THE PERALKALINE GRANITE-RELATED KHALDZAN-BUREGTEY RARE-METAL (ZR, NB,REE) DEPOSIT, WESTERN MONGOLIA

The Khaldzan-Buregtey rare metal (Nb, Zr, REE, etc.) deposit in peralk aline granitoids is located in western Mongolia, 45 km northeast of th e town of Kobdo. The deposit, which was discovered by the authors, is composed of dome-shaped stocks of rare metal peralkaline granite cover ing 0.85 km2 and represents the late plutonic phases of the Devonian ( 380-390 Ma) multiphase Khaldzan-Buregtey peralkaline granite massif. T he formation of the massif took place during tectonic extension, as sh own by the development of a major dike complex synchronous with granit e 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 granit e, (4) dikes of pantellerite, (5) rare metal peralkaline granite, (6) gabbroids, (7) miarolitic peralkaline rare metal granite, and (8) syen ite and leucite basalt. Field relationships of the rare metal peralkal ine granite bodies indicate an intrusive igneous origin. The rare meta l peralkaline granites consist of K-Na feldspar, quartz, albite, arfve dsonite, aegirine, fluorite, and diverse rare metal minerals ranging u p tp 25 vol percent. Ore minerals are represented by elpidite, gittins ite, and zircon; pyrochlore (Nb); rare metal fluorcarbonate and monazi te (REE); and polylithonite (Li). The zirconium minerals often occur i n the endocontact and apical parts of rare metal granite bodies and ar e reflcted in the negative correlation of Zr and Nb contents in the se venth-phase granites. The rare metal peralkaline granites of the depos it differ from those of other massifs by their high and variable Ca an d F contents. The granites are rich in Zr (up to 5.3wt %), Nb (up to 0 .8 wt %), REE (up to 0.4 wt %), Y (up to 0.3 wt %), Be, Sn, Rb, and so metimes Li, Pb, Zn, Hf, and Ta. They are depleted in Sr and Ba and hav e an almost flat REE distribution pattern, with a small; negative Eu a nomaly. The initial Sr-87/Sr-86 ration of fluorite from rare metal per alkaline granite is 0.70433+/-7. The epsilon(Nd) value for these rocks at 386 Ma ranges from 4.3 to 5.7. The granitoids of the massif are co nsidered to originate from two sources, resulting in different Nd isot ope compositions (epsilonNd=5.87.6 for the first three phases, and eps ilon(Nd)=4.3 for rare metal granites of the fifth and seventh phases), whereas the Sr isotope signatures are virtually indistinguishable. The rare metal peralkaline granites contain primary melt inclusions which were homogenized in a bomb under a pressure of 3 kbars, and the glass analyzed by electron and ion microprobe. This enabled direct determin ation of the composition of the peralkaline rare metal granite magma ( F, H2O in mass %, other elements in ppm): H2O=2.2, Li=230, Be=340, B=3 40, F=2.5, P=520, Ti=4,210, Rb=3,770, Sr=5, Y=2,240, Zr=26,530, Nb=5,7 50, Sn=660, and Ce=1,450. The proportion of crystals in the rare metal granite magma, the initial magma composition, and the combined partit ion coefficients have been evaluated. The initial magmas were rich in rare elements. Calculated partition coefficients indicate that the rar e metal magma was generated by differentiation of the most primitive p antellerite magma which we surmise originated from fractional melting of the basaltic crust. Rare metal granite magma crystallization was ac companied by saturation in Zr minerals and their accumulation near the contact and in the apical part of the magma chamber.