Multiple Sulfur Isotope Records of the 3.22 Ga Moodies Group, Barberton Greenstone Belt - Archive ouverte HAL
Article Dans Une Revue Geosciences Année : 2020

Multiple Sulfur Isotope Records of the 3.22 Ga Moodies Group, Barberton Greenstone Belt

Résumé

The Moodies Group, the uppermost unit in the Barberton Greenstone Belt (BGB) in SouthAfrica, is a ~3.7-km-thick coarse clastic succession accumulated on terrestrial-to-shallow marinesettings at around 3.22 Ga. The multiple sulfur isotopic composition of pyrite of Moodies intervalswas newly obtained to examine the influence of these depositional settings on the sulfur isotope record.Conglomerate and sandstone rocks were collected from three synclines north of the Inyoka Fault of thecentral BGB, namely, the Eureka, Dycedale, and Saddleback synclines. The sulfur isotopic compositionof pyrite was analyzed by Secondary Ion Mass Spectrometry (SIMS) for 6 samples from the threesynclines and by Isotope Ratio Mass Spectrometry (IR-MS) for 17 samples from a stratigraphic sectionin the Saddleback Syncline. The present results show a signal of mass-independent fractionation ofsulfur isotopes (S-MIF), although t-tests statistically demonstrated that the Moodies S-MIF signals(mostly 0% < D33S < +0.5%) are significantly small compared to the signal of the older Paleoarchean(3.6–3.2 Ga) records. These peculiar signatures might be related to initial deposition of detrital pyriteof juvenile origin from the surrounding intrusive (tonalite–trondhjemite–granodiorite; TTG) andfelsic volcanic rocks, and/or to secondary addition of hydrothermal sulfur during late metasomatism.Moreover, fast accumulation (~0.1–1 mm/year) of the Moodies sediments might have led to a reducedaccumulation of sulfur derived from an atmospheric source during their deposition. As a result, thesulfur isotopic composition of the sediments may have become susceptible to the secondary additionof metasomatic sulfur on a mass balance point of view. The sulfur isotopic composition of Moodiespyrite is similar to the composition of sulfides from nearby gold mines. It suggests that, after theMoodies deposition, metasomatic pyrite formation commonly occurred north of the Inyoka Fault inthe central BGB at 3.1–3.0 Ga.

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Dates et versions

hal-02891741 , version 1 (12-11-2020)

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Masafumi Saitoh, Sami Nabhan, Christophe Thomazo, Nicolas Olivier, Jean-François Moyen, et al.. Multiple Sulfur Isotope Records of the 3.22 Ga Moodies Group, Barberton Greenstone Belt. Geosciences, 2020, 10 (4), pp.145. ⟨10.3390/geosciences10040145⟩. ⟨hal-02891741⟩
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