Paper Number: 4028
The source of sulfur in the Mesoarchaean gold deposits of the Barberton Greenstone Belt, southern Africa
Agangi, A.1, Reddy, S.M.1, Hofmann, A.2, Eickmann, B.2, Marin-Carbonne, J.3
1Curtin University, Bentley, WA, Australia.
2University of Johannesburg, Johannesburg, South Africa
3Université J. Monnet, Saint-Etienne, France
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The source of metal-bearing fluids in structurally-controlled ore deposits is controversial, as both metamorphic and igneous sources have been proposed [1]. In gold deposits of Neoarchaean terranes in Western Australia and Canada, felsic magmatism coeval with mineralisation has been interpreted as the source of gold [2]. The Palaeoarchaean Barberton Greenstone Belt of southern Africa contains some of the oldest gold deposits known. In these deposits, Au is hosted by late extensional faults that truncate the main compressional structures in the greenstone belt [3]. The ore, which is hosted in meta-volcanic and meta-sedimentary rocks, is dominated by pyrite and arsenopyrite, and gold either occurs finely disseminated in sulfide minerals (“invisible gold” present in solid solution or nanoinclusions), or is present as free gold grains in quartz-carbonate veins. The ore fluids were H2O-CO2-rich, and S may have been an important complexing agent for the transport of Au. The estimated age of mineralisation (3.0 – 3.1 Ga) broadly overlaps with felsic magmatism in the area [4, 3]. We used multiple S isotopes (32S, 33S and 34S) to track the possible sources of S in sulfides from Barberton gold deposits.
Sulfur in sulfide and sulfate minerals from Archaean terrains has δ33S and δ34S values that characteristically deviate from mass-dependent fractionation. This is interpreted as a consequence of UV radiation-induced photolytic reactions of S gases in a low-oxygen atmosphere [5]. MIF-S can be used to distinguish “atmospheric” S sources from endogenous (volcanic, crustal etc.) sources. Analyses of S isotopes in pyrite hosted in meta-mafic and meta-sedimentary rocks from two deposits of the Barberton Greenstone Belt indicate Δ33S = -0.6 to +1.0 ‰, and δ34S = 0 to +8 ‰. These values overlap with available analyses of pyrite and, to a lesser extent, barite from several sedimentary units in the greenstone belt, and clearly indicate derivation of S from the surrounding rocks. These analyses do not allow to constrain the origin of the major components of the ore fluids (H2O and CO2), which may have been derived from sources external to the greenstone belt (e.g. magmas). However, the fact that S – possibly the main complexing agent for Au – was derived from the supracrustal succession places constraints on the ultimate source of Au.
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