Geology and mineralization of Nb, P, Fe and light rare earth elements of the Araxá alkaline-carbonatite complex, Minas Gerais state, Brazil

Abstract

New 40Sr/39Ar dating revealed that the magnetite phlogopitite at the center of the Araxá complex reached the surface at 98 Ma, and generated volcanoes with mineralized volcanic tuffs and crater lakes. At the bottom of the crater lakes, micro-graywacke deposited. Based on the mineral composition, hydrothermal structures, and δ18O and δD values, a hydrothermal episode occurred between ∼98 and 86 Ma Ma. The δ18O of phlogopites of phlogopities vary between 4.8‰ and 5.7‰, and those of magnetitites are 4.3‰ and 6.8‰. The δ18O of the water in equilibrium with phlogopites of the phlogopitites vary between 2.3‰ and 3.2‰, and those with magnetitite are 4.3‰ and 1.8‰. The δD of phlogopites of phlogopitites vary between −86.5‰ and −95.2‰, and those of magnetitite are −84.9‰, and −85.8‰. The δD of the water in equilibrium with phlogopites of the phlogopitites vary between −26.8‰ and −35.5‰, and with magnetitite are −25.6‰, and −26.26‰. During the hydrothermal event, magnetite phlogopitites were hydrothermally altered into phlogopite magnetitite, magnetite apatitite (epifoskorites), and mineralized with pyrochlore. At 84 Ma, the phlogopitic core was permeated by veins and micro-veins of Sr-rich norsethite carbonatites. At 83.5 Ma, calcitic carbonatites and micro-phlogopitites originating from the same mantle source as the magnetite phlogopitite intruded, which resulted in the formation of a carbonatite ring around the core and the assimilation of part of the phlogopitic core. This magmatism generated a new hydrothermal event of limited coverage that mineralized phlogopitites and carbonatites with monazite and massive pyrite ± chalcopyrite. The magmatism ended at 77 Ma, when phosphorous-saturated calcitic carbonatite surrounded the core. The saturation of phosphorous favored the crystallization of apatite, which generated phosphate deposits.