Ongonite–elvan magmas of the Kalguty ore-magmatic system (Gorny Altai): composition, fluid regime, and genesis

The Kalguty ore-magmatic system (OMS) is a complex combination of a granite pluton, a hydrothermal Mo–W deposit, pegmatites, greisens, and a belt of rare-metal (RM) and ultra-rare-metal (URM) elvan and ongonite dikes.Studies of melt inclusions (MI) in quartz phenocrysts in the dike rocks have demonstrated that quenched glass has major element contents close to those of the dike rocks but rare elements (Li, Rb, Be, Cs) and P contents. This suggests that the MI represent magma at the stage preceding the dike emplacement. The MI in quartz from the URM rocks are poorer in Si, Fe, Mg, and REE than those in quartz from the RM rocks but richer in Cs, Rb, Nb, and Ta, like the URM rocks themselves. This indicates that the melts had segregated into RM and URM ones before the studied quartz phenocrysts began to crystallize. The composition of MI glass corresponds to “the albite trend” of differentiation, suggesting that the initial melt compositions were ongonitic, while their K enrichment and formation of elvan magma followed the crystallization of the quartz phenocrysts.According to our estimates, the melt contained 6–7 wt.% H2O. The quartz phenocrysts crystallized in a heterogeneous medium consisting of a silicate melt and an aqueous fluid. The latter was a high-density supercritical fluid with 3–12 wt.% NaCl equiv. Variations in the gas and salt compositions of the fluid inclusions (FI) are attributed to the interaction between fluids of magmatic and hydrothermal systems. This possibility is confirmed by ample evidence of their coeval formation.Quartz crystallization from the RM melts took place at 630–650 °C, whereas quartz from the URM melts formed at 20–30 °C lower temperatures. Quartz phenocrysts crystallized at 4.5–5.5 kbar. Additional estimates with regard to the mineral composition and quartz compressibility yielded values of 3–6.5 kbar.A petrogenetic model of some crystallization stages of the dike rocks within the Kalguty OMS was constructed on the basis of the results obtained in this study. The melts which formed the dikes of the East Kalguty belt are derivatives of the same magma which formed the major-stage granite pluton. Quartz is present as intratelluric phenocrysts, which crystallized at considerably greater depths than those of the dike emplacement. Differentiation of the parental magma was accompanied by rare element and P accumulation. The compositions of the FI and MI confirm that the magma and hydrothermal system of the Kalguty OMS exchanged their substances. It is associated with the increasing K content of the melts and the subsequent elvan crystallization as well as considerable variations in the salt and gas compositions of the magmatic fluid inclusions.