Geology, geochemistry, and mineralogy of saprolite and regolith ores with Nb, P, Ba, REEs (+ Fe) in mineral deposits from the Araxá alkali-carbonatitic complex, Minas Gerais state, Brazil

Abstract

The Araxá Complex is a significant source of mineral deposits, including Nb, P, Ba, LREE, and Fe. These deposits are formed by weathering phlogopite-rich rocks and norsethite carbonatite in the Complex core and from the weathering of carbonatites in the surrounding ring. The main types of ore in the Complex include rust-colored, brown, and orange ore. The rust-colored ore contains over 2% CaO and MgO and is mineralized with pyrochlore, barite, and fluorapatite in the phlogopititic core and only fluorapatite in the carbonatitic ring. Brown ore has less than 2% MgO and over 2% CaO and is mineralized with pyrochlore, barite, fluorapatite, and magnetite in the core and only fluorapatite in the ring. Orange ore has less than 2% MgO and CaO and is mineralized with pyrochlore, barite, monazite, and magnetite in the core and goyazite in the ring. Pyrochlore and iron minerals were concentrated from the rock to the orange ore, while fluorapatite was concentrated from the rock to the brown ore. The weathering of fluorapatite in orange ore leads to the formation of authigenic gorceixite, goyazite, and monazite. The neogenesis of barite in the Complex occurs due to the dissolution of norsethite, followed by the chemical bonding of Ba with S from sulphuric solutions. The ore deposits are up to 200 m thick, stratiform, and horizontal, formed by meteoric water that flowed vertically downwards, then horizontally, radially, and centrifugally. The weathering of 100 m of rock resulted in the formation of approximately 33 m of orange ore, 9 m of brown ore, and 4 m of rust-colored ore in the Complex core and 35 m of orange ore, 7 m of brown ore, and 10 m of rust-colored ore in the carbonatite ring. Lake sediments accumulate on the orange regolith top surface.