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

Abstract

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) complexly 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.