Tracing the chemical evolution of primary pyrochlore from plutonic to volcanic carbonatites: the role of fluorine

Abstract

Three Angolan carbonatites were selected to evaluate the change in composition of pyrochlores during magmatic evolution: the Tchivira carbonatites occur in a plutonic complex, the Bonga carbonatites represent hypabyssal carbonatites and the Catanda carbonatites are volcanic in origin. In Tchivira pyrochlore, zoning is poorly developed; fluorine is dominant at the Y site; chemical zoning may arise as a result of substitutions for Nb in the B site; and the rare earth element (REE), U, Th and large-ion lithophile element (LILE) contents are very low. Pyrochlores from Bonga show oscillatory zonation; the F and Na contents are lower than those in the pyrochlores from Tchivira; and as substitution of Na at the A site increases, the Th, U, REE contents and inferred vacancies also increase. Pyrochlores from Catanda display complex textures. They generally have a rounded corroded core, which is mantled by two or three later generations. The core composition is similar to the Bonga pyrochlores. The rims are enriched in Zr, Ta, Th, Ce and U, but depleted in F and Na. In pyrochlores from the Angolan carbonatites, the F and Na contents decrease from plutonic to volcanic settings and there is enrichment of Th, U and REE in the A site and Ta and Zr in the B site. Zoning may be explained by changes in the activity of F, due to the crystallization of fluorite or apatite in the plutonic and hypabyssal carbonatites, or to volatile exsolution in the volcanic carbonatites.

Carbonatitic lavas in Catanda (Kwanza Sul, Angola): Mineralogical and geochemical constraints on the parental melt

“…Pyrochlore occurs as small euhedral grains, less than 20 µm in diameter, often slightly corroded. It is generally well zoned and up to three generations of pyrochlore may be distinguished in some grains (Bambi et al, 2012;. Brucite is widely present in the Huilala-Ungongué samples, typically as pseudomorphs after primary grains of periclase.…”

Pyrochlore Chemistry from the Bonga Carbonatite‐type Nb Deposit, Huila Province, Angola: Implications for Magmatic‐Hydrothermal Processes of Carbonatite.

“…Only a few researchers have worked on the of carbonatite and pyrochlore in this deposit (Issa Filho et al, 1991;Coltorti et al, 1993;Bambi et al, 2012). Furthermore, the research on pyrochlore is limited in primary (magmatic) pyrochlore.…”

Role of vein-phases in nanoscale sequestration of U, Nb, Ti, and Pb during the alteration of pyrochlore

“…Leaching of A-site cations was reported for pyrochlore from granitic pegmatites, nepheline syenites, and carbonatites (e.g., Lumpkin and Ewing, 1995;Lumpkin, 2001;Chakhmouradian and Mitchell, 2002;Bambi et al, 2012 were not involved in the alteration processes described here. They have, however, been reported in the other localities (Lumpkin and Ewing, 1995;Nasraoui and Bilal, 2000;Chakhmouradian and Mitchell, 2002;Bonazzi et al, 2006).…”