The role of magmatic-to-carbohydrothermal processes in rare earth mineralization in Hogenakkal carbonatites, India

Abstract

The Paleoproterozoic Hogenakkal complex (India) consists of silicate-rich-, silicate-poor calcite carbonatite (carbonatite-I and II) and clinopyroxenite emplaced in granulite country rocks. Carbonatite-I contains abundant clinopyroxenite and K-feldspar pegmatite xenoliths. Silicate minerals are the primary governing factors in the formation of a distinctive lithology-specific REE mineralization. Carbonatite-I consists of magmatic Mg-Fe-Sr-bearing calcite (Cal-1), and REE-poor apatite (Ap-1) together with minor REE-rich apatite (Ap-2). The interaction of the carbonatite-forming magma with the clinopyroxenite and K-feldspar pegmatite xenoliths releases K, Al, and Si, forming phlogopite and pargasite-edenite. The late-magmatic carbonatite-II consists of Sr-rich calcite (Cal-2), Ap-2 and minor Ap-1. The carbohydrothermal REE-mineralization in both the carbonatites is induced by a magma-derived fluid dominated by REE, OH−, CO32−, and SO42−. The high modal abundance of silicate minerals and the continuous leaching of Si and Al from clinopyroxene, K-feldspar xenocrysts, and phlogopite by REE-enriched fluid facilitated the formation of allanite-(Ce) and actinolite in carbonatite-I. In contrast, in the silicate-poor carbonatite-II, the same fluid with low Si-saturation resulted in monazite-(Ce) + celestine-baryte±strontianite precipitation. The carbohydrothermal fluid is also responsible for the alteration of apatite to monazite-(Ce) by a dissolution-reprecipitation mechanism releasing fluorine. Consequently, localized crystallization of F-rich hydroxylbastnäsite-(Ce) is common in carbonatite-II. Complex intra-grain compositional and textural variations of monazite-(Ce) and hydroxylbastnäsite-(Ce) suggest further interaction of these minerals with evolving fluid. The waning phase of the carbohydrothermal activity is marked by an assemblage consisting of pure calcite (Cal-3) + quartz±magnetite in carbonatite-I. The Ba-Sr-REE mineralization and the overall mineral assemblages of the Hogenakkal carbonatites represent a transition from a magmatic to a carbohydrothermal stage.