Abstract
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.
Highlights
Granitic pegmatites present a broad diversity of mineral species of great economic interest, and are an important source of rare metals, such as Li, Rb, Cs and Ta
A mining company explored some mineralised pegmatites for muscovite and beryl during the Portuguese colonial times, up to the 1980s but wars prevented any operation during decades. In this contribution we aim to describe the characteristics of the Giraúl pegmatites in Angola with special emphasis on their structure, distribution of critical minerals, mineralogy and mineral chemistry of Nb-Ta minerals in order to discuss about the processes that control the enrichment in critical elements
Hafnon shows a clear but moderate enrichment trend in zircon, from the less evolved pegmatites towards the most evolved pegmatites; there is an increase in the ratios Hf/(Zr + Hf) from every pegmatite as the crystallisation process becomes more advanced
Summary
Granitic pegmatites present a broad diversity of mineral species of great economic interest, and are an important source of rare metals, such as Li, Rb, Cs and Ta. Pegmatites are, in addition, a source for high-quality industrial minerals (mainly feldspars, kaolinite, quartz, micas, and spodumene for ceramics) or gemstones such as topaz, tourmaline, aquamarine, spodumene, among others [2]. The study of Nb-Ta minerals is of great interest from both economic and scientific point of view. New technologies, especially those for the manufacture of electronic devices of small dimensions, have caused an increase in the need for tantalum [3,4].
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