Quartz-albite-microcline metasomatic rocks in the Main Sayan Fault Zone: Evolution of metasomatism and composition of accessory minerals

Abstract

Quartz-albite-microcline metasomatic rocks (qualmites) localized in the Main Sayan Fault Zone at the boundary of Irkutsk oblast and Buryatia (Eastern Sayan, middle reaches of the Kitoi River) were formed after dynamometamorphic biotite and biotite-amphibole granite gneisses. The zone of alkaline metasomatism up to 250–300 m thick extends for about 12 km along the Main Sayan Fault. Riebeckite-aegirine and hornblende-clinopyroxene qualmites were formed during the early alkaline stage of metasomatism. Biotite-magnetite qualmites, which are occasionally superimposed on riebeckite-aegirine rocks, are products of the following stage of increasing acidity. The 40Ar/39Ar age of amphibole from metasomatic rocks is 321 ± 5 Ma. The metasomatic rocks are enriched by 2–4 times in Zr, Nb, Y, REE, Be, Th, and U relative to unaltered granite gneisses and contain rare-metal mineralization (fergusonite-(Y), betafite, Nb- and Y- bearing titanites, gadolinite-(Y), zircon, thorite, allanite-(Ce), chevkinite-(Ce), etc.). The composition of accessory minerals was studied on a LEO-1430VP SEM. Their composition and mineral assemblages show that meta-somatic alteration and Ti-Nb-Zr-REE mineralization were formed synchronously. The stage of acid solution neutralization was characterized by crystallization of epidote and andradite and by replacement of chevkinite with allanite and titanite in metasomatic rocks. Hydro- and fluorcarbonates of LREE, phosphates (monazite-(Ce)), and fluorides (F-bearing thorianite?) were formed during the final low-temperature stage.