Provenance and genesis of the auriferous palaeoplacer deposits in the Palaeoproterozoic Moeda Formation, Minas Gerais, Brazil
Final Report Abstract
A first combined U-Pb and Lu-Hf isotopic dataset has been generated for detrital zircon grains from the auriferous palaeoplacer deposits of the Moeda Formation, Minas Gerais, Brazil. These data not only allowed to constrain the sedimentation age to 2.68 Ga but also gave important information about possible source areas and indicate a major reworking of older crust in the hinterland of the Moeda Formation metasediments. Rutile grain analysis yielded atypically high REE and Th concentrations. In combination with U-Pb isotopic data, these data revealed a major metamorphic overprint combined with different stages/intensities of fluid-rock interactions at 2.25 Ga. During investigation of heavymineral concentrates hydrothermal gold and detrital gold micro-nuggets were discovered for the Moeda Formation metasedimentary rocks, which corroborate the notion of a modified palaeoplacer deposit. Mineralogical evidence from gold–tourmaline intergrowths on authigenic pyrite indicates that the transporting fluid was boron-rich but poor in aqueous sulphide species. The analysis of the data from the Moeda Formation in conjunction with available data from palaeoplacer deposits worldwide led to a new genetic model for the world’s largest known concentration of gold in the Earth’s crust and thus to a potential solution to one of the most intensely debated issues in economic geology – the principal gold concentration mechanism during a Mesoarchaean gold super-event, for which a strong link between gold concentration and microbial evolution from anoxygenic to oxygenic photosynthesisers could be established.
Publications
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(2014): A Giant Mesoarchean Crustal Gold-Enrichment Episode: Possible Causes and Consequences for Exploration. Society of Economic Geologists, Special Publication 18, 209-234
Frimmel, H. E.
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(2014): Depositional age and sediment source of the auriferous Moeda Formation, Quadrilátero Ferrífero of Minas Gerais, Brazil: New constraints from U–Pb–Hf isotopes in zircon and xenotime. Precambrian Research 255, 96-108
Koglin, N., Zeh, A., Cabral, A.R., Seabra Gomes Jr., A.A., Corrêa Neto, A.V., Brunetto, W.J., Galbiatti, H.
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(2015): First whiffs of atmospheric oxygen triggered onset of crustal gold cycle. Mineralium Deposita 50, 5-23
Frimmel, H. E., Hennigh, Q.
