Sequence of crystallisation of pegmatites: the Angola case

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Sequence of crystallisation of pegmatites: the Angola case

The Precambrian basement of Angola contains several pegmatite fields, although most of them have not been studied yet. Some of them include pegmatites devoid of rare elements, as those found close to Caxito, about 100 km ENE Luanda. These pegmatites are exploited for the production of industrial minerals. Moreover, some rare element pegmatite fi elds occur close to Namibe, in the desertic part of the southwest of the country. These pegmatites were prospected for Be and Ta in the 1960's. The distribution of the pegmatite types, as well as the exceptional quality of the outcrops, due to lack of weathering or soil cover, allow a complete sampling in order to study the evolution of a pegmatite field, from parental granites to barren pegmatites, beryl-columbite-phosphate pegmatites and spodumene pegmatites. Furthermore, the internal evolution of each pegmatite type has been also studied

Subsolidus processes in the Longonjo and Bailundo carbonatites (Angola)

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Nb and REE Distribution in the Monte Verde Carbonatite-Alkaline-Agpaitic Complex (Angola)

The Angolan alkaline-carbonatite complex of Monte Verde has a semi-circular shape and is comprised of a central intrusion of foidolite rocks surrounded by concentrically arranged minor bodies of other alkaline rocks and carbonatite magmatic breccias. This rock association is hosted by fenitized Eburnean granites. Concentric swarms of alkaline dykes of late formation, mostly of nepheline trachyte composition, crosscut the previous units. Most high-field strength elements (HFSE) and rare earth elements (REE) are concentrated in pyrochlore crystals in the carbonatite and alkaline breccias. Magmatic fluornatropyrochlore is replaced and overgrown by five secondary generations of pyrochlore formed during subsolidus stages and have higher Th, REE, Si, U, Sr, Ba, Zr, and Ti contents. The second, third, and fourth pyrochlore generations are associated with late fluids also producing quartz and REE rich minerals; whereas fifth and sixth pyrochlore generations are linked to the fenitization process. On the other hand, minerals of the rinkite, rosenbuschite, wöhlerite, eudialyte groups, as well as loparite-(Ce), occur in accessory amounts in nepheline trachyte, recording low to moderate agpaicity. In addition, minor REE-bearing carbonates, silicates, and phosphates crystallize as late minor secondary minerals into carbonatite breccia and alkaline dykes. In conclusion, the scarcity of HFSE and REE minerals at the Monte Verde carbonatite-alkaline-agpaitic complex suggests low metallogenetic interest and economic potential for the outcrops analysed in this study. However, the potential for buried resources should not be neglected

Óxidos de Nb-Ta de las pegmatitas graníticas de Giraúl, Angola

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Minerales de Nb y REE en las carbonatitas de Longonjo y Bailundo (Angola)

Bailundo y Longonjo son dos de los mayores macizos carbonatíticos de Angola, en los que Lapido-Lourelio (1973) y Alberti et al. (1999) mencionaron la presencia de minerales de Nb, pero las condiciones políticas impidieron durante más de 40 años el desarrollo de estudios de detalle. El objetivo de este trabajo consiste en aportar nuevos datos referentes a la mineralogía de los elementos raros, así como de los procesos subsolidus implicados en el enriquecimiento en Nby REE de las carbonatitas

Minerales de Nb y REE en las carbonatitas de Longonjo y Bailundo, Angola

Bailundo y Longonjo son dos de los mayores macizos carbonatíticos de Angola, en los que Lapido-Lourelio (1973) y Alberti et al. (1999) mencionaron la presencia de minerales de Nb, pero las condiciones políticas impidieron durante más de 40 años el desarrollo de estudios de detalle. El objetivo de este trabajo consiste en aportar nuevos datos referentes a la mineralogía de los elementos raros, así como de los procesos subsolidus implicados en el enriquecimiento en Nby REE de las carbonatitas.Peer ReviewedPostprint (published version

Evolution of pyrochlore in carbonatites: the Angola case

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The distribution of rare metals in the LCT pegmatites from the Giraúl field, Angola

The Giraúl granitic pegmatite field, 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 as Ta and Hf. Pegmatites are concentrically zoned with an increased grain size towards a quartz core. Tourmaline, garnet and beryl 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 ; the Ta/(Ta+Nb) ratios in Nb-Ta oxide minerals and the Hf/(Hf+Zr) ratios in zircon also increase with the grade of evolution of the pegmatites and inside 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 exsolved fluids exsolved from the magma, along with Hf and other incompatible elements as Sn, U, Pb, Sb and Bi.Peer ReviewedPostprint (published version

Zonación del pirocloro primario en carbonatitas de Angola

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