The evolution of fluid flow systems prior to, during, and post MVT mineralization in the Givetian-Frasnian carbonates of the Emanuel Range, Western Australia

摘要

The Emanuel Range is part of the extensive Devonian Reef complex, which overlies the Lennard Shelf, a section of shallow basement at the northern end of the Canning Basin in Western Australia. The reef complexes formed in an extensional tectonic environment and host numberous Mississippi Valley-type (MVT) Zn-Pb accumulations. The distribution, relative timing, and origin of calcite, dolomite, and MVT sulphide generations in the southeastern Emanuel Range indicate that the fluid flow systems operating during Middle Devonian to Middle Carboniferous burial changed significantly through time. The topographic flow of metoric water from norht to south, driven through alluvial fans shed from the highlands of the Precambrian Kimberley block to the north of the Emanuel Range, was the longest-lived fluid flow system. The topographic flow system operated intermittently during periods of lowstand in the Givetian and early Frasnian and continued throughout subsequent burial and uplife in the Middle Carboniferous. During early to intermediate burial depths (< 500 m), stratigraphically focused compactionally driven fluids of predominantly modified marine origin entered the reefs from a depocentre to the south of the Emanuel Range. During later burial (1-1.5 km) in the Late Devonian and Early Carboniferous, more deeply sourced fluids were driven laterally into the Emanuel Range from the Fitzroy Trough to the south. The introduciton of compactionally driven, hot metalliferous hydrocarbon-bearing brines into the sequence occurred episodically, precipitating MVT sulphides. Between hyrothermal pulses, formation waters decreased in temperature and salinity, due to mixing with cooler surface waters. Hydrothermal brines flowed laterally from south to north until encounter with the Cajdebut Fault, which impeded brine flow. Further brine movement was focused through dilatant faults with a NNE orientation (thought to be related to transfer zones; Vearncombe, J.R., Dentith, M., Doerling, S., Reed, A., Cooper, R., Hart, J., Muhling, P., Windrim, D., Woad, G., 1995. Structural controls on Mississippi Valley-type mineralization, the southeastern Lennard Shelf, western Australia. In: Sangster, D.E.(Ed.), Carbonate-Hosted Lead-Zinc Deposits. Society of Economic Geologists, Special Publication, vol.4. 1995, pp.74-94.) and near-vertical hydraulic fractures related to high fluid pressures. Through time, hyrothermal pulses became both less frequent and less intense, and topographically driven flow again dominated.