Distribution and Evolution of Zirconium Mineralization in Peralkaline Granites and Associated Pegmatites of the Khan Bogd Complex, Southern Mongolia

The western part of the Khan Bogd complex comprises (in order of emplacement): microcline-phyric granite, peralkaline aegirine-arfvedsonite granite (main intrusive phase) and aplite-pegmatite veins confined predominantly to the apical parts of the intrusion. These rocks are interpreted to represent products of extreme fractional crystallization what involved an increase in index of peralkalinity (from 1.0 in the porphyritic granite to 1.3 in the main phase to 2.5 in the pegmatite) and whole-rock Zr content (864 to 1130 to 16900 ppm). The hydrothermal stage involved replacement of the primary elpidite by late-stage Ca-rich elpidite or armstrongite, and then precipitation of minor gittinsite and abundant zircon. With the exception of zircon, all secondary zirconosilicates exhibit relative enrichment in light REE.