Hydrothermal and supergene uranium mineralization at the Osamu Utsumi mine, Poços de Caldas, Minas Gerais, Brazil

Abstract

Abstract The Osamu Utsumi uranium deposit in the Mesozoic alkaline complex of Pocos de Caldas (MG, Brazil) is a product of subsequent hydrothermal and supergene processes. The lithology of the open-pit mine is composed of a sequence of volcanic and subvolcanic phonolites and nepheline syenite intrusions similar to those of the alkaline complex; volcanic breccia pipes of about 80 m in diameter also occur, characterized by UThZr-REE mineralization concentrated in the matrix. A strong hydrothermalism, related to the formation of the breccias, has resulted in the potassic alteration and pyritization of the phonolites and syenites, with a low-grade mineralization of disseminated pitch-blende. The potassic alteration has transformed all feldspars into pure potash feldspars, nepheline into illite and kaolinite, and clinopyroxenes, which are the primary REE-bearers, into mixtures of TiO2-rich minerals, clay minerals and pyrite. The enrichment of K, S, U, Th, Pb, Rb, Ba and Mo was accompanied by a strong depletion in Ca, Na, Mg and Sr. The fluid inclusion data indicate temperatures around 250°C and a KClH2O mixture with approximately 7 wt.% KCl for the hydrothermal fluids. For fluids in the breccia pipes that transported additional Zr, Hf, F and minor REEs, the inclusions indicate boiling and give temperatures of 210°C with 40–45 wt.% KCl for a KClNaClH2O brine containing FeSO4 and KF. Uraninite, pitchblende and brannerite are the main U-carriers, and monazite the major REE-bearing mineral in the high-grade mineralization of the breccias. Ultramafic dykes (dated to 76 Ma) with carbonatitic affiliation put a younger age limit to the hydrothermal event. As a result of supergene weathering below a lateritic soil cover 20 to 40 m thick and a saprolite zone 15 to 60 m thick, the pyrites in the rocks have been oxidized to varying depths of 80 to 140 m below surface, resulting in a redox front marked by a contrasting color change from oxidized (reddish-brown) to reduced (bluish-grey). In the vicinity of water-bearing fissures these redox fronts have penetrated to greater depth. Due to mobilization in the oxidized zone and precipitation immediately below the redox front, a secondary mineralization of pitchblende, partly occurring as nodules associated with secondary pyrite, has been developed. This secondary pyrite is sometimes related with CdS, and has a δS of −13% compared to δS values of −3.63 to +1.24% for hydrothermal pyrites: the low δS values of the former are attributed to bacterial action. In the immediate vicinity of the redox front, dissolution of the potash feldspar becomes apparent and the kaolinite content begins to increase. The marked color change at the redox front is caused by the presence of hydrous ferric oxides that, with time, evolve from amorphous Fe-hydroxides to goethite and hematite. On the oxidized side alunite-jarosite minerals are frequent. The porosity increases by 5 to 8% (almost doubled) at the redox front. Gibbsite is present when potash feldspar has been removed at the saprolite-laterite contact. Uranium is partly retained in the oxidized zone by adsorption onto poorly crystalline phases and due to incorporation into hydrothermal refractory minerals. Many of the REEs are associated with clay-sized phosphates (including crandallite group minerals) and seem to be hardly mobilized by the oxidizing weathering fluids. Only a slight general loss of REEs was observed between oxidized and reduced rock, with the degree of loss being greater for the light REEs. Certain indications for a fractionation of Ce and Eu from the other REEs are present.