Pyrochlore Group Minerals from the Qaqarssuk Carbonatite Complex

Abstract

The pyrochlore group minerals (PyGM) found in the Qaqarssuk carbonatite complex are located either in late carbonatite or in glimmerite and fenite adjacent to late stage lanthanide-rich carbonatite, often together with apatite, magnetite and zircon. No PyGM are found in carbonatites of the main intrusive phase. This indicates that Nb and Ta have been concentrated in, and precipitated from, the residual fluids.

Elements like Nb, Ta, U, Th and lanthanides are likely to have been soluble in the carbonatite magma as complexes with ligands like $PO_4^{3 - }$, F- and $CO_3^{2 - }$ possibly also influenced by Na⁺ and K⁺. The PyGM were precipitated where there was a decrease in concentration of the complexing ligands (1) simultaneously with precipitation of apatite, (2) in contact with or in the metasomatized wall rock (brown glimmerite and fenite), where there has been a high gradient of the complexing ligands and loss of alkalis during the metasomatic alteration of the wall rock.

The PyGM in the Qaqarssuk carbonatite complex show a wide compositional variation. Pyrochlore with a composition close to the end member NaCaNb₂O₆ (O, OH, F) is the most common PyGM species and species like bariopyrochlore, uranpyrochlore and betafite are present. A new mineral species, a Ba-betafite analogue, with Ba as the dominating A-cation and Ba exceeding 20% of total A-cations, has been found.

The composition of the PyGM varies with the rock type in which the minerals are found. U- and Ta-rich PyGM are found in fenite, PyGM with high Th and Ce are found in glimmerite and “pure” pyrochlore or zoned pyrochlore with a “pure” rim is found in carbonatite. The compositional variation in the PyGM with rock type is interpreted as precipitation of the most mobile elements in the most distal environment, the most mobile being those with the largest capacity for retaining the anions with which they form soluble complexes. With decreasing concentration of e. g. $PO_4^{3 - }$, F⁻ and $CO_3^{2 - }$ away from the intruding carbonatite, the cations as U, Th and lanthanides will be precipitated in PyGM in a sequence of increasing field strength.

A similar zonation can be observed in compositionally zoned pyrochlore grains with a U- and Ta-rich core, a Th-, Ce- or Ba-rich intermediate zone and a pure pyrochlore rim. The zoning of PyGM grains is interpreted as the result of a more or less continuous precipitation of pyrochlore with a composition in equilibrium with a fluid with increasing concentration of the complexing ligands, during the intrusion of carbonatite.