Zr-Nb-REE Mineralization in Peralkaline Granites from the Amis Complex, Brandberg (Namibia): Evidence for Magmatic Pre-enrichment from Melt Inclusions

Abstract

The Amis Complex is a late intrusive phase associated with the Brandberg anorogenic granite intrusion. It consists of peralkaline, arfvedsonite-bearing granitic and pegmatitic dikes and sills and is characterized by locally extreme enrichments in REE and rare metals with high charge-ionic radius ratios, such as Zr and Nb. The highest concentrations (e.g., 1.7 wt % Zr, 0.3 wt % Nb, 0.5 wt % total REE) are found in aegirine-albite aplites that formed around arfvedsonite pegmatite cores. Quartz-hosted melt inclusions in coarse-grained arfvedsonite granite and pegmatite give unequivocal evidence that the peralkaline composition and rare metal enrichments are primary magmatic features. Electron microprobe and SIMS analyses of rehomogenized inclusion glasses reveal their peralkaline high Si rhyolite compositions (avg molar [Na + K]/Al = 1.1) and very high contents of H2O (max 10.6 wt %) and F (2.2 wt %). Inclusion glasses also have high concentrations of incompatible trace elements including Nb (min 803 ppm), Zr (min 3,187 ppm), and REE + Y (Σ = min 1,271 ppm). The REE, Zr, and Nb compositions of the aegirine-albite aplites plot along the same linear enrichment trends as the melt inclusion glasses, and Y/Ho ratios mostly display unfractionated near-chondritic values. The chemical and textural features of the aegirine-albite aplites are compatible with an origin by rapid crystallization (compositional quenching) after volatile loss from a residual peralkaline granitic melt of a composition like that of the melt inclusions. Interaction of postmagmatic fluorine-rich aqueous fluids with the arfvedsonite granites caused local remobilization of REE and secondary mineralization of fluorocarbonate phases, such as bastnaesite and fractionation of Y and Ho.