Magmatic and metasomatic processes during formation of the Nb-Zr-REE deposits Khaldzan Buregte and Tsakhir (Mongolian Altai): Indications from a combined CL-SEM study

Cathodoluminescence (CL) imaging and spectroscopy, as well as backscattered electron imaging, were used to assign the occurrence of several mineral ph ases and rock structures in altered nordmarkites and calcite-bearing granit es from the Nb-Zr-REE deposits from Khaldzan Buregte and Tsakhir (Mongolian Altai) to three events: (1) intrusion of barren nordmarkites; (2) intrusio n of small bodies of calcite-bearing granites with metasomatic alteration o f the wall-rocks; and (3) alteration by F-rich fluids. Unusual red and yellow CL caused by Fe3+ and Mn2+ emission centres were det ected in microcline and albite. Fe3+ centres were also established (along w ith others) in quartz, zircon, and possibly in fluorite. Magmatic and metasomatic rock structures and internal structures of the min erals coexist in the samples. The primary magmatic features were in part pr eserved during alteration. In contrast, the internal and the centre structu res may be changed during alteration even in non-replaced mineral phases. E uhedral minerals may be formed by secondary processes as shown for lath-sha ped albite. The occurrence of pseudomorphs, the inheritance of elements dur ing replacement, and the mechanical effects of secondary minerals on earlie r mineral phases during metasomatic growth are proposed as criteria for the reconstruction of the mineral succession in altered rocks. Snowball struct ures may be formed as a result of metasomatic alteration rather than as a m agmatic intergrowth.