Unraveling the Magmatic-to-Carbothermal Processes in the Ba-Sr-REE Mineralization of the Sevattur Carbonatites, India

Abstract

The Neoproterozoic Sevattur carbonatite complex (India) consists of calcite carbonatite, silicate-poor- and silicate-rich (banded and blue) dolomite carbonatite. The trace element composition of the calcite and ferroan dolomite shows that most of them have magmatic characteristics. The predominance of dolomite carbonatite over calcite carbonatite and the absence of a genetically related silicate suite at Sevattur suggest a dolomitic parental magma derived by the direct melting of the mantle. Regardless of the prevalent magmatic character of the Sevattur carbonatites they host a plethora of Ba-Sr-REE minerals formed throughout the orthomagmatic-to-carbothermal stages under varying activities of ${\mathrm{F}}^{-}$,${\mathrm{CO}}_3^{2-}$, ${\mathrm{PO}}_4^{3-}$and ${\mathrm{SO}}_4^{2-}$. The orthomagmatic stage begins with the crystallization of calcite carbonatite containing xenocrystic diopside, antecrystic Y-bearing calcite (Cal-I), and primary benstonite, apatite, and richterite in a Sr-Mg-rich calcite (Cal-II) matrix. Fractionation of apatite, carbocernaite, and cordylite-(Ce) together with the enrichment of Mg, Fe, Na, and F in late-magmatic stages, forms the dolomite carbonatite. The decomposition of benstonite to norsethite, Ba-Ca carbonate, and strontianite occurs within dolomite carbonatite at the same stage with minimal wall-rock assimilation. Interaction between the xenocrystal clinopyroxenite and the primary magma during late-magmatic stages has resulted in the formation of aegirine and phlogopite in calcite carbonatite. Similarly, the prolonged assimilation of clinopyroxenite by the evolving magma leads to the formation of magnesio-riebeckite and magnesio-arfvedsonite within the banded- and blue carbonatites. The increasing activities of ${\mathrm{PO}}_4^{3-},{\mathrm{SO}}_4^{2-}$and H2O during the carbothermal stage, destabilizes precursor carbocernaite and cordylite-(Ce), resulting in an assemblage consisting of synchysite-(Ce), ancylite-(Ce), daqingshanite, hydroxylbastnäsite-(Ce), ferriallanite-(Ce), monazite-(Ce), and baryte in dolomite carbonatite. In the banded carbonatite, the carbothermal stage is characterized by ferri-winchite, tremolite-actinolite, and phlogopite crystallization. Further contamination and differentiation of the carbothermal fluid have resulted in the crystallization of quartz and albite in blue carbonatite. Phlogopitization within the blue carbonatite and clinopyroxenite results from late K enrichment in the carbothermal fluids.