Subsolidus processes as a key factor on the distribution of Nb species in plutonic carbonatites: The Tchivira case, Angola

Melgarejo, Joan Carles; Costanzo, Alessandra; Bambi, Aurora C.J.M.; Gonçalves, António Olímpio; Neto, Andrés B.

doi:10.1016/j.lithos.2012.06.024

Cited by 32 publications

(33 citation statements)

References 45 publications

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“…Commonly, the transition passes through a number of intermediate areas of Ba-, Sr-, and REE-pyrochlores [28,63]. The pyrochlores reflect the general pattern of imposed metasomatic processes in such areas, including the Petyayan-Vara ore field rocks.…”

Section: Discussionmentioning

confidence: 98%

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

et al. 2018

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This article is devoted to the geology of titanium-rich varieties of the Petyayan-Vara rare-earth dolomitic carbonatites in Vuoriyarvi, Northwest Russia. Analogues of these varieties are present in many carbonatite complexes. The aim of this study was to investigate the behavior of high field strength elements during the late stages of carbonatite formation. We conducted a multilateral study of titanium-and niobium-bearing minerals, including a petrographic study, Raman spectroscopy, microprobe determination of chemical composition, and electron backscatter diffraction. Three TiO 2 -polymorphs (anatase, brookite and rutile) and three pyrochlore group members (hydroxycalcio-, fluorcalcio-, and kenoplumbopyrochlore) were found to coexist in the studied rocks. The formation of these minerals occurred in several stages. First, Nb-poor Ti-oxides were formed in the fluid-permeable zones. The overprinting of this assemblage by residual fluids led to the generation of Nb-rich brookite (the main niobium concentrator in the Petyayan-Vara) and minerals of the pyrochlore group. This process also caused niobium enrichment with of early generations of Ti oxides. Our results indicate abrupt changes in the physicochemical parameters at the late hydro (carbo) thermal stage of the carbonatite formation and high migration capacity of Ti and Nb under these conditions. The metasomatism was accompanied by the separation of these elements.

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Section: Discussionmentioning

confidence: 98%

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

et al. 2018

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“…10d), which is reflected in the zoning of varieties II and III. Comparison of the compositions with the trends for pyrochlore from the carbonatites of the Tchivira massif, Angola (Melgarejo et al, 2012), showed high convergence (Fig. 10c), though the cloud of compositions is displaced toward the peak Sr + Ba + Pb with respect to the trend formed in the diagram by pyrochlore with predominant Sr, which forms reaction rims.…”

Section: Minerals and Formsmentioning

confidence: 94%

“…In the new classification, the mineral names were determined with regard not only to the predominant element at the A-site but also to the anion or vacancy at the Y-site (Atencio et al, 2010). The pyrochlore group minerals are characterized by reaction rims (sometimes, very complicated), in which minerals with different predominant elements at the A-site replace one another (Melgarejo et al, 2012). Such rims are a record of the geologic events following the mineral formation.…”

Section: Minerals and Formsmentioning

confidence: 99%

“…This variety can be called "strontioplumbopyrochlore" (Hogarth, 1977) or "kenoplumbopyrochlore" (Atencio et al, 2010). Plumbopyrochlore is described in the alkaline rocks of the Kola Peninsula (Chakhmouradian and Mitchell, 2002) and in the carbonatites of Angola (Melgarejo et al, 2012), but their PbO contents (38 wt.%) are much higher than that in pyrochlore III from the Tomtor ores. It might be that, actually, an alternation of barium-strontium pyrochlore and plumbopyrochlore is observed and the zoning is too fine for their analytical discrimination.…”

Section: Minerals and Formsmentioning

confidence: 99%

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Main minerals of abnormally high-grade ores of the Tomtor deposit (Arctic Siberia)

Лазарева

Жмодик

Добрецов

et al. 2015

Russian Geology and Geophysics

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The Tomtor massif of Paleozoic ultramafic alkaline rocks and carbonatites is located in the northern part of the Sakha Republic (Yakutia). The massif (its total area is ~250 km 2 ) is ~20 km in diameter, with a rounded shape and a concentrically zoned structure. The core of the massif consists of carbonatites surrounded by a discontinuous ring of ultramafic rocks and foidolites. The outer part is composed of alkali and nepheline syenites. All rocks are weathered and covered with eluvium, which is the thickest after carbonatites enriched in phosphates and REE. The weathering profile consists of four layers, from the top: kaolinite-crandallite, siderite, goethite, and francolite. The highest-grade ores are observed in the bedded deposit which fills depressions in "sagging" eluvium. The ores are laminated and cryptogranular, with high Nb, Y, Sc, and REE contents (on average, 4.5% Nb 2 O 5 , 7-10% REE 2 O 3 , 0.75% Y 2 O 3 , and 0.06% Sc 2 O 3 ). The highest-grade ores are natural Nb and REE concentrates. The total REE content in some layers is >10%. The morphologic features of the highest-grade phosphate ores from the northern part of the Burannyi site were studied. The ore-forming minerals belong to the pyrochlore group, crandallite group (goyazite), and monazite-Ce. The pyrochlore group minerals occur mainly as crystals that were completely replaced by barium-strontium pyrochlore and/or plumbopyrochlore but retained the original faces; also, they occur as numerous conchoidal fragments. The grains of the pyrochlore group minerals sometimes have a zonal structure, with an unaltered pyrochlore core and a reaction rim. Goyazite occurs predominantly as colloform grains. According to SEM and TEM data, monazite occurs in the ores as ~50 nm particles, which cover the outer part of halloysite tubes (800-3000 nm long and 300 nm in diameter) as a dense layer and make up peculiar biomorphic aggregates. The mineralogical data, the occurrence of biomorphic aggregates, and the close association of organic remains with ore minerals suggest that the high-grade ores of the Tomtor deposit, including the Burannyi site, resulted from a hydrothermal-sedimentary process with a presumably important role of bioaccumulation of REE phosphates.

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“…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.…”

Section: Igneous Textures: Key Examplesmentioning

confidence: 99%

Calcite and dolomite in intrusive carbonatites. I. Textural variations

2015

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Carbonatites are nominally igneous rocks, whose evolution commonly involves also a variety of postmagmatic processes, including exsolution, subsolidus re-equilibration of igneous mineral assemblages with fluids of different provenance, hydrothermal crystallization, recrystallization and tectonic mobilization. Petrogenetic interpretation of carbonatites and assessment of their mineral potential are impossible without understanding the textural and compositional effects of both magmatic and postmagmatic processes on the principal constituents of these rocks. In the present work, we describe the major (micro)textural characteristics of carbonatitic calcite and dolomite in the context of magma evolution, fluid-rock interaction, or deformation, and provide information on the compositional variation of these minerals and its relation to specific evolutionary processes.

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Subsolidus processes as a key factor on the distribution of Nb species in plutonic carbonatites: The Tchivira case, Angola

Cited by 32 publications

References 45 publications

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

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

Main minerals of abnormally high-grade ores of the Tomtor deposit (Arctic Siberia)

Calcite and dolomite in intrusive carbonatites. I. Textural variations

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