Alekseeva, and Marina published in 2017 that the pyrochlore-group minerals in ongonite and zwitter of the Verkhneurmiysky granite pluton in the Badzhal district, Russian Far East are reported. The composition, genesis, and crystallization sequence are characterized. Three rare species of pyrochlore, pyrochlore-I and pyrochlore-II in ongonite, and pyrochlore-III in zwitter are found for the first time in the Russian Far East. Pyrochlore-I is a possible new member of the pyrochlore group, bismuth-bearing “oxyferropyrochlore”; pyrochlore-II is an uranium-bearing and iron-bearing hydrokenopyrochlore; and pyrochlore-III is a lead-and iron-bearing hydrokenopyrochlore.
All three pyrochlore species result from an alteration of early accessory minerals: niobium wolframite, samarskite, ishikawaite, wolframoixiolite, scheelite, and fergusonite. The Far East pyrochlore is characterized by extremely high Ta, W, and Fe contents. Compositional variations in pyrochlore are caused by substitution of Nb, Ta, W at the B site of the crystal structure and exchange of U, Pb, Bi, and Fe at the A site. The Ta, Bi, and Th concentrations decrease during transition from the late magmatic to greisen stage; the mineral forming significance of Nb, W, U, Y, Pb, and H2O increases as followed: Ta, Nb, Bi, Fe, Th, As, P, Ca, Ti → Nb, Ta, W, U, Fe, HREE, Sc, Mn, Na, H2O → Nb, Pb, W, Fe, Y, H2O.
The authors concluded that:
1) The specific features of the composition, genesis, and formation sequence of the pyrochlore-group minerals in ongonite are reported for the first time. Three rare pyrochlore species, pyrochlore-I, pyrochlore-II, and pyrochlore-III, are found in zwitter in the Russian Far East for the first time.
Pyrochlore-I, a probable new mineral species of the pyrochlore group, is Bi-bearing “oxyferropyrochlore”;
Pyrochlore-II is U-and Fe-bearing hydrokenopyrochlore;
Pyrochlore-III is Pb- and Fe-bearing hydrokenopyrochlore.
2) All three species of the Far East pyrochlore were formed by the multistage metasomatic replacement of the early accessory minerals: niobium wolframite, samarskite, wolframixiolite, scheelite, and fergusonite.
3) The pyrochlore-group minerals from the Verkhneurmiysky pluton are characterized by extremely high Ta, W, and Fe. Variations in the chemical composition
of pyrochlore are caused by substitutions among Nb, Ta, and W in the B framework of the crystal structure and exchange among U, Pb, Bi, and Fe at site A.
4) The compositional evolution of the pyrochlore-group minerals in the transition from the late magmatic to greisen stage of the Verkhneurmiysky pluton reflects decreasing Ta, Bi, and Th concentrations and, as mentioned above, the authors emphasized the increasing mineral-forming significance of Nb, W, U, Y, Pb, and H2O: Ta, Nb, Bi, Fe, Th, As, P, Ca, Ti → Nb, Ta, W, U, Fe, HREE, Sc, Mn, Na, H2O → Nb, Pb, W, Fe, Y, H2O.
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