Niobium and rare earth minerals from the Virulundo carbonatite, Namibe, Angola

Abstract

The Virulundo carbonatite in Angola is one of the largest in the world and contains pyrochlore as an accessory mineral in all of the carbonatite units (calciocarbonatites, ferrocarbonatites, carbonatite breccias and trachytoids). The primary magmatic pyrochlore is fluorine dominant and typically contains about equal molar quantities of Ca and Na at the A site. High-temperature hydrothermal processes have resulted in the pseudomorphic replacement of the primary pyrochlore by a second generation of pyrochlore with less F and Na. Low-temperature hydrothermal replacement of the first and second generation pyrochlore, associated with quartz-carbonate-fluorite vein formation in the carbonatite, has produced a third generation of pyrochlore, with a high Sr content. The Sr appears to have been released by low-temperature hydrothermal replacement of the primary magmatic carbonates. Finally, supergene alteration processes have produced late-stage carbonates, goethite, hollandite and rare earth element (REE) minerals (mainly synchysite-(Ce), britholite-(Ce), britholite-(La), cerite-(Ce)). Cerium separated from the other REEs in oxidizing conditions and Ce 4+ was incorporated into a late generation of supergene pyrochlore, which is strongly enriched in Ba and strongly depleted in Ca and Na.

Ti-Nb Mineralization of Late Carbonatites and Role of Fluids in Its Formation: Petyayan-Vara Rare-Earth Carbonatites (Vuoriyarvi Massif, Russia)

“…(1) Fluorcalciopyrochlore with the fully filled A-sites is the main variety in magmatic calcite [29,[46][47][48] and dolomite [31,49] U-Th and Ca-F-varieties represent early generations of the Petyayan-Vara pyrochlore and are commonly interpreted as magmatism products. However, the structural position of the pyrochlore localized in mineralized veinlets supports its fluid nature, which is also indicated by some of its chemical features.…”

Britholite, monazite, REE carbonates, and calcite: Products of hydrothermal alteration of allanite and apatite in A-type granite from Stupné, Western Carpathians, Slovakia

“…Britholites in granitic rocks are uncommon but have been described as late-magmatic crystals in alkaline granites from Scotland and Northern Ireland in the United Kingdom (MacDonald et al, 2013) and in hydrothermally altered S-type, tin-bearing granites of the Ervedosa area in Portugal (Gomes and Neiva, 2000). Moreover, low-temperature britholite-(Ce) and britholite-(La) in association with synchysite-(Ce), cerite-(Ce), goethite, hollandite, and barite precipitated during late-stage supergene alteration processes in Virulundo carbonatite, Angola (Torró et al, 2012). The above-mentioned postmagmatic and commonly low-temperature origin of BGM is in accordance with textural relationships in the Stupné granite.…”

“…The presence of britholite + synchysite paragenesis (Torró et al, 2012) indicates a possibility of contemporaneous origin of both fluorapatite and allanite alteration mechanisms.…”

Calcite and dolomite in intrusive carbonatites. I. Textural variations

“…The extent and potential significance of this secondary rare-metal mineralization in carbonatites strongly affected by dolomitization (e.g., Aley in Canada) are yet to be ascertained. From the exploration standpoint, dolomitization is also important because it is accompanied by the replacement of primary Nb minerals (predominantly, pyrochlore) by secondary phases, such as fersmite (CaNb 2 O 6 ), columbite (FeNb 2 O 6 ), cation-deficient pyrochlores, or Nb silicates (Voloshin et al 1990;Subbotin and Subbotina 2000;Melgarejo et al 2012;Torró et al 2012;Chakhmouradian et al 2015a). Clearly, these changes affect not only the Nb and Ta grade, but also the amenability of ore to beneficiation and metal recovery.…”