Processes of formation of fluorite-leucophane-melinophane-eudidymite ores of the Ermakovka F-Be deposit (western Transbaikalia)

Fluorite-leucophane-melinophane-eudidymite ores of zone XVIII of the Ermakovka F-Be deposit were studied by geological, mineralogical, and thermobarogeochemical methods. Contents of Be and impurity elements (Li, Na, Mg, Al, Si, Cl, K, Mn, Fe, Cu, Zn, Nb, Mo, Ag, Sn, W, and Pb) in fluid inclusions in fluorite of this zone have been first determined by LA-ICP-MS. It is shown that fluorite-leucophane-melinophane-eudidymite ores were formed by alkaline high-F low-salt (6.0-12.5 wt.% NaCl equiv) solutions with a relatively low content of Be (0.0002-1.04 g/kg of solution). Fluorite and beryllium minerals were deposited in ores in a wide range of P-T conditions. The early fluorite-phenakite paragenesis formed at high temperatures (480-650 °C) and high pressures (> 3 kbar). At the late low-temperature stage, phenakite was replaced by Na-Be silicates (eudidymite and melinophane-leucophane) at < 220 °C and < 770 bars. The Be-ore deposition was due to the destruction of a predominant beryllium fluoride-carbonate complex as a result of the crystallization of fluorite during the metasomatic replacement of limestones. Eudidymite and melinophane-leucophane formed at low temperatures under high activity of Na and Ca and low activity of Be and F in highly alkaline solutions.