Gonçalves, and 7 co-authors, reported in 2019 that the Giraúl granitic pegmatite field in Angola is composed of five pegmatite types, the most evolved belong to the beryl-columbite, beryl-columbite-phosphate and spodumene types. Pegmatites are concentrically zoned with increased grain size toward a quartz core; the most evolved pegmatites have well-developed replacement units. These pegmatites are rich in Nb-Ta oxide minerals and the field has a moderate interest for critical elements such as Ta and Hf. Tourmaline, garnet and micas occur as accessory minerals.
The abundance of Zr and Nb-Ta minerals increases with the evolution of the pegmatites, as well as the proportions of beryl and Li-rich minerals. The Ta/(Ta + Nb) ratio in Nb-Ta oxide minerals and the Hf/(Hf + Zr) ratio in zircon also increase with the evolution of the pegmatites and within each pegmatite body from border to inner zones, and especially in the late veins and subsolidus replacements. Textural patterns and occurrence of late veins with Ta-rich minerals suggest that Nb and especially Ta can be enriched in late hydrothermal fluids exsolved from the magma, along with Hf and other incompatible elements as Sn, U, Pb, Sb and Bi.
The authors concluded that Hafnon shows a clear but moderate enrichment trend in zircon, from the less evolved pegmatites towards the most evolved pegmatites; moreover, there is an increase in the ratios Hf/(Zr + Hf) from every pegmatite as the crystallization process becomes more advanced. An identical behavior at the scales of field and pegmatite, but also crystal, is observed in the Ta/(Ta + Mn) values. The Nb-Ta crystals of the Giraúl pegmatites follow an evolutionary trend represented by variations in the Ta/(Ta + Nb) ratio, whereas the variation in the Mn/(Mn + Fe) ratio is limited. This evolution is attributed to a higher solubility of the tantalite end-member respect to columbite in the evolved pegmatite-forming melts, at the field scale, and to remobilizations by hydrothermal exsolved fluids, at the scale of body, at least in
the late episodes.
The Mn/(Mn + Fe) value increases markedly during the late stages of replacement in type-IV pegmatites. Variation in this ratio depends on the other (Fe, Mn)-bearing minerals that coexist with these minerals. Oscillatory zoning, present in most of columbite-group crystals, indicates a growth under disequilibrium conditions. Crystals with convoluted patterns formed by metasomatic processes.
Stibio-columbite, bismuto-columbite and members of the pyrochlore supergroup, as well as other Ta-rich minerals as tapiolite also formed as late minerals such as indicated by their location in veins. They formed from the reaction of hydrothermal fluids with previous Nb-Ta oxide minerals. Pyrochlore-supergroup minerals exhibit a wide compositional variation. The Nb-Ta content is similar to that of neighboring CGM in U-dominant pyrochlore but it is higher in Pb and Sb-dominant pyrochlore. This fact and the richness in U, Sb, Pb, Ca, Sr indicate the metasomatic origin of these minerals. Thus, pyrochlore-supergroup minerals formed from the reaction of hydrothermal fluids with the pre-existing Nb-Ta oxides. Nevertheless, the Ta enrichment suggests that Nb and Ta were transported in the hydrothermal fluids.
As a whole, the relatively low enrichment of Hf in zircon and the moderate enrichments of Mn and Ta n the most evolved pegmatites suggest that the Giraúl field has only a moderate potential as resource for critical elements.
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