaird, J.A. (2018): The geochemical evolution of Nb-Ta-Sn oxides from pegmatites of the Cape Cross–Uis pegmatite belt, Namibia. Mineralogical Magazine, 82, (in press).

Abstract:
The Cape Cross-Uis pegmatite belt, Damara Orogen, north-central Namibia hosts multiple Ta-Nb- and Sn-oxide-bearing pegmatites. Columbite-group minerals, tapiolite, cassiterite and minor ixiolite and wodginite occur in abundance within pegmatites and display various compositional and internal structural mineralogical variations. Ta-Nb oxides display various zonation patterns indicative of multiple crystallization phases, whereas cassiterite is dominantly homogeneous with minor euhedral columbite-group mineral inclusions. Ta-Nboxides are mostly Fe-rich, with fractionation patterns in the columbite quadrilateral being sub parallel to the Ta/(Ta+Nb) axis; increasing Ta/(Ta+Nb) with little change in Mn/(Mn+Fe), which is consistent with classical trends in beryl to spodumene type rare-element pegmatites. In addition, these trends suggest that co-crystallizing minerals compete with Ta-Nb-oxides for elements such as Mn, preventing Ta-Nb-oxides from attaining Mn-rich compositions during the fractionation process. Cassiterite shows similar fractionation patterns wit Fe>Mn and notable increases in the Ta content. Minor element substitution in Ta-Nb-oxides shows sharp decreases with increasing fractionation supporting the hypothesis that newly stabilized co-occurring minerals compete with columbite-group minerals for certain elements. Tapiolite shows the same minor element trend, however, only for the elements Sn and Ti suggesting cassiterite was a dominant competing mineral phase. Although crystallization of Ta-Nboxides from an aqueous fluid at the late-stages of pegmatite petrogenesis is highly debated, significantly elevated values of Ta in metasomatized country rock, compared to unaltered country rock, may give new insight into the debate, suggesting that Ta may indeed partition into and be transported by an exsolved aqueous fluid. However, further studies of the country rock metasomatic contacts are required as currently the dataset is limited. The degree of fractionation as depicted by Ta-Nb- and Sn-oxides within pegmatites, indicate that a zonation from primitive to evolved pegmatites surrounding granites is not present and that pegmatites are not likely to have been related to granites in the typical parent-daughter type relationship, confirming similar observations in Fuchsloch et al. (2018
Marco E. Ciriotti

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