Advances in genetic understanding of sediment hosted base metal and gold deposits
|Location||International Geological Congress,oslo 2008|
|Author||Large, Ross R.; Cooke, David; McGoldrick, Peter; Scott, Rob; Selley, Dave|
|Holding Date||08 October 2008|
Continental margin and intra-continental basins are hosts to the major global resources of sediment-hosted base metal and gold deposits. Research in the last ten years has greatly improved our understanding of basin scale fluid flow and fluid chemical processes involved in the transport and deposition of copper, zinc-lead-silver and gold in ore deposits within these sedimentary basins. The presence or absence of evaporitic strata, and its position within the stratigraphy, is critical to the development of basinal brines, their fluid flow patterns and their capacity to transport particular metals.
At the early stages of basinal development, in shallow clastic and evaporitic-carbonate dominated rift basins, circulation of oxidised saline marine fluids, below a cap rock of evaporites, is the ideal regime for the formation of stratiform copper deposits such as those in Zambia and the Kuperschiefer. Oxidised saline fluids sink into the basement along rift faults, leach copper and cobalt from the basement lithologies, and recirculate under convective flow, driven by the geothermal gradient, to deposit stratiform copper sulfides due to fluid mixing and reduction. Ore deposition occurs by syn-diagenetic replacement of organic-rich shales and mixing with organic rich fluids trapped within footwall clastic packages, particular at pinch out positions.
At later times in sedimentary basin development (several 10’s of million years later), within the tectonically quiet sag phase of siltstone, shale and carbonate deposition, overlying the clastic rift package, stratiform zinc-lead-silver deposits may form adjacent to deep penetrating basin-scale rift faults. Saline marine fluids sink into the basin along a system of recharge faults to depths of 2 to 7 km. During episodes of tectonic activity, the saline fluids move laterally through the basin along clastic aquifer and/or permeable volcanic units leaching zinc, lead and silver to become powerful ore-forming fluids. These fluid rise to the surface along deep penetrating discharge faults, and exhale metal-rich brines into the basin waters, or into porous sediments below the basin floor. Two types of fluids may be generated in this way, depending on the lithological composition of the sedimentary basin. Oxidised metalliferous basinal fluids (SO4 > H2S) are generated in basins dominated by clastics, carbonates and volcanics, whereas reduced metalliferous basinal brines (H2S > SO4) are generated in basins dominated by shales and quartzites. Oxidised fluids deposit Zn-Pb due to reduction on the seafloor by mixing and reaction with organic-rich seawater and bottom muds. Reduced fluids deposit Zn-Pb due to mixing with seawater and rapid cooling. Distal or proximal SEDEX deposits result from these processes, depending on the fluid type.
Sedimentary basins that are dominated by organic-rich shales, siltstones and carbonates, but contain no oxidised evaporitic sequences, generate basinal fluids that are excessively reduced (H2S and organic-C rich), with moderate to low salinity. Such fluids do not have the capacity to transport significant Zn, Pb, Cu or Ag (due to their low salinity and high H2S content), but can transport Au and As as bisulfide complexes. Basin fluid convection and related exhalation of these strongly reduced, organic-C and H2S-rich fluids, leads to concentrations of stratiform gold, arsenic and other trace elements (Zn, Mo, Se, Ag, Pb, V, Ni) within black mudstone facies, by adsorption and organo-metallic complexation in the basin-floor muds. Formation of world-class basin-hosted gold deposits is commonly a two-stage process, which involves syngenetic concentration of Au-As in organic-rich sediments as described above, followed by basin inversion, leading to metamorphic re-concentration and up-grading of gold in structural sites such as anticlinal cores, shear zones and breccia zones.