The regional geology of the Poços de Caldas alkaline complex: mineralogy and geochemistry of selected nepheline syenites and phonolites

Abstract

The Mesozoic Poços de Caldas alkaline complex, the largest known in South America, is circular-shaped with a mean diameter of about 33 km, and developed during continental break-up and drift. It comprises a suite of alkaline volcanic and plutonic rocks (mainly phonolites and nepheline syenites) with average amounts of U, Th and rare-earth elements (REEs). The evolutionary history began with major early volcanism involving ankaratrites, phonolite lavas and volcanoclastics, followed by caldera subsidence and nepheline syenite intrusions forming minor ring dykes, various intrusive bodies and circular structures. Finally, the addition or concentration of strongly incompatible elements led to the formation of eudialyte nepheline syenites and phonolites. Magmatic evolution included deuteric processes indicating a volatile-rich parent magma of upper mantle origin, without appreciable crustal contamination. These processes extended over a large temperature range and resulted in the formation of pegmatitic veins and comprised mineral assemblages including rare metal silicates such as giannettite, incipient alkali exchange reactions of feldspars, various zeolites, fluorite and hematite. Geochemically, the resulting rocks are enriched in potassium when compared to global nepheline syenites and phonolites. Mobilization and concentration of U, Th and REEs did not apparently occur at this stage. At one place (Morro do Ferro) the intermediate nephelinic suite was affected by a possible carbonatite intrusion and the formation of a stockwork of magnetite veins. Very intensive hydrothermal K- and S-rich alteration, associated with contemporaneous volcanic breccias, occurred locally. These processes led to the formation of several important radioactive and REE-rich anomalies. Two of these, the Th-REE occurrence of Morro do Ferro and the U-Zr-REE-Th occurrence of the Osamu Utsumi uranium mine, comprise the study sites of the Poços de Caldas Analogue Project. Later major stages in the evolution of the Poços de Caldas complex involved the emplacement of mafic-ultramafic dyke rocks and the onset of lateritic and allitic weathering, resulting (at the uranium mine) in supergene geochemical redistribution and the formation of redox fronts sometimes related to uranium enrichments. The end of the magmatic and hydrothermal-mineralizing events is likely fixed by the Ar-Ar dating of a lamprophyre dyke at the uranium mine (76 Ma). This study was focused towards the major rock types of the regional nephelinic suite relative to those experiencing more local hydrothermal and final weathering-related alteration. In the studied intrusive, subvolcanic and volcanic nepheline syenites and phonolites, very little variation was observed. This lack of differentiation may be seen as an argument for a short emplacement history of these rock bodies. Present radiometric age measurements suggest a time span of about 10 Ma for igneous activity at the caldera.