Karst-bauxite formation during the Great Oxidation Event indicated by dating of authigenic rutile and its thorium content.

Aluminium (Al)-rich palaeosols-i.e., palaeobauxite deposits-should have formed in karst depressions in carbonate sequences as a result of acidic solutions from oxidative weathering of sulfide minerals during the Great Oxidation Event (GOE), but no GOE-related karst-palaeobauxite deposits have so far been recorded. Here, we report results of in situ uranium-lead (U-Pb) dating of detrital zircon and spatially associated rutile from a metamorphosed Al-rich rock within a dolomite sequence in the Quadrilátero Ferrífero (QF) of Minas Gerais, Brazil, known as the Gandarela Formation. Rutile grains are highly enriched in thorium (Th = 3-46 ppm; Th/U ratio = 0.3-3.7) and yielded an isochron, lower-intercept age of ca. 2.12 Ga, which coincides with the final phase of the GOE-i.e., the Lomagundi event. The rutile age represents either authigenic growth of TiO2 enriched in Th, U and Pb during bauxite formation, or subsequent rutile crystallisation during metamorphic overprint. Both cases require an authigenic origin for the rutile. Its high Th contents can be used as a palaeoenvironmental indicator for decreased soil pH during the GOE. Our results also have implications for iron (Fe)-ore genesis in the QF. This study demonstrates that in situ U-Th-Pb-isotope analyses of rutile can place tight constraints on the age and nature of palaeosols.