Geology, geochemistry, and ore deposits of the Bau gold mining district, Sarawak, Malaysia

Abstract

District-scale zoning of ore deposits and structural-tectonic setting of Bau was investigated. Regional tectonic studies, structural analysis, and field mapping complemented by aeromagnetic interpretation were integrated to a structural model of Bau. Ores were studied with optical and electron microscopy, followed by major and trace element, fluid inclusion, and lead and sulfur isotope analyses. In the Late Triassic, Bau was in an island arc - back-arc basin environment Following Early Jurassic deformation and uplift, an active margin developed. Subduction of the West Pacific oceanic plate under the NW Kalimantan block began. Erosion of the Triassic Serian Volcanics produced extensive turbidite flows of the Pedawan Formation since the Latest Jurassic. Coeval development of rudist patch reefs on an unstable shelf edge of the overriding plate lasted until Cenomanian. Accretion of the turbidites ended in the Latest Cretaceous. Early Tertiary molasse deposition ended with a Mid-Eocene event. A 200-km-Iong, crustal-scale complex fault system involving dextral strike-slip and wrench faulting, termed the Bau Trend, developed during Mid-Miocene post-subduction regional extension. The principal mineralization event at Bau took place at 12-10 m.a., when I-type, calc-alkaline, reduced granodiorites intruded along the Bau Trend and its intersection with seven parallel, ENE fracture zones, providing channel ways to distribute hydrothermal fluids laterally away from the Bau Trend. Deformation preceding mineralization produced high structural permeability of the host rocks. Central Bau is underlain by an ENE trending, 5x12 km broad plutonic body at depth, inferred from aeromagnetic data. Bau displays district-scale zoning, from proximal porphyry-copper and skarn deposits, via intennediate Cordilleran-Vein base metal mineralization, sediment-hosted precious metal deposits, to distal disseminated Au-As and Ba-Hg-TI deposits. The deposits are hosted in, from proximal to distal, porphyritic granodiorites, limestones, and turbiditic shales. Gradual changes in geochemical, mineralogical, and isotopic compositions across the district indicate consanguinity between them. Epithermal gold mineralization at Bau is most similar to disseminated, sediment-hosted gold deposits of Nevada, except for higher grades, visible occurrence of gold, both base and precious metal signatures, purely structural controls, less radiogenic lead, and magmatic sulfur isotope signatures.