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

Abstract

The western part of the Khan Bogd complex, in the south Gobi desert, southern Mongolia, 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 involving alkali feldspar, quartz and, to a lesser extent, ferromagnesian silicates and, in the final stages of magma evolution, release of a silica-saturated orthomagmatic fluid. Geochemically, this evolutionary path involved an increase in peralkalinity index (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, respectively). In the porphyritic granite, zircon with a low Hf content (Zr/Hf in the range 55-75) is the principal Zr phase, whereas alkali-Ca zirconosilicate minerals are characteristically absent. Zirconium mineral parageneses in the peralkaline granites and associated aplite-pegmatite bodies are texturally and compositionally complex products of Zr enrichment during the differentiation of magma, followed by reaction of the granitic rocks with a calcic CO 2-F-rich fluid. In the peralkaline unit and pegmatites, Ca-poor elpidite is an early magmatic host of Zr, along with appreciable levels of the rare-earth elements (n 3 10 3 ppm REE + Y). 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 [(La/Yb) CN ≈ 0.1], all secondary zirconosilicates exhibit relative enrichment in light REE [(La/Yb) CN in the range 0.3-5.0]. Enrichment of the light REE in the fluid due to the sequestration of heavy REE in zircon led to the deposition of light-REE fluorocarbonates associated with calcite and minor fluorite. The fluid probably separated at the final stages of the evolution of the peralkaline granitic magma, but a crustal input, indicated by Ca enrichment of the fluid, cannot be ruled out. The observed variations in the modal composition of the secondary paragenesis can be explained by changes in fluid regime, and the activity of silica and other dissolved species during the hydrothermal stage.

The Early Jurassic Bokan Mountain peralkaline granitic complex (southeastern Alaska): Geochemistry, petrogenesis and rare-metal mineralization

“…Primitive mantle-normalized incompatible trace element abundances of the peralkaline granitic rocks. (d) Bokan granites with relatively flat to slightly LREE enriched patterns; (e) Bokan granites with LREE enrichment and a positive MREE/HREE slope; (f) Typical peralkaline granites: Egypt -average of peralkaline granites of eastern Egypt (Abdel-Rahman, 2006), Mong -average of the main phase granite of the Khan Bogd complex, southern Mongolia (Kynicky et al, 2011), Lac -peralkaline granite of the Lac Brisson, Labrador, Canada (Linnen and Cuney, 2004;Pillet et al, 1992); Khald -average of mineralized granite of the Khaldzan-Buregtey complex, western Mongolia (Kovalenko et al, 1995). Normalizing values are after Sun and McDonough (1989).…”

Hydrothermal metasomatism of a peralkaline granite pegmatite, Khaldzan Buragtag massif, Mongolian Altai; complex evolution of REE-Nb minerals

“…As noted by Macdonald et al (2016), the HaldzanBuragtag deposit has certain features in common with other peralkaline granitic intrusions which host significant abundances of rare-metal minerals, such as Amis, Namibia (Schmitt et al 2002), Tamazeght, Morocco (Salvi et al 2000), Strange Lake, Canada (Salvi and Williams-Jones 2006;Gysi et al 2013), and Khan Bogd, Mongolia (Kynický et al 2011). These features include evidence for the presence of late-stage Ca and HREE metasomatism.…”

The Origin of Skarn-Hosted Rare-Metal Mineralization in the Ambohimirahavavy Alkaline Complex, Madagascar

“…Owing to a recent increase in the demand for these metals, they are the focus of exploration by mining companies, particularly in the western hemisphere. Studies of rare-metal deposits associated with peralkaline igneous rocks have shown that the mineralization results from igneous or hydrothermal processes, although in most cases both processes played a role in mineralization Williams-Jones, 1990, 2006;Kovalenko et al, 1995;Kempe et al, 1999;Schmitt et al, 2002;Kynicky et al, 2011;Sheard et al, 2012;Gysi and Williams-Jones, 2013). Thus, a topic of some debate is the relative importance of these two processes.…”