Palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Basin: oxygen‐deficient atmosphere or hydrothermal alteration?

摘要: Petrographic and sulphur isotope studies support the long-held contention that rounded grains of pyrite in siliciclastic sequences Late Archaean Witwatersrand Supergroup originated as placer grains. The areconcentrated at sites where detrital heavy minerals are abundant within quartz-pebble conglomerates quartzose sandstones. Depositional with are: (1) matrix bar-type, clast-supported conglomerates; (2) on scoured or winnowed surfaces; (3) stratification planes. internally compact porous, truncation internal structure outer margins indicating fragmentation rounding pyritic source-rocks during erosion sediment transport. A large range textures reflects source-rock lithologies, known varieties linked to sedimentary-hosted diagenetic pyrite, volcanic-hosted massive sulphide deposits hydrothermal pyrite. Laser ablation analysis reveals a broader δ 3 4 S values (- 5.3 +6.7‰) than previously reported conventional bulk-grain analyses 1 +4‰). Rounded from Steyn Reef has significant variation 4.7 establishes heterogeneous compositions, even adjacent having diverse isotopic signatures. heterogeneity supports origin for Euhedral overgrowths which undoubtedly authigenic have restricted 0.5 + 2.5‰), chemically distinct probably products metamorphism alteration. indicates was stable mineral transport early atmosphere. Its distribution three (Witwatersrand Supergroup, Ventersdorp Contact Black Reef), other not Witwatersrand-type Au-U ore deposits, implies deposition redox-sensitive Archaean. Consequently, strong indicators an oxygen-poor While confirming gold palaeoenvironmental significance negates its link mineralization.