Distribution and evolution of zirconium mineralization in peralkaline granites and associated pegmatites of the Khan Bogd complex, southern Mongolia

journal article - other

English

The western part of the Khan Bogd complex, located in South Gobi (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 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). In the porphyritic granite, zircon with a low Hf content (Zr/Hf = 55-75) is the principal Zr phase, whereas alkali-Ca zirconosilicate minerals are characteristically absent. Zirconium mineralogy in the peralkaline granites and associated aplite-pegmatite bodies is a texturally and compositionally complex product of Zr enrichment during the differentiation of magma, followed by reaction of the granitic rocks with a calcic CO2-F-rich fluid. In the peralkaline unit and pegmatites, Ca-poor elpidite is an early magmatic Zr host also containing appreciable levels of rare-earth elements (n103 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 0.1-0.3], all secondary zirconosilicates exhibit relative enrichment in light REE [(La/Yb)CN = 1.3-5.3]. Enrichment of the fluid in light REE due to the sequestration of heavy REE in zircon led to the deposition of light-REE fluorocarbonates (bastnäsite, parasite and röntgenite) associated with calcite and minor fluorite. The fluid probably separated at the final stages of the evolution of 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.

gittinsite, Mongolia, zircon, Gobi desert, elpidite, granitic pegmatite, peralkaline granite, Khan Bogd, Zr mineralization, armstrongite

KYNICKÝ, J. -- CHAKHMOURADIAN, R. A. -- CHENG, X. -- KRMÍČEK, L. -- GALIOVÁ, M.

2011

31. 8. 2011

Mineralogical Association of Canada

Kanada

0008-4476

CANADIAN MINERALOGIST

49

4

Canada

763

781

19