10:00 AM - 10:15 AM
[BCG06-05] Mesoarchaean continental weathering and Au transport recorded in Witwatersrand pyrite
Keywords:Archaean, Gold, Witwatersrand, Pyrite
The Witwatersrand Basin of South Africa hosts the world’s largest accumulation of Au known. Gold is hosted in thin, but laterally continuous quartz-pyrite conglomerates (reefs) that overlie regional scale erosional surfaces. The most recent models for Au mineralisation emphasize the role of craton-scale weathering and erosion of continental rocks, which operated since the emergence of the craton from ca. 3.1 Ga. According to these models, dissolved gold deriving from extensive weathering of mafic-ultramafic volcanic greenstone successions was transported by acid surface waters to coastal flood plains where sulfate-reducing bacteria incorporated it into sedimentary diagenetic pyrite.
In order to test these genetic scenarii, and better constraint the source of sulfur (S) here, we present new petrographic observations, trace element and multiple S-isotope Secondary Ion Mass Spectrometry analyses (SIMS)of pyrite from different Witwatersrand reefs that complement previous datasets (Hofmann et al., 2009; Large et al., 2013; Agangi et al., 2015). The reefs studied include the Dominion, Carbon Leader, Basal, Vaal, and Black reefs, spanning in age from ca. 3.0 to 2.6 Ga. The new enhanced dataset allows the visualisation of secular variations in Au and other trace elements in pyrite formed over a protracted period spanning from the Mesoarchaean to the Neoarchaean. Gold contents in reworked sedimentary-diagenetic pyrite grains in the conglomerates (extending up to 7 ppm, median 1.5 ppm) and high Au/Ag (mostly ranging from 0.1 to 1) exceeding the values of Mesoarchaean-Neoarchaean marine pyrite indicate an efficient trapping of continentally derived Au in coastal plains. The large range of δ34S (from --9.6 to +21.4‰) indicated that various S sources are involved. Multiple S-isotope analyses of massive detrital and sedimentary-diagenetic pyrite indicate muted mass-independent fractionation (Δ33S range from -0.8 to +0.7 ‰), which mirror previous analyses of Mesoarchaean pyrite worldwide (Maynard et al., 2013). Further, a predominance of near-zero or mildly negative Δ33S values (average -0.18 ±0.22‰) in diagenetic pyrite from the reefs indicates preferential derivation of S from photolytic sulfate, and fixation by microbial sulfate reducers. Such Δ33S distribution contrasts with the predominant positive values of marine shales across the Archean, and may reflect the activation of a continental flux of S.
We interpret the data in a context of large-scale craton emergence above sea level, triggering weathering, erosion, and transport of Au and other metals by rivers and run-off. Slow submergence affecting large portions of the craton would have accommodated such thick sedimentary successions. The efficient trapping of Au by fluvial muds hosting microbial activity is considered a key step in forming the Au deposits of the Witwatersrand Basin.
References
Hofmann, A., et al., 2009. Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis. Earth and Planetary Science Letters 286, 436-445.
Large, R.R., et al., 2013. Evidence for an intrabasinal source and multiple concentration processes in the formation of the Carbon Leader Reef, Witwatersrand Supergroup, South Africa. Economic Geology 108, 1215-1241.
Agangi, A., et al., 2015. Gold accumulation in the Archaean Witwatersrand Basin, South Africa — Evidence from concentrically laminated pyrite. Earth-Science Reviews 140, 27-53.
Maynard, J.B., et al., 2013. Mass-independently fractionated sulfur in Archean paleosols: A large reservoir of negative Δ33S anomaly on the early Earth. Chemical Geology 362, 74-81.
In order to test these genetic scenarii, and better constraint the source of sulfur (S) here, we present new petrographic observations, trace element and multiple S-isotope Secondary Ion Mass Spectrometry analyses (SIMS)of pyrite from different Witwatersrand reefs that complement previous datasets (Hofmann et al., 2009; Large et al., 2013; Agangi et al., 2015). The reefs studied include the Dominion, Carbon Leader, Basal, Vaal, and Black reefs, spanning in age from ca. 3.0 to 2.6 Ga. The new enhanced dataset allows the visualisation of secular variations in Au and other trace elements in pyrite formed over a protracted period spanning from the Mesoarchaean to the Neoarchaean. Gold contents in reworked sedimentary-diagenetic pyrite grains in the conglomerates (extending up to 7 ppm, median 1.5 ppm) and high Au/Ag (mostly ranging from 0.1 to 1) exceeding the values of Mesoarchaean-Neoarchaean marine pyrite indicate an efficient trapping of continentally derived Au in coastal plains. The large range of δ34S (from --9.6 to +21.4‰) indicated that various S sources are involved. Multiple S-isotope analyses of massive detrital and sedimentary-diagenetic pyrite indicate muted mass-independent fractionation (Δ33S range from -0.8 to +0.7 ‰), which mirror previous analyses of Mesoarchaean pyrite worldwide (Maynard et al., 2013). Further, a predominance of near-zero or mildly negative Δ33S values (average -0.18 ±0.22‰) in diagenetic pyrite from the reefs indicates preferential derivation of S from photolytic sulfate, and fixation by microbial sulfate reducers. Such Δ33S distribution contrasts with the predominant positive values of marine shales across the Archean, and may reflect the activation of a continental flux of S.
We interpret the data in a context of large-scale craton emergence above sea level, triggering weathering, erosion, and transport of Au and other metals by rivers and run-off. Slow submergence affecting large portions of the craton would have accommodated such thick sedimentary successions. The efficient trapping of Au by fluvial muds hosting microbial activity is considered a key step in forming the Au deposits of the Witwatersrand Basin.
References
Hofmann, A., et al., 2009. Multiple sulphur and iron isotope composition of detrital pyrite in Archaean sedimentary rocks: A new tool for provenance analysis. Earth and Planetary Science Letters 286, 436-445.
Large, R.R., et al., 2013. Evidence for an intrabasinal source and multiple concentration processes in the formation of the Carbon Leader Reef, Witwatersrand Supergroup, South Africa. Economic Geology 108, 1215-1241.
Agangi, A., et al., 2015. Gold accumulation in the Archaean Witwatersrand Basin, South Africa — Evidence from concentrically laminated pyrite. Earth-Science Reviews 140, 27-53.
Maynard, J.B., et al., 2013. Mass-independently fractionated sulfur in Archean paleosols: A large reservoir of negative Δ33S anomaly on the early Earth. Chemical Geology 362, 74-81.