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An Atmospheric Source of S in Mesoarchaean Structurally-Controlled Gold Mineralisation of the Barberton Greenstone Belt

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An Atmospheric Source of S in Mesoarchaean Structurally-Controlled Gold Mineralisation of the Barberton Greenstone Belt An atmospheric source of S in Mesoarchaean structurally-controlled gold mineralisation of the Barberton Greenstone Belt Andrea Agangi, Axel Hofmann, Benjamin Eickmann, Johanna Marin-Carbonne, Steven Reddy To cite this version: Andrea Agangi, Axel Hofmann, Benjamin Eickmann, Johanna Marin-Carbonne, Steven Reddy. An atmospheric source of S in Mesoarchaean structurally-controlled gold mineralisation of the Barberton Greenstone Belt. Precambrian Research, Elsevier, 2016, 285, pp.10-20. 10.1016/j.precamres.2016.09.004. hal-01407404 HAL Id: hal-01407404 https://hal.archives-ouvertes.fr/hal-01407404 Submitted on 19 Oct 2018 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. An atmospheric source of S in Mesoarchaean structurally-controlled gold mineralisation of the Barberton Greenstone Belt ⇑ Andrea Agangi a, , Axel Hofmann b, Benjamin Eickmann b, Johanna Marin-Carbonne c, Steven M. Reddy a a Department of Applied Geology, Curtin University, Bentley, WA, Australia b Department of Geology, University of Johannesburg, Johannesburg, South Africa c Université Lyon, UJM –Saint Etienne, Laboratoire Magma et Volcans CNRS UMR 6524, UBP, IRD, 23 rue Paul Michelon, 42100 St Etienne, France The Barberton Greenstone Belt of southern Africa hosts several Mesoarchaean gold deposits. The ores were mostly formed in greenschist facies conditions, and occur as hydrothermal alteration zones around extensional faults that truncate and post-date the main compressional structures of the greenstone belt. Ore deposition was accompanied by the intrusion of porphyries, which has led to the hypothesis that gold may have been sourced from magmas. Because the transport of Au in the hydrothermal fluids is widely believed to have involved S complexes, tracing the origin of S may place strong constraints on the origin of Au. We measured multiple S isotopes in sulfide ore from Sheba and Fairview mines of the Barberton Greenstone Belt to distinguish ‘‘deep” S sources (e.g. magmas) from ‘‘surface” S sources (i.e. rocks of the volcano-sedimentary succession that contain S processed in the atmosphere preserved as sulfide and sulfate minerals). Ion probe (SIMS) analyses of pyrite from ore zones indicate mass-independent frac- tionation of S isotopes (D33S=À0.6‰ to +1.0‰) and the distribution of the analyses in the D33S–d34S space matches the distribution peak of previously published analyses of pyrite from the entire volcano-sedimentary succession. Notwithstanding that the H2O–CO2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks, the fact that S bears an atmo- spheric signature suggests the hypothesis that the source of Au should also be identified in the supracrus- tal succession of the greenstone belt. Our findings differ from conclusions of previous studies of other Archaean shear-hosted Au deposits based on mineralogical and isotopic evidence, which suggested a magmatic or mantle source for Au, and imply that there is no single model that can be applied to this type of mineralisation in the Archaean. 1. Introduction either present in the mineral structure or as sub-microscopic inclu- sions (‘‘invisible gold”) (Craig et al., 1998). In this kind of The Palaeoarchaean Barberton Greenstone Belt of southern structurally-controlled gold deposits, the mineralising fluids are Africa hosts some of the oldest gold mineralisation known typically aqueo-carbonic and have low to moderate salinity (Anhaeusser, 1976; de Ronde et al., 1991; Dirks et al., 2013; (Goldfarb et al., 2001; Mikucki and Ridley, 1993), and are inter- Dziggel et al., 2010)(Fig. 1). These deposits have proved to be an preted to originate from a deep source (Salier et al., 2005). The ori- important source of Au since their discovery in the 1880s, and have gin of mineralising fluids is controversial, especially in Archaean produced more than 345 tons of Au (Anhaeusser, 1976; Dirks et al., deposits, and mineralogical, elemental and isotopic evidence 2009)(Fig. 1). Most deposits are hosted in greenschist facies rocks, seems to point towards either metamorphic or igneous sources, where gold mineralisation is structurally controlled and occurs or a combination of these (Hutti mine, India; Rogers et al., 2013; along extensional faults cross-cutting the main compressional Western Australia, Doublier et al., 2014; Wang et al., 1993). Prop- structures of the greenstone belt, which extend for several tens agation of these fluids along crust-scale structures is believed to be of km along strike (Dirks et al., 2013). The ore is dominated by pyr- responsible for the formation of deposits in a single region over a ite and arsenopyrite, and gold is mostly finely dispersed in sulfides, range of depths and temperatures (from <200 to >500, and possibly 6700 °C) (‘‘crustal continuum model”; Barnicoat et al., 1991; ⇑ Corresponding author. Groves, 1993; Phillips and Powell, 2009; Kolb et al., 2015). Deposi- E-mail address: [email protected] (A. Agangi). tion of Au would have occurred by reaction of the mineralising 1 31˚E 31˚30`E A Nelspruit Batholith 3106 Ma Salisbury 25˚30`S Kop 3109 Ma Stentor Pluton New Consort 3180 Ma Lily Kaap Valley Pluton Sheba 3227 Ma Fairview Saddleback-Inyoka Fault System Abbotts Agnes Nelshoogte Bellevue Pluton LEGEND Transvaal Group Sediments Potassic granites 26˚S Syenites, granodiorites Piggs Peak TTG Intrusions Batholith Dalmein Barberton Moodies Group Greenstone Belt Pluton Fig Tree Group Mpuluzi Batholith Onverwacht Group Kaapvaal 20 km Major gold deposits Craton W Fairview (cross section) E S Sheba (cross section) N B Mamba E 11 Level adit M ureka Cats cavear chert No 1 incline Ulundi syncline garet Blue Rock Fig Tree Group e tombi (greywacke, shale) lin In icline Commitment reef Ulundi syncline Eureka SynclineKidson 22 Level e antic Golden Quarry spital ant Ho No 2 incl tkoppi r in Zwa e 46CMR 23 Level line No 3 in MRC cli 33ZKA Sheba fault ne 33ZKB Eureka Sync Le Roux Reef Rossiter Reef 35 Level Eldorado ancline es Hope Reef in Moodies Group 62-11 Chert Moodies Group y anticl (quartzite) Talc carbonate schist da (quartzite) Sheba fault DykesLevel 74 Birth Gold mineralisation 500m 0 Shafts 0500m samples Fig. 1. (A) Geological map of the Barberton Greenstone Belt and distribution of the main gold deposits (modified from de Ronde et al., 1992). (B) Cross sections of Fairview and Sheba mines (modified from Barberton gold mines, 2014). fluid with the host rocks or by fluid mixing (Bateman and can be expressed as D33S=d33S À 1000 Á [(1 À d34S/1000)0.515 À 1], Hagemann, 2004; Evans et al., 2006), or vapour separation (de as ‰ variation. Among the products of this reaction, water-soluble Ronde et al., 1992; Mikucki and Ridley, 1993). The origin of S is sulfate with D33S < 0 and relatively insoluble elemental S with an important aspect in the study of deposits hosted in Archaean D33S > 0 can then be separated by bodies of water upon deposition greenstone belts and other structurally controlled Au deposits, on the Earth’s surface, incorporated into the sediments, and pre- À since hydrosulfide complexes [Au(HS)2 and AuHS] are believed served in the rock record in the form of sulfide and sulfate miner- to be the main Au transporting agents (Benning and Seward, als. In the Barberton Greenstone Belt MIF-S has been described in 1996; Pokrovski et al., 2014; Seward, 1973; Simon et al., 1999). pyrite and barite from several stratigraphic units (Grosch and Therefore, identifying the source of S can help constrain the origin McLoughlin, 2013; Montinaro et al., 2015; Philippot et al., 2012; of Au in these deposits, which has so far remained elusive Roerdink et al., 2012, 2013). (Tomkins, 2013; Gaboury, 2013; Kendrick et al., 2011; Pitcairn Following the discovery of MIF-S in sedimentary environments, et al., 2006). MIF-S signal has also been found in Neoarchaean VMS deposits In order to distinguish deep (magmatic- or mantle-related) (Jamieson et al., 2013), in diamond-hosted sulfide inclusions from sedimentary sources of sulfur, multiple S isotope analyses (Farquhar et al., 2002; Thomassot et al., 2009), in the Palaeopro- can be used. Mass-independent fractionation of S isotopes (MIF- terozoic Rustenburg Layered Suite of the Bushveld complex S) is a common feature of Archaean and early Palaeoproterozoic (Penniston-Dorland et al., 2012) and in olivine-hosted sulfide (>2.4 Ga) sedimentary and diagenetic sulfur minerals (sulfides inclusions in Cainozoic plume-related ocean island basalt magmas and sulfates; Ono et al., 2003). This S isotope signature is believed (Cabral et al., 2013). These findings have revealed a feedback to originate from ultraviolet radiation-induced reactions of S gas between surface and deep S cycle, indicating that S processed in species (e.g. SO2,SO3) in anoxic atmosphere, and thus to be a dis- the atmosphere during the Archaean can be stored in the crust or tinctively atmospheric signature (e.g. Farquhar et al., 2000). MIF-S the mantle, and be recycled back to the surface through different 2 processes, even after a long time. Thus, using MIF-S signal as a mar- and structural style, and occur as auriferous quartz-carbonate ker of Archaean atmospheric processes has opened up new ways of veins and sulfide bodies (Agangi et al., 2014; Anhaeusser, 1986; testing the hypothesis that ore deposits, even if non-sediment- Schouwstra, 1995). Ore assemblages include predominant pyrite hosted, can have sourced at least part of their S from sediments and arsenopyrite, with minor chalcopyrite, Ni–As sulfide and spha- or other deposits that carry MIF-S (Bekker et al., 2009; Fiorentini lerite.
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