REE-Nb mineralization in trachytes and rhyolites of the Khulam volcano-plutonic complex, North Caucasus

Abstract

The paper presents the frst data on the composition of REE and Nb mineralization in the Middle Jurassic moderately alkaline felsic and moderately acid rocks of the Khulam volcano-plutonic complex at the territory of Kabardino-Balkaria, central part of Northern Caucasus. The igneous rocks of the Khulam Complex (intrusive and subvolcanic facies) belong to the contrasting diferentiated bimodal association: (gabbro + basaltic trachyandesite) – (trachyte + rhyolite). Their main petrological and geochemical characteristics and ore mineralization are discussed. The trachytes and rhyolites of the Khulam Complex exhibit the high contents of Zr (up to 3100 ppm), Nb (up to 400 ppm), and ?REE (>1000 ppm), which are caused by specifc process of fractional crystallization of magmatic melts of elevated alkalinity during the formation of acid and moderately acid rocks. The Nb mineralization includes ilmenorutile and Nb-bearing brookite; REE minerals are monazite-(Ce), REE-bearing fuorapatite and REE fuorocarbonates of the bastnasite–vaterite homologous series. Monazite has magmatic origin and its formation is related to the enrichment of residual magmas in incompatible elements during fractional crystallization. The fuorocarbonates and ilmenorutile formed later as a result of autometamorphism and low-temperature hydrothermal alteration of felsic rocks. Keywords: fuorocarbonates, rare earth elements, niobium, ilmenorutile, Khulam Complex, North Caucasus, Kabardino-Balkarian Republic.