Two billion years of evolution enclosed in hydrothermal rutile : recycling of the São Francisco Craton Crust and constraints on gold remobilisation processes.
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Date
2019
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Abstract
The Quadrilátero Ferrífero in the São Francisco Craton, Brazil is known for itsworld-class gold deposits hosted by
the Rio das Velhas greenstone belt (RdV). As in other greenstone belts, gold is also concentrated in late Archean/
early Paleoproterozoic supracrustal units; in this casewithin the Moeda Formation. This unit has been compared
to theWitwatersrand basinwhere there has been a long-termdebate over the nature of the gold andwhether it is
detrital (placer), hydrothermal or hybrid (modified placer). Presently, links between gold in the Moeda Formation
and RdV are tenuous. To enlighten this area of ambiguity, a new approach using chemical, isotopic andmineral
inclusion data from detrital rutile found in the gold-bearing horizons of the Moeda Formation is presented.
Grain textures and mineral inclusions have contributed to establishing a primary hydrothermal origin for the rutile,
related to As-Fe-Cu-Sb-Pb-Wrich mineralising fluids. Fe, Nb and V indicate that the likely source of the goldrelated
rutiles is the Archean banded iron formations within the RdV, where most of the lode-gold occurs. Average
Nb/Ta values of these rutiles are lower than average continental crust ratios which suggests fractionation
driven by fluid precipitation and/or competing mineral phases. U-Pb data yield primary crystallization ages
older than deposition ages (N2.58 Ga), supporting the detrital nature of these rutiles. Rutiles record either resetting
or new growth at 2.1 Ga and at ca. 500 Ma, duringwhich goldwas remobilised. Hence, this study supports a
modified placer model for the gold-bearing horizons of the Moeda Formation. These fluid-assisted gold
remobilisation events can be found in other correlated Paleoproterozoic gold-bearing horizon units that represent
intracratonic basins related to the rifting of the Kenorland Supercontinent. This study reinforces the suitability
of rutile and mineral inclusions as a powerful tool to elucidate processes and provenance.
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Keywords
Paleoplacer, Mineral inclusion, Gold remobilisation
Citation
PEREIRA, I. et al. Two billion years of evolution enclosed in hydrothermal rutile : recycling of the São Francisco Craton Crust and constraints on gold remobilisation processes. Gondwana Research, v. 68, p. 69-92, 2019. Disponível em: <https://www.sciencedirect.com/science/article/pii/S1342937X18303058>. Acesso em: 08 fev. 2019.