Allanite-(Y) and Allanite-(Ce) Paragenesis in Tourmalinite of the Severnyi Pluton, Chukchi Peninsula, and the Relationship between Yttrium and Lanthanides in Allanite - 2016

Rare Earth Deposits & Mining

Alekseev and two co-authors reported in 2016 on the morphology, optical properties, and composition of allanite-(Ce) and allanite-(Y) from rare-metal tourmalinite of the Severnyi granitic pluton in the Chukchi Peninsula, as well as on the composition and structure of host metasomatic rocks and the assemblage of rock-forming and accessory minerals.

The hydrothermal origin of both allanite species and their stable combination have been established. Allanite-(Y) is partly replaced with allanite-(Ce), and metasomatic rims 2–10 μm wide enriched in LREE around allanite-(Y) have also been identified. The degree of isomorphic Y+HREE substitution for LREE is estimated at 16%, on average; the maximum (Y+HREE)/LREE ratio does not exceed 0.25. It is assumed that the transition of allanite-(Y) into allanite-(Ce) might be caused by an increase in acidity and decrease in aqueous fluid temperature at the late stage of the hydrothermal process.

The authors concluded that: 

1. The hydrothermal origin of allanite-(Y) has been established.
2. The allanite-(Y) and allanite-(Ce) paragenesis has been established in tourmalinite from the Severnyi granitic pluton in the Chaun District of the Chukchi Peninsula.
3. A restricted (Y + HREE)–LREE isomorphism in allanite has been confirmed.