The origin of low-Be quartz-fluorite field at the Ermakovskoe high-grade F-Be-ore deposit

With geological, mineralogical, and petrographic methods, thermometry, and microprobe analysis of fluid inclusions, the structure and formation conditions of a specific orebody (zone 12) of the Ermakovskoe F-Be deposit have been established. This orebody is similar in fluorite content to other orebodies of the deposit (28.7 and 24.6%, respectively) but has the lowest average content of Be (∼0.1% BeO against 1.19%). It is shown that in contrast to high-grade apocarbonate ores, the Ermakovskoe deposit formed mainly through the filling of the cavities of host rocks (syenites and schists) rather than the rock replacement. Calcium necessary for the formation of fluorite was supplied, most likely, as complexes Na2CaF4 and Ca2Cl3F by moderately saline (11.5–12.5 wt.% NaCl equiv.) F-rich (=0.3 mole/kg) solutions. The fluorite was deposited as a result of the solution cooling from ≥430 to 250 °C. Microprobe analysis of fluid inclusions showed that the solutions of ore zone 12 were similar in Be concentration to near-neutral and alkaline solutions that produced high-grade apocarbonate ores of the deposit but were of acid composition (pH≈4 at 350 °C). Cooling of such solutions to 100 °C is not accompanied by the decrease in the solubility of prevailing Be fluorcomplexes, which accounts for the low BeO content in the orebody of zone 12.