Ore Formation Conditions of the Aunik F-Be Deposit (Western Transbaikalia)

Abstract

The paper presents the results of studying of the ore composition, ore-forming solutions, formation conditions, and sources of Be mineralization based on the example of the Aunik F-Be deposit, which is part of the Western Transbaikalia beryllium-bearing province; the main factors responsible for the beryllium mineralization are also assessed. The ores of the deposit are represented by feldspar-fluorite-phenakite-bertrandite metasomatites that formed after carboniferous limestones during its metasomatic replacement by hydrothermal solutions bearing F, Be, and accompanying admixture elements. It is established that the early phenakite-fluorite assemblage formed from high-fluoride CO₂-bearing solutions with high alkalinity and a salinity of ~10.5–12 wt % NaCl eq. in the temperature range ~370–260°C at pressures from 1873 to 1248 bar. Later fluorite and bertrandite were formed by solutions with a salinity of 6.4–7.7 wt % NaCl eq. in the temperature range of ~156–110°C and pressure range of 639–247 bar. Isotopic composition studies of the ore assemblage minerals have confirmed the apocarbonate nature of the main ores at the deposit, making it possible to establish the igneous nature of the ore-forming paleohydrothermal solutions, the source of which were subalkaline leucogranites. The main factors controlling the formation of F-Be ores were the decrease in F activity in solutions owing to bonding of Ca and F in fluorite and the reduction in temperature during ore deposition. The increased alkalinity of the ore-forming solutions controlled the low solubility of Be complexes, which resulted in a low Be content in ores and a relatively small-scale mineralization at the deposit.