Niobium-tantalum minerals in the Skoddefjellet NYF granitic pegmatite, Svalbard Archipelago, Norway: Primary versus secondary assemblage

Abstract

The first documented occurrence of Nb-Ta mineralization in a granitic pegmatite from the Svalbard Archipelago, Arctic Caledonides and the relationships between primary and secondary Nb,Ta-bearing phases are discussed. The Skoddefjellet NYF pegmatite located in southwest Spitsbergen (Wedel Jarlsberg Land) is characterized by enrichment in Nb, U, Zr, Y, REE and Pb, low Zr/Hf (∼8-10), and (∼8-12) Nb/Ta ratios. Columbite-(Fe) and columbite-(Mn) (locally enriched in Ti, Y, Sc and Ca) and Nb,Ta, Y-bearing titanite (≤3.8 wt. % Nb2O5, ≤1.9 wt. % Ta2O5, ≤2.9 wt. % Y2O3) represent the primary magmatic Nb-Ta minerals. The secondary assemblage comprises Nb,Ta-bearing rutile (≤23 wt. % Nb2O5, ≤1.7 wt. % Ta2O5), ilmenite, fersmite and pyrochlore group minerals: including yttropyrochlore-(Y), plumbopyrochlore, yttrobetafite-(Y) and plumbobetafite. Internal zoning of columbite group minerals is expressed by distinct rimward increase of the Mn/(Mn+Fe) ratio (0.07 to 0.83) and low Ta/(Ta+Nb) ratio (0.03 to 0.10), suggesting alkaline environment and increasing fluorine activity during the pegmatite evolution. The formation of secondary phases is related to the hydrothermal stage of the pegmatite. Notably, the formation of Nb,Ta-bearing rutile and ilmenite seems to be a product of Nb,Ta-bearing titanite alteration.