Composition and genesis of albitite-hosted antecrystic pyrochlore from the Sevattur carbonatite complex, India

Abstract

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.

Compositional variation and genesis of pyrochlore, belkovite and baotite from the Sevattur carbonatite complex, India

“…In places, this mantle is partially disrupted leading to extensive metamictisation together with alteration resulting in the formation of U-poor Si-rich pyrochlore (Pcl-V). Compositionally, Pcl-III (comparable to ‘Pcl-II’ of Dey et al , 2021) is characterised by a very high U content (31–33.1 wt.% UO 2 ; 0.49–0.53 apfu) compared to other pyrochlore (Table 2). Pcl-III is also enriched in Ti, Ca and Na (11.7–14.0 wt.% TiO 2 ; 5.5–10.3 wt.% CaO; 0.9–1.8 wt.% Na 2 O) (Table 2) and depleted in Ba (3.7–5.7 wt.% BaO) compared to Pcl-I and Pcl-II, whereas the Nb content is comparable to that of Pcl-II.…”

“…2a–f). These pyrochlores are also characterised by a mantle of Ba-bearing potassium feldspar (Dey et al , 2021) (Fig. 2d).…”

“…Carbonatite-hosted pyrochlores are relatively large (0.1–1 mm) (Fig. 2a–c), whereas the majority of the albitite-hosted pyrochlores (Dey et al , 2021) are small (<50 μm), and large grains (>100 μm) are seldom present (Fig. 2d–f).…”

“…During field investigations, we found freshly exposed albitite veins cross-cutting the blue carbonatite in the western and south-eastern walls of the vermiculite mine (12.422°N, 78.528°E) (Dey et al , 2021). These veins are of variable thickness (2–12 cm) and do not have surficial exposures.…”

“…Albitite-hosted U-rich pyrochlore (Pcl-III) with Ba-potassium feldspar mantles has been described in detail by Dey et al (2021), and only a summary of this paragenesis is included in this work for comparison with pyrochlore occurring in the associated carbonatites. These pyrochlore are restricted to albitite and present in two textural modes: either surrounded by an orbicular mantle of Ba-bearing potassium feldspar; or as an inclusion within the amphibole (Figs 2f, 3d).…”