The micro-chemistry of uraniferous laterites from Brazil: A natural example of inorganic chromatography

Numerous small deposits of U-rich laterites are found in the northwestern border of the Paraná Basin. Detailed mineralogy and chemistry of the uranium lateritic ores from the Iporá-Amorinopolis region were studied using classic petrographic techniques, X-ray diffraction and fluorescence, atomic absorption, infrared and Mössbauer spectrography, autoradiography, electron microprobe and scanning electron microscopy with energy-dispersive spectroscopy. The ore mineral assemblage is formed by goethite, lepidocrocite, hematite, quartz, calcedony, variscite, wavelite, apatite, collophane, barite, gypsite, renardite, meta-autunite I, uranophane, fourmarierite, koninchite, ranquilite, phospho-uranilite, meta-uranocircite I, meta-uranocircite II, metatyuyamunite, sklodowskite, parsonite and an hydrated Ca-silicate of uranium. The mineralization can be weakly disseminated in the laterite matrix with U probably adsorbed in the lepidocrocite; in pockets and impregnations of yellowish uranium phosphated within the laterite (U₃O₈10%) and in nodules with U₃O₈ ranging from 25% to 55%. The nodules are the most common ore type, being typically concentric with a white porous barite-gypsite-apatite core, followed by a compact greenish-yellow rim with colloform uranophane and ranquilite, a yellow aggregate of meta-autunite I, renardite and koninchite and an outer brownish-yellow shell of goethite with disseminated fourmarierite, renardite and koninchite. Size, shape and chemistry of the nodules indicate replacement of brachiopods and chitinozoa that are abundant in the Ponta Grossa Fm. by a primary uraninite-pyrite assemblage that is found in drill cores from fresh rock. The U enrichment is accompanied by enrichment of Ba, Ca, Sr, Pb, Ce and Nd in sulphate, phosphate, hydrous silicate or vanadate phases. The efficient separation of metals like Ce and Nd in individual phases on the surface of barite illustrates a clear example of a natural chromatographic separation. Such a process may be an important mechanism to explain the varying microchemistry within a laterite profile where each successive microsystem represents a chromatographic column through which the solubilities of metals will vary slightly in the continuum of Eh- and pH-values.