The peralkaline granite-related Khaldzan-Buregtey rare metal (Zr, Nb, REE) deposit, western Mongolia

Abstract

The Khaldzan-Buregtey rare metal (Nb, Zr, REE, etc.) deposit in peralkaline granitoids is located in western Mongolia, 45 km northeast of the town of Kobdo. The deposit, which was discovered by the authors, is composed of dome-shaped stocks of rare metal peralkaline granite covering 0.85 km and represents the late plutonic phases of the Devonian (380-390 Ma) multiphase Khaldzan-Buregtey peralkaline granite massif. The formation of the massif took place during tectonic extension, as shown by the development of a major dike complex synchronous with granite emplacement. The massif consists of peralkaline granitoids, as well as gabbro and basalt, and is characterized by a radial-circular shape. Eight intrusive phases are recognized and consist of (1) nordmarkite, (2) peralkaline granite, (3) dikes of fine-grained peralkaline granite, (4) dikes of pantellerite, (5) rare metal peralkaline granite, (6) gabbroids, (7) miarolitic peralkaline rare metal granite, and (8) syenite and leucite basalt. Field relationships of the rare metal peralkaline granite bodies indicate an intrusive igneous origin. The rare metal peralkaline granites consist of K-Na feldspar, quartz, albite, arfvedsonite, aegirine, fluorite, and diverse rare metal minerals ranging up to 25 vol percent. Ore minerals are represented by elpidite, gittinsite, and zircon; pyrochlore (Nb); rare metal flurocarbonate and monazite (REE); and polylithionite (Li). The zirconium minerals often occur in the endocontact and apical parts of rare metal granite bodies and are reflected in the negative correlation of Zr and Nb contents in the seventh-phase granites. The rare metal peralkaline granites of the deposit differ from those of other massifs by their high and variable Ca and F contents. The granites are rich in Zr (up to 5.3 wt %), Nb (up to 0.8 wt %), REE (up to 0.4 wt %), Y (up to 0.3 wt %), Be, Sn, Rb, and sometimes Li, Pb, Zn, Hf, and Ta. They are depleted in Sr and Ba and have an almost flat REE distribution pattern, with a small negative Eu anomaly. The initial 87 Sr/ 86 Sr ratio of fluorite from rare metal peralkaline granite is 0.70433 + or - 7. The end value for these rocks at 386 Ma ranges from 4.3 to 5.7...