Evolution of Nb-mineralization in the Chuktukon carbonatite massif, Chadobets upland (Krasnoyarsk Territory, Russia)

Abstract

This paper reports on new study of the niobium mineralization of the Chuktukon carbonatites and weathering crust. The Chuktukon massif is located within the Chadobets upland in the southwestern part of the Siberian Platform. Primary Nb-minerals of carbonatites are pyrochlore group minerals (mainly fluorcalciopyrochlore), newly discovered mineral - rippite (K₂(Nb,Ti)₂(Si₄O₁₂)O(O,F)), and Nb-rutile. Fluorcalciopyrochlore has oscillatory zonation with variations in content of Nb₂O₅ (59-67 wt.%), TiO₂ (2.8-5 wt.%), SiO₂ (up to 3.3 wt.%), CaO (15.9-17.7 wt.%), Na₂O (7-8.2 wt.%) and F (4.5-5.13 wt.%). Rippite is Nb-rich silicate (~44 wt.% of Nb₂O₅) and contains inclusions of primary fluorcalciopyrochlore. Nb-rutile form oscillatory and sectorial hourglass-like zoned crystals with variable content of Nb₂O₅ (9-14.7 wt.%). The carbonatites underwent strong hydrothermal alteration and subsequent weathering, and, as a result, in the hydrothermally altered carbonatites and weathering crust the primary Nb-minerals become unstable, dissolve and decompose, and Nb-bearing goethite (up to 4.3 wt.% of Nb₂O₅) formed. Primary fluorcalciopyrochlore is replaced by the secondary Sr-, Pb-, LREE- and Ba-rich pyrochlore group minerals.