Volcanic facies architecture of the Cambro-Ordovician Seventy Mile Range Group, Northern Queensland, Australia

The Cambro-Ordovician Seventy Mile Range Group occurs within the Mount Windsor Subprovince in northern Queensland, Australia. It extends approximately 165 km east-west, consists of compositionally and texturally diverse submarine volcanic and sedimentary fades and hosts massive sulfide deposits. The Seventy Mile Range Group has undergone greenschistgrade regional metamorphism and three phases of deformation. It comprises four formations, the middle two of which are dominantly volcanic (Mount Windsor and Trooper Creek Formations). Detailed volcanic fades mapping has been undertaken along spaced traverses throughout the Seventy Mile Range Group. The main areas studied include Warawee, Brittania, Highway /Reward, Mt. Farrenden, Mt. Windsor, Lion town and Thalanga. This research has prompted a review of the existing stratigraphic framework. The Mount Windsor and Trooper Creek Formations are amalgamated into one lithostratigraphic unit, informally termed the Mount Windsor-Trooper Creek unit. The Mount Windsor-Trooper Creek unit overlies nonvolcanic sandstone and mudstone of the Puddler Creek Formation. The contact with the Puddler Creek Formation is either narrow (<100 m) or broad (up to 1000 m) and interfingering or intrusive. The Mount Windsor-Trooper Creek unit is conformably overlain by non-volcanic sandstone and mudstone of the Rollston Range Formation. Twenty-three principal lithofacies have been identified in the Mount Windsor-Trooper Creek unit and are organised into five groups: 1) coherent rhyolite, dacite and basaltic andesite; 2) monomictic volcanic breccia and conglomerate; 3) polymictic volcanic breccia and conglomerate; 4) volcanic sandstone and mudstone; and 5) non-volcanic sandstone and mudstone. The volcanic succession is interpreted to comprise lavas and intrusions, syn-eruptive pyroclastic sediment gravity flow deposits, bomb-rich fire-fountain deposits, syn-eruptive resedimented pyroclastic and autoclastic deposits and post-eruptive and reworked volcaniclastic deposits. Transport and deposition of monomictic and polymictic volcanic breccia and conglomerate involved water-supported sediment gravity flows. Coherent and associated autoclastic rhyolite fades are interpreted to be dominantly intrusions. Regionally, the environment of deposition was submarine and below-storm-wave-base. The products of both intrabasinal submarine and extrabasinal shallow-marine or subaerial volcanic eruptions are preserved in the Mount Windsor-Trooper Creek unit. The volcanic fades associations represent three and possibly four main intrabasinal volcano types: 1) rhyolite lava and intrusion volcanic complexes, 2) basaltic andesite fire-fountain and lava volcanoes, 3) dacite dome volcanoes, and 4) possibly rhyolite lava-intrusion \tuff'' volcanoes. Rhyolite lava and intrusion volcanic complexes are the most voluminous and common type of volcano identified. The basaltic andesite fire -fountain eruptions generated a distinctive monomictic breccia composed of fluidal and blocky basaltic andesite clasts. The fluidal clasts resemble subaerial volcanic bombs and are interpreted to be the products of submarine fire fountaining of relatively low-viscosity lava. The stratigraphic thickness of the Mount Windsor-Trooper Creek unit varies across the belt from ~1100 mat Thalanga to >7 km in the Mt. Farrenden and Mt. Windsor east areas. Thickness variations of the volcanic stratigraphy are a product of accumulation of mass-flow volcaniclastic fades in local depocentres and emplacement of voluminous felsic lavas and intrusions at intrabasinal volcanic centres. Lavas intrusions and monomictic volcaniclastic fades can be divided into three geochemically distinct suites. Suite I ranges in composition from rhyolite to basaltic andesite and is interpreted to comprise a cogenetic suite related by variable degrees of crystal fractionation and crustal assimilation. Suite II consists principally of rhyolites which are geochemically distinct from Suite I rhyolites. Suite III consists of trachyandesites. Suite I occurs at all stratigraphic levels within the Mount Windsor-Trooper Creek Suite II is restricted to the lower Mount WindsorTrooper Creek unit and Suite III is largely restricted to the Puddler Creek Formation. All suites occur across the length of the Seventy Mile Range Group. New U-Pb zircon ages for felsic volcanic rocks from the Mount Windsor-Trooper Creek unit are 467.9 ± 4.1 Ma 466.7 ± 2.6 Ma and 465.4 ± 1.4 Ma. Volcanic-hosted massive sulfide deposits occur in the middle to upper part of the Mount Windsor-Trooper Creek unit. They have previously been interpreted to be spatially and temporally associated with emplacement of rhyolite lavas and shallow-level intrusions (e.g. Thalanga and Highway /Reward) or hosted by volcaniclastic facies (e.g. Lion town). The volcanic fades architecture of the Seventy Mile Range Group is similar to that of the Early Proterozoic Skellefte District in Sweden where numerous polymetallic massive sulfide deposits are hosted within rhyolite volcanic centres and volcaniclastic deposits."