The Neoproterozoic Sevattur complex is composed essentially of calcite and dolomite carbonatites together with pyroxenites and diverse syenites. This work reports the compositions and paragenesis of different pyrochlore generations hosted by albitite veins in this complex. The pyrochlore are distinctive, being exceptionally rich in uranium (26 to 36 wt.% UO2). Five types of pyrochlore (Pcl-I to Pcl-V) are recognised on the basis of composition and texture. With the exception of Pcl-V, the majority of the pyrochlore (Pcl-II to Pcl-IV) are surrounded by a thick orbicular mantle of Ba-rich potassium feldspar. This mantle around Pcl-V is partially-broken. Pcl-I is restricted to the cores of crystals, and associated with Pcl-II and -III and is relatively rich in Nb (0.53–0.62 apfu) together with more A-site vacancies (0.37–0.71 apfu) compared to Pcl-II to Pcl-IV. Other pyrochlore (Pcl-II to Pcl-IV) are characterised by elevated Ca and Ti compared to Pcl-I, which are related to the (3Nb5+ + Na+ → 3Ti4+ + U4+) and (2Nb5+ → 2Ti4+ + Ca2+) substitutions, respectively. These substitutions represent replacement of Pcl-II to Pcl-IV. Alteration and Ba-enrichment in all the pyrochlore are marked by interaction with an externally-derived Ba-rich hydrothermal fluid following the (2Nb5+ → 2Ti4+ + Ba2+) substitution. This substitution, coupled with extensive metamictisation leads to the formation of Ba-rich (15.9–16.3 wt.% BaO) patchy-zoned Pcl-V. The orbicular mantles around Pcl-I to Pcl-IV have prevented extensive metamictisation and extensive secondary alteration compared to Pcl-V, where mantling is partially disrupted. The compositional and textural variation suggests that Pcl-II to Pcl-IV form by nucleation on Pcl-I, and are transported subsequently as antecrysts in the host albitite.

The existing classification of pyrochlore group minerals is essentially based on the dominant valence rule. However, coupled heterovalent-homovalent substitutions at the A-, B-, and Y-sites commonly result in charge-imbalanced endmember formulae. The application of the site total charge (STC) method permits the determination of a charge-balanced endmember. Species names are assigned by using the dominant constituent rule. According to the current IMA nomenclature scheme, some previously established pyrochlore species, such as kalipyrochlore, strontiopyrochlore, bariopyrochlore, plumbopyrochlore, ceriopyrochlore, yttropyrochlore, bismutopyrochlore, and uranpyrochlore, are all grouped as zero-valent-dominant pyrochlores, resulting in the loss of petrogenetic information. In this work, the zero-valent-dominant pyrochlores of the pyrochlore group (sensu stricto) are classified into R+-, R2+-, R3+-, and R4+-pyrochlores where the respective cations (R) are the dominant valencies at the A- and Y-sites (for R+-pyrochlores) after vacancies (□) and H2O. The endmember charge arrangements are determined by the STC method to obtain charge-balanced endmember formulae for all possible zero-valent pyrochlore species. It is recommended that suitable adjectival modifiers be used along with the species name to emphasize the abundance of certain cations, which may or may not be reflected in the endmember formula. This approach would facilitate the usage of pyrochlore group minerals for all practical petrological and exploration purposes. It is considered that pyrochlores with significant A-site vacancies do not necessarily reflect formation in a supergene environment, as such pyrochlores can also form in hydrothermal parageneses.

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