Rutile alteration and authigenic growth in metasandstones of the Moeda Formation, Minas Gerais, Brazil – A result of Transamazonian fluid–rock interaction

Abstract

Microstructures, trace elements and U–Pb ages of rutile grains are reported from two metasandstone samples of the Moeda Formation, which is the basal unit of the Minas Supergroup in the Quadrilátero Ferrífero, Minas Gerais, Brazil. The Moeda metasandstones, deposited between 2.68 and 2.65 Ga, are intercalated with Witwatersrand-like, pyritiferous and auriferous metaconglomerate layers. Rutile from both samples shows unusually high contents of Th (1–142 ppm) and rare-earth elements (ΣREE = 1–132 ppm), with chondrite-normalized middle REE humps, and reveals post-depositional U–Pb ages of ≤2.25 Ga; only three grains yielded significantly older ages of up to 2.72 Ga, pointing to a detrital origin. The combined data set suggests that rutile formation and alteration at 2.25 Ga was caused by distinct fluid-mediated processes, which erased nearly all provenance information. Detrital, rounded rutile grains in sample A were altered by a coupled dissolution–reprecipitation process (type 1), resulting in highly porous grains. These show variable contents of rutile-compatible and -incompatible trace elements: Th (1–48 ppm), Zr (25–796 ppm), U (12–167 ppm), Pb (9–52 ppm), Cr (900–12,878 ppm), W (143–3939 ppm), Nb (1148–6168 ppm), Sb (6–63 ppm), as well as variable ratios of Zr/Hf, Y/Ho and Th/U. Rutile grains in sample B are mostly authigenic (type 2), and formed after dissolution of pre-existing detrital rutile grains and subsequent transport and homogenization of trace elements. This interpretation is supported by the anhedral shape of some grains and the finding of numerous, randomly oriented muscovite laths. The authigenic population has significantly lower, and less variable contents of trace elements compared to type-1 rutile: Th (0.5–16 ppm), Zr (21–67 ppm), U (5.8–24 ppm), Pb (6-18 ppm), Cr (1379–3153 ppm), W (9.4–70 ppm), Nb (921–3058 ppm), Sb (3.3–10 ppm), and distinctively lower ratios of Zr/Hf (7.9) and Th/U (0.2), but variable Y/Ho ratios. Significant contents of Th and REE are explained by co-precipitation with TiO2 aggregates prior to structural ripening. The close association of rutile with muscovite, the rutile MREEN humps and the results of Zr-in-rutile thermometry suggest that rutile formation and alteration in the investigated sandstones was mediated by K(Na)–Si–P-bearing aqueous fluids at temperatures of about 490 °C. The rutile MREEN humps are interpreted to result from the tetrad effect during leaching of spatially associated detrital zircon grains. Uranium–Pb ages of ≤2.25 Ga indicate that the fluid overprint started at the onset of the Transamazonian orogeny, perhaps related to the opening of the Sabará foreland basin.