Mineralogical characterization and origin of uranium mineralization in Witwatersrand-like metaconglomerate of the Moeda Formation, Quadrilátero Ferrífero, Brazil

Abstract

The Moeda Formation is the basal unit of the Minas Supergroup in the Quadrilátero Ferrífero, Brazil, and has quartz-pebble metaconglomerate that hosts Au–U mineralization, commonly assumed to be detrital in origin. The Au–U mineralization occurs immediately above an unconformity with the underlying Archean Rio das Velhas greenstone sequence. In this work, we focus on the Moeda metaconglomerate in its U-bearing portions in the Gandarela syncline: the southern sector, near the syncline axis; and the western limb, near the Palmital gold deposit. The metaconglomerate has mica seams that host zircon, xenotime, rutile, nodules of organic matter and sulfide minerals. The sulfide assemblage includes four generations of pyrite along with Fe–Ni–Cu–Co–Zn–Pb–Mo sulfides. A Sb–As sulfosalt assemblage of gersdorffite, stibnite and ullmannite is late in relation to pyrite and other sulfides. Gold is argentiferous with minor Hg. Uranium minerals include uraninite, coffinite, brannerite and uranyl arsenates. An early generation of uraninite and brannerite occurs as inclusions in organic-matter nodules, mainly in the southern sector. A late generation of uraninite formed due to a low-temperature hydrothermal overprint. Coffinite and late-stage brannerite are products of hydrothermal alteration of zircon and rutile, respectively. Uranyl arsenates have Ni as the prevalent cation; they formed as replacement of arsenic minerals, such as gersdorffite, by oxidized fluids containing U. Microstructural data and mineral chemistry favor a hydrothermal hypothesis analogous to an unconformity-related setting, rather than a detrital, placer-deposit model for the occurrence of U in the Moeda metaconglomerate.