A geological, geochemical and textural study of an LCT pegmatite: implications for the magmatic versus metasomatic origin of Nb-Ta mineralization in the Moose II pegmatite, Northwest Territories, Canada

Abstract

*Corr esponding author The internal zonation and chemical evolution of Nb–Ta oxides and muscovite have been characterized in the Moose II pegmatite, Northwest Territories, Canada, to distinguish primary magmatic mineralization from that formed during late-stage metasomatic processes. In addition, muscovite associated with Nb–Ta oxides was examined in order to assess the evolution of the pegmatite melt and the nature of the late-stage fluids. Detailed mapping shows that the studied body (430 × 61 m) is a highly fractionated, irregularly zoned, spodumene-subtype rare-element pegmatite dominated by coarse-grained to megacrystic albite, K-feldspar, and spodumene, with intergranular assemblages of quartz, K-feldspar, albite, spodumene, muscovite, and minor amblygonite–montebrasite. Monomineralic core zones (quartz and amblygonite–montebrasite), aplitic albite ‘pods,’ and units characterized by phyllic alteration occur in the central portions of the pegmatite. Columbite–tantalite minerals occur throughout the pegmatite, excluding the quartz cores, and patterns of internal zoning include: (1) normally zoned ferrocolumbite with early Ta–Ti-rich, and later Nb–W-rich overgrowths; (2) oscillatory zoned Ti-rich ferrotantalite with Nb-rich patchy replacement; (3) reversely zoned ferrocolumbite, with Ta-rich cores and (4) com plexly zoned Ti-rich ferrotantalite with abundant late Nb- and Ta-rich patches and sieve textures. The textures and chemical patterns demonstrate an evolution from columbite to tantalite, whereby the Ta/(Ta + Nb) ratio increased with progressive fractionation (0.13–0.59) but the Mn/(Mn + Fe) ratio remained nearly constant (0.15–0.31). The chemical evolution of the Nb- Ta oxides from columbite to tantalite is consistent with crystallization from a magmatic to late-stage magmatic Na-rich melt, with a sequence of crystallization progressing from those of the wall zone, to the first intermediate zone, and finally the late aplitic albite zones. Minor remobilization of Nb, and to a lesser extent Ta, was responsible for some of the replacement features found in the Nb–Ta oxides. Textural observations and trace-element analyses of fine-grained, secondary muscovite found throughout the pegmatite indicate hydrothermal metasomatism by a late F- and Nb-rich vapor-like “supercritical” fluid.