Palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Basin: oxygen-deficient atmosphere or hydrothermal alteration? GAVIN L. ENGLAND 1 , BIRGER RASMUSSEN, BRYAN KRAPEZ and DAVID I. GROVES Centre for Global Metallogeny, Department of Geology, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia ABSTRACT Petrographic and sulphur isotope studies support the long-held contention that rounded grains of pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Supergroup originated as placer grains. The grains are concentrated at sites where detrital heavy minerals are abundant within quartz-pebble conglomerates and quartzose sandstones. Depositional sites with abundant pyrite are: (1) within the matrix of bar-type, clast-supported conglomerates; (2) on scoured or winnowed surfaces; and (3) on stratification planes. The grains are internally compact or porous, with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source-rocks during erosion and sediment transport. A large range in textures reflects source-rock lithologies, with known varieties linked to sedimentary-hosted diagenetic pyrite, volcanic-hosted massive sulphide deposits and hydrothermal pyrite. Laser ablation sulphur isotope analysis of pyrite reveals a broader range in d 34 S values () 5Æ3 to +6Æ7&) than that of previously reported conventional bulk-grain analyses () 1 to + 4&). Rounded pyrite from the Steyn Reef has significant variation in d 34 S values () 4Æ7 to +6Æ7&) that establishes heterogeneous sulphur compositions, with even adjacent grains having diverse isotopic signatures. The heterogeneity supports a placer origin for rounded pyrite. Euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted d 34 S values () 0Æ5 to + 2Æ5&), are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration. The placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere. Its distribution in three sequences (Witwatersrand Supergroup, Ventersdorp Contact Reef and Black Reef), and in other sequences not linked to Witwatersrand-type Au-U ore deposits, implies deposition of redox-sensitive detrital heavy minerals during the Late Archaean. Consequently, rounded grains of detrital pyrite are strong indicators of an oxygen-poor atmosphere. While not confirming a placer origin for gold in Witwatersrand Au-U ore deposits, the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization. Keywords Archaean atmosphere, mineralization, pyrite, sulphur isotopes, Witwatersrand. INTRODUCTION The origin of rounded grains of pyrite in silici- clastic sequences of the Late Archaean Witwa- tersrand Supergroup of South Africa has long 1 Present address: Department of Geology and Geo- physics, Grant Institute, University of Edinburgh, Edinburgh EH9 3JW, UK (E-mail: Gavin.England@ glg.ed.ac.uk) Sedimentology (2002) 49, 1133–1156 Ó 2002 International Association of Sedimentologists 1133