Sulfur sources of buckshot pyrite in the Auriferous Conglomerates of the Mesoarchean Witwatersrand and Ventersdorp Supergroups, South Africa Citation

23 24 Large rounded pyrite grains (>1 mm), commonly referred to as ‘buckshot’ pyrites, are a characteristic feature of 25 the auriferous conglomerates (reefs) in the Witwatersrand and Ventersdorp supergroups. Detailed petrographic 26 analyses of the reefs indicated that the vast majority of the buckshot pyrites are of reworked sedimentary origin, 27 i.e., that the pyrite originally formed in the sedimentary environment during sedimentation and diagenesis. 28 Forty-five of these reworked sedimentary pyrite grains from the Main, Vaal, Basal, Kalkoenkrans, Beatrix and 29 Ventersdorp Contact reefs were analyzed for their multiple sulfur isotope compositions (δ 34 S, Δ 33 S and Δ 36 S) to 30 determine the source of the pyrite sulfur. In addition, four epigenetic pyrite samples (pyrite formed after 31 sedimentation and lithification) from the Middelvlei and the Ventersdorp Contact reefs were measured for 32 comparison. The δ 34 S, Δ 33 S and Δ 36 S values of all 45 reworked sedimentary pyrite grains indicate clear 33 signatures of mass-dependent and mass-independent fractionation and range from −6.8 to +13.8‰, −1.7 to 34 +1.7‰ and −3.9 to +0.9‰, respectively. In contrast, the four epigenetic pyrite samples display a very limited 35 range of δ 34 S, Δ 33 S and Δ 36 S values (+0.7 to +4.0‰, −0.3 to +0.03‰ and −0.3 to +0.1‰, respectively). 36 Despite the clear signatures of mass-independent sulfur isotope fractionation, very few data points plot along 37 the primary Archean photochemical array suggesting a weak photolytic control over the data set. Instead, other 38 factors command a greater degree of influence such as pyrite paragenesis, the prevailing depositional 39 environment and non-photolytic sulfur sources. In relation to pyrite paragenesis, reworked syngenetic 40 sedimentary pyrite grains (pyrite originally precipitated along the sediment-water interface) are characterized by 41 negative δ 34 S and Δ 33 S values, suggesting open system conditions with respect to sulfate supply and the 42 presence of microbial sulfate reducers. On the contrary, most reworked diagenetic sedimentary pyrite grains 43 (pyrite originally precipitated below the sediment-water interface) show positive δ 34 S and negative Δ 33 S values, 44 suggesting closed system conditions. Negligible Δ 33 S anomalies from epigenetic pyrite suggest that the sulfur 45 was sourced from a mass-dependent or isotopically homogenized metamorphic/hydrothermal sulfur reservoir. 46 Contrasting sulfur isotope compositions were also observed from different depositional environments, namely 47 fluvial conglomerates and marine-modified fluvial conglomerates. The bulk of the pyrite grains from fluvial 48 conglomerates are characterized by a wide range of δ 34 S values (−6.2 to +4.8‰) and small Δ 33 S values (±0.3‰). 49 This signature likely represents a crustal sulfate reservoir derived from either volcanic degassing or from 50 weathering of sulfide minerals in the hinterland. Reworked sedimentary pyrite grains from marine-modified 51 fluvial conglomerates share similar isotope compositions, but also produce a positive Δ 33 S/δ 34 S array that 52 overlaps with the composition of Archean barite, suggesting the introduction of marine sulfur. These results 53 demonstrate multiple sources of sulfur, including atmospheric, crustal and marine sulfur reservoirs. The 54 prevalence of the mass-dependent crustal sulfur isotope signature suggests that sulfate concentrations were 55 probably much higher in terrestrial settings in comparison to marine environments, which were sulfate-deficient. 56 However, the optimum conditions for forming sedimentary pyrite with a crustal sulfur isotope signature were 57 probably not during fluvial progradation but rather during the early phases of flooding of low angle 58 unconformities, i.e., during retrogradational fluvial deposition, coupled in some cases with marine 59 transgressions, immediately following inflection points of maximum rate of relative sea level fall. 60