Alkaline granites and Be (Phenakite-bertrandite) mineralization: An example of the Orot and Ermakovka deposits

The Orot (Or) and Ermakovka (Er) intrusions of aegirine granites with various resources of accompanying Be mineralization in Transbaikalia were studied to reproduce Be behavior during the crystallization and degassing of alkaline granite magma. Data on the petrography and geochemistry of the rocks and the microthermomety and microprobe analysis of fluid and melt inclusions in them indicate that the intrusions were formed by discrete magma portions derived from a single magmatic source during successive stages of its differentiation. The intrusions crystallized at temperatures higher than 1030–1070°C (Or) and 840–640°C (Er), and the melts contained elevated concentrations of H₂O and F: from 2.1 to 3.5% F and from <1 to 1% H₂O for the former intrusion and from 3.9 to 6.7% F and from <2.6 to 4.1% H₂O for the latter. Fluids were released from the magma during a late crystallization stage for the Orot intrusion and an intermediate stage for the Ermakovka intrusion. Early in the course of this process, the fluids were halide-sulfate brines with the Cl: F ratio higher for the Orot intrusion and lower for the Ermakovka intrusion. A temperature decrease resulted in the exsolution of the fluids into two immiscible phases, one of which contained low and the other high concentrations of salts. The magmatic brines and low-salinity solutions of both intrusions were enriched in Be (up to 1.1 g/kg), which is comparable with the concentration of this element in the emanations of Be-bearing pegmatites in the Pamirs and is manyfold higher than C _Be in magmatic fluids related to granite intrusions with W-Sn mineralization. The Be ore mineralization of the Orot and Ermakovka deposits was produced by solutions whose composition and Be concentrations were analogous to those in the low-salinity phase of the corresponding magmatic fluids. The brines of the Ermakovka intrusion were enriched in Mo (up to 17 g/kg) and, to a lesser extent, Mn, Ce, and La and produced uneconomic monazite-molybdenite ore mineralization. Based on available data and results of our calculations, we arrived at the conclusion that the very high alkali concentrations in the melts of both intrusions (ASI < 1), their high F concentrations (up to 4.1%), and the absence of magmatic Be-bearing minerals facilitated Be selective extraction by the separated fluids in the form of its most soluble F-complexes. The high oxidation of the melt predetermined the predominance of hexavalent S and Mo compounds, which could be efficiently extracted by the fluid phase in the form of sulfates and molybdates of alkali metals. The differences in the ore potentials of the Orot and Ermakovka intrusions were caused by the different H₂O, F, and perhaps, also Be concentrations in the parental melts, which was, in turn, caused by their different degrees of differentiation during the preintrusive stages of their magmatic evolution.