Dating hydrothermal processes related to the formation of the Lagoa Real uranium deposits using in situ U–Pb dating of andradite and titanite

Abstract

To understand the mechanisms and origin of ore deposits, it is essential to know their age. However, the processes responsible for ore formation do not always leave a geochronological record, with hydrothermal processes often either not affecting zircon or causing only partial resetting. The Lagoa Real uranium province hosts the most important U resource in Brazil. The U mineralization is hosted by Na-rich rocks known as albitite, which also contain variable amounts of garnet, clinopyroxene, biotite, amphibole, magnetite and titanite. These rocks are presumed to be the result of hydrothermal alteration of the Lagoa Real intrusive suite, a group of A-type granitic rocks emplaced between 1740 and 1720 Ma. Despite the economic importance of these rocks, determining the age of metasomatism and U mineralization has been extremely difficult, with previous attempts using a variety of methods and targeting the whole rock, titanite, uraninite, heavy-mineral concentrates and zircon, yielding a wide variety of ages in the range of 1.5 to 0.9 Ga. This study is the first to directly date metasomatic minerals (garnet and titanite) with the in situ U–Pb dating method. Garnet and titanite from an unmineralized sample gave U–Pb ages of ca. 545 Ma and ca. 520 Ma, respectively. U–Pb dating of zircon from two other unmineralized samples yielded ages of 1740–1720 Ma, interpreted as igneous ages. Electron-microprobe dating of uraninite from mineralized samples produced ages of 530–520 Ma. Altogether, these results suggest that both metasomatism and U mineralization occurred as a result of fluids associated with the gravitational collapse phase (ca. 530–480 Ma) of the Araçuaí–West Congo orogen. This study illustrates the utility of U–Pb dating of andradite garnet for determining the age of mineralization related to metasomatism.