FLUORITE PROPERTIES AND FORMATION CONDITIONS AT THE BERYLLIUM-FLUORITE DEPOSITS IN THE VOZNESENSK ORE DISTRICT (FAR EAST REGION)

Abstract

The article discusses the formation and properties of fluorite in the Voznesensk and Pogranichnyi deposits (the Voznesensk ore district, Far East region, Russia). Evolution of the postmagmatic process was mainly influenced by an intense apocarbonate metasomatosis, which caused the decrease in the F acidity and activity, an increase in the alkalinity and oxidizing potential of fluids, and an inheritance of REE distribution patterns of limestones at terminal stages of ore formation. Fluorite, the most abundant ore mineral, was accumulated under different conditions and at all successive megastages of the postmagmatic process: early and late albitite, greisen, and late hydrothermal. There is a significant positive and negative correlation between REE distribution patterns; intensity of Yb2+ and Eu2+ band maxima in photoluminescence spectra; intensity of the TR2+, Y2+, and F02i in thermoluminescence curves; and concentration of paramagnetic Mn2+ and O- centers. All of these fluorite parameters change at different megastages and stages of the deposit formation, which also depends on spatial zoning. Earliest varieties of fluorite (in albitites after granite apophyses and in preore explosive breccia with the albite matrix) inherit the REE distribution pattern of late granite phases: they are enriched in heavy lanthanoids (HLn) and bivalent ions and depleted in Eu. Apocarbonate ores formed at the late albitite and greisen megastages show three generations of metasomatic fluorite (Met I, Met II, and Met III). Fluorite Met I associates with albite and Li-phengite and is close in its properties to fluorite from breccia. It is also characterized by a deficiency in the Eu content, relatively high REE content (particularly HLn), Mn2+ centers, and elevated intensity of the Yb2+ maxima and TR2+, Y2+, and F02i peaks. In fluorite Met II, associated with muscovite and tourmaline, all these parameters show lowered values and Eu deficiency is insignificant. Fluorite Met III in external zones of apocarbonate metasomatic aureoles is close in REE distribution patterns to the carbonate substrate: light lanthanoids prevail over heavy ones, and the negative Eu anomaly is leveled. Fluorite from enclosing explosive breccia with the euclase-micaceous matrix is intermediate between fluorite from preore breccia and fluorite Met I from apocarbonate ores. Thus, evolution of fluorite properties in the Voznesensk ore district points to succession of granite- and ore-forming processes. The type and degree of succession of the magmatic process and fluid-related ore formation depend on the local structural setting, especially on the isolation degree of fluid-conducting and ore-accumulating structures relative to the magmatic chambers and other sources of fluid flows. These factors are responsible for the formation of different mineral types (phenacite-fluorite or chrysoberyl-fluorite), as well as for spatial ore zoning, which is reflected in variations of typomorphic properties that can serve as indicators of ore shoots and impoverished parts of ore lodes. Ore mineralization in the Voznesensk deposit formed in a system of vertical fissures open to intragranite fluid sources and screened by the schist sequence and by thick porphyrite dikes and fault zones in flanks. This stimulated the formation of a high-grade compact phenacite-fluorite ore lode at both the greisen and albitite megastages. The wide development of fluorite Met I marks the location of ore shoots relative to areas of wedging out and pinching of ore lodes along the deposit strike. At deep-seated levels, properties of fluorite Met I become close to those of fluorite Met II, which points to the decreasing intensity of apocarbonate metasomatosis and ore accumulation. Fluorite Met III is poorly developed. The ore-forming process in the Pogranichnyi deposit was mainly controlled by low-angle structural elements - limestone beds, a system of conformable fractures, a granite massif roof- and, to a lesser extent, by subvertical fractures. Partial screening of fluids by low-angle contact of the granite massif and dispersion of the flow within the sequence of enclosing rocks along fractures resulted in the reduction of the albitite megastage beyond the granite massif and less compact localization of ores of the chrysoberyl-fluorite type in the form of a series of low-angle lodes. Consequently, the composition of apocarbonate ores is mainly dominated by relatively low-temperature fluorites Met II and Met III.