Pyrochlore group minerals in phoscorites and carbonatites from the Kovdor complex, Kola

Zaitsev A.N.¹, Ivashenkova O.V.¹, Williams C.T.², Spratt J.²

¹ St. Petersburg State University, St. Petersburg, Russia

² Natural History Museum, London, UK

 

The Kovdor alkaline complex in the Devonian Kola Alkaline province (Russia) contains diverse phoscorites and carbonatites. Phoscorites include apatite-forsterite, apatite-forsterite-magnetite, calcite-forsterite-magnetite, calcite-forsterite-magnetite with tetraferriphlogopite and dolomite-forsterite-magnetite varieties. Carbonatites are represented by two calcite (with phlogopite or tetraferriphlogopite) and dolomite varieties.

Pyrochlore group minerals occur as euhedral to anhedral crystals up to 2 mm in size and are present in calcite-forsterite-magnetite, calcite-forsterite-magnetite with tetreferriphlogopite and forsterite-magnetite-dolomite phoscorites, and calcite with tetraferriphlogopite and dolomite carbonatites.

On the basis of Hogarth’s classification, the pyrochlore group minerals at Kovdor are pyrochlore, uranpyrochlore, bariopyrochlore and strontiopyrochlore. Several compositional varieties have been identified:

(1) REE- and Ta-rich pyrochlore in calcite-forsterite-magnetite phoscorites, with 13.2-20.7 wt.%, Ta₂O₅ and 4.7-9.1 wt.% REE₂O₃,

(2) REE-poor, Ta-rich uranpyrochlore in both phoscorites and carbonatites containing tetraferriphlogopite (5.8-18.1 wt.% Ta₂O₅, 11.6-21.1 wt.% UO₂ and 0.5-1.3 wt.% REE₂O₃),

(3) REE-poor, Ta-rich pyrochlore with bariopyrochlore rims in dolomite-forsterite-magnetite phoscorites, and with similar contents of rare earths in both minerals (0.6-1.0 wt.% REE₂O₃), and

(4) REE-bearing, Ba-rich pyrochlore in dolomite carbonatites, with 1.1-3.4 wt.% REE₂O₃.

The earliest pyrochlore group minerals to crystallise at the Kovdor phoscorites and carbonatites were pyrochlore, uranoan pyrochlore, and uranpyrochlore, whereas bario- and strontiopyrochlore appear at the end of phoscorite-carbonatite formation and during hydrothermal alteration of early formed pyrochlore-uranpyrochlore.

This work was supported by St. Petersburg State University, “Human Capital” program (8313) and the Natural History Museum.