PECULIARITIES OF THE FORMATION OF DIVERSE VEINLET BERYLLIUM MINERALIZATION AT THE ERMAKOVKA DEPOSIT (WESTERN TRANSBAIKALIA)

Abstract

Based on geological, mineralogical, and petrographic data and results of thermometric and microanalytical (electron probe microanalysis and atomic-emission spectroscopy) studies of fluid inclusions (FI), the major factors have been established which determined the composition and Be content of various types of veinlet mineralization at the Ermakovka F-Be deposit. Despite the great difference in the composition of vein filling (ores of types I and II), the parageneses in all near-fracture metasomatites are the same. In limestones, they form diffusion-metasomatic fringes composed of predominant fluorite, subordinate K-feldspar, calcite, quartz, apatite, spodiophyllite, and phenakite (up to 4%). In aluminosilicate schists, the veinlet fringes are ≤0.6 cm thick; they are dominated by K-feldspar, albite, and, to a lesser extent, dolomite and calcite, bear minor fluorite, and lack phenakite. These schists bear nearly barren veinlet mineralization of type II, which is due to the effect of the wallrock lithology on the composition of near-fracture metasomatites. Beryllium mineralization of type II occurs only in carbonates, though it is also poorer in Be than the ores of type I because of the low content of phenakite in the vein filling. As the composition of veinlets does not depend on the lithology of the host rocks (in contrast to the composition of near-fracture metasomatites), the minerals seem to have deposited in fractures as a result of cooling of hydrothermal solutions. But the ores of types I and II are characterized by close temperature ranges of formation (∼ 300-150 and 360-90°C, respectively), which indicates that temperature was not responsible for their compositional difference. The composition of products of the interaction between limestone and fracture solution evidences that both types of mineralization were formed by nearly neutral F-rich fluids (L1 (type I) and L2 (type II)), which also contained K, Na, Al, Si, P, and Be. Study of Fl showed that they are enriched in CaCO3 (∼3%) but differ slightly in salt composition (Teut = -21.5...-27 and -35.6...-36.6°C for L1 and L2, respectively) and concentration (4.7-10.8 and 10.7-11.1 % equiv. NaCl). Moreover, L1 are richer in Be and CO2 than L2 (up to 3.8 g/kg against ≤0.1. g/kg and 4.1-181.1 wt.% against <3.6 wt.%, respectively). As the stability of Be compounds depends on temperature and F- and CO32- activities, the conclusion is drawn that it is the high content of CO2 in L1 solutions that is responsible for their enrichment in Be relative to L2: At high activity of CO32-, Be exists in fluoride solutions not only as BeF3- and BeF42- but also as BeCO3 F-; the solubility of this complex at 300°C is nearly two orders of magnitude higher than those of the former two complexes. This is consistent with the great (more than 10 times) difference in Be contents between the two types of veinlet mineralization but leaves unexplained the cause of their opposite fluorite/K-feldspar ratios.