Relationship of F-Be mineralization to granites and syenites at the Ermakovka deposit (Western Transbaikalia)

Abstract

The paper presents the mineralogical and geochemical characteristics of two groups of hydrothermalites and their relation with igneous rocks of the Ermakovka deposit. The first group includes F-Be ore bodies, occurring outside the granite massif. The second group is presented in veinlets with sulfates, phosphates, kaolinite, muscovite and hematite. It contains Rare Earth Elements (REE) mineralization (monazite, florencite, xenotime) and occurs within the massif. There are two different massifs of igneous rocks (granite and syenite) at the area of the deposit which have ages (226-227Ma) close to the age of hydrothermalites (225Ma). Each of them could be a source of F-Be-containing veins. This paper reviews the ore potential of these rocks. Itincludes a mineralogical study isotopic analyses of Sr, Nd and O, and trace, including rare-earth element compositions and age determination (U-Pb) of F-Be ores. The data obtained by us evidence that the fluid source of F-Be ores were syenites rather than granites. This is proven by the absence of Be-mineralization in granites and schlieren pegmatites, and a sharp difference in composition of their fluid phases. A reductive fluid specification forming F-Be ores (containing CH4, H2, N2, CO2 and H2S), contrasts sharply with fluid specification of granites. The granites are characterized by high oxygen fugacity, due to ferric iron, sulfates and phosphates. Besides isotopic composition of oxygen in quartz (7.4 and 5.1 ‰ δ18О V-SMOW respectively), initial Sr ratios (0.7056-0.7065 and 0.707-0.709 respectively) and REE compositions are different.