University of Tasmania
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A Cambro-Ordovician submarine volcanic succession hosting massive sulfide mineralisation: Mount Windsor Subprovince, Queensland

posted on 2023-05-26, 05:05 authored by Doyle, MG
The Seventy Mile Range Group is a Cambro-Ordovician, dominantly submarine volcano sedimentary succession that occurs within the Mount Windsor Subprovince of northern Queensland, Australia. Detailed facies analysis of the Mount Windsor Formation and Trooper Creek Formation, between Coronation homestead and Trooper Creek prospect (approximately 15 km strike length), has clarified the facies architecture in this part of the Seventy Mile Range Group. The rocks have been affected by regional greenschist facies metamorphism and deformation, and hydrothermal alteration is intense around VHMS deposits. The Mount Windsor Formation comprises associations of rhyolitic to dacitic autoclastic breccia and coherent facies, 100-500 m thick, which form submarine lavas, domes and syn-volcanic intrusions. The Mount Windsor Formation (300-3500 m thick) is conformably overlain by the Trooper Creek Formation (500-4000 m thick). The Trooper Creek Formation is divided into two members, the Kitchenrock Hill Member and the overlying Highway Member. The new stratigraphic scheme is based on mappable compositional and lithological variations which reflect changing provenance. The Kitchenrock Hill Member (90-110 m thick) includes rhyolitic to dacitic syn- and posteruptive volcaniclastic facies, minor syn-sedimentary intrusions and rare siltstone units. Some volcaniclastic units contain rounded clasts with geochemical and petrographic properties which suggest they were sourced from the Mount Windsor Formation. Rounded clasts were reworked prior to deposition and imply that the source areas were subaerial or shallow marine. The Highway Member comprises compositionally and texturally diverse volcano-sedimentary facies, including rhyolitic to basaltic lavas and intrusions, andesitic scoria- and bomb-rich breccia, dacitic to rhyolitic pumice breccia, and volcanic and non-volcanic sandstone and siltstone. The presence of turbidites, hyaloclastite and fossils within the Kitchenrock Hill Member and Highway Member suggest that the depositional setting for the Trooper Creek Formation was largely submarine and below storm wave base. The exception is in the upper part of the Highway Member at Trooper Creek prospect, where microbialites, gypsum molds, and traction current structures indicative of wave activity, collectively suggest a depositional environment above stonn wave base. The lithofacies exposed at Trooper Creek prospect suggest that shoaling of the succession occurred in response to construction of a small, submarine andesitic volcano which temporarily emerged above sea level. Growth of the edifice involved: (I) eruption of lava and intrusion of synsedimentary sills; (2) strombolian-style volcanism in a near-stonn-wave-base environment; (3) hydrovolcanic interactions above storm wave base and possibly subaerially; and (4) post-eruptive and possibly syn-eruptive degradation of the volcanic edifice. When subsidence due to compaction and/or tectonism outpaced accumulation, the depositional environment returned to below storm wave base. Syn-eruptive pumiceous and crystal-rich sediment gravity flow deposits in the Trooper Creek Formation were sourced from rhyolitic to dacitic eruptions at volcanic centres which are either; located outside the study area, not exposed, or not preserved. The abundance of pyroclasts (principally pumice, shards and crystals) reflects the importance of explosive magmatic and/or phreatomagmatic eruptions, and suggests that the source vents were in shallow water or subaerial settings. Some key units may be traceable for over 30 km, and are an important framework for exploration within the Trooper Creek Formation. The Highway Member includes the products of intrabasinal, non-explosive silicic to intermediate eruptions that formed lava- and intrusion-dominated volcanic centres. The volcanic centres are: (1) dominated by syn-sedimentary sills and cryptodomes; (2) comprise thick (> 5 km) lava complexes; or (3) form lava-lobe hyaloclastite domes, up to 500 m thick and at least 1.5 km long. The Cu-Au-rich Highway-Reward massive sulfide deposit is hosted by one small syn-sedimentary intrusion-dominated volcanic centre. Detailed mapping of contact relationships and phenocryst populations suggest the presence of more than thitteen distinct porphyritic units in a volume of I x I x 0.5 km. The peperitic upper margins of more than 75% of these units suggests that they were emplaced as syn-sedimentary sills and cryptodomes. Evidence for partial extrusion of lava is limited to one rhyolite. The shape, distribution and emplacement mechanisms of the units were influenced by: (1) the density of magma relative to wet sediment; (2) the location and shape of previously emplaced lavas and intrusions; and (3) possibly synvolcanic faults which may have acted as conduits for rising magma. Deformation and disruption of bedding, resedimentation, dewatering and Iow-grade metamorphism of the enclosing sediment accompanied emplacement of the intrusions. The resulting patterns of permeability and porosity in the volcanic succession are inferred to have strongly influenced the location and evolution of the syn-genetic hydrothermal system which formed the Highway-Reward massive sulfide deposit. The pyrite-chalcopyrite pipes and marginal pyrite-sphalerite-galena-barite mineralisation are largely syn-genetic sub-seafloor replacements of the host sediment, cryptodomes and volcaniclastic deposits. Hematite alteration and ironstones are regionally distributed and are not preferentially associated with mineralisation. Most ironstones have geochemical and lithofacies characteristics which suggest they are the deposits from low temperature fluids circulating around lavas and intrusions and within the proximal facies association of shallow submarine volcanoes. At Handcuff, some ironstone lenses have REE and trace element signatures which suggest they may be associated with as yet undiscovered massive sulfide mineralisation. Many ironstones are sub-seafloor replacements of pumiceous breccia and sandstone beds. The top of the Highway Member marked the end of intrabasinal volcanism and volcanicdominated sedimentation. This was then followed by a phase of post-eruption erosion of the subaerial to submarine source areas that led to an influx of variably rounded volcanic and basement-derived detritus into the submarine basin, during deposition of the overlying Rollston Range Formation. This research provides insights into understanding comparable modern and ancient submarine volcanic successions and assessing prospective host sequences for massive sulfide mineralisation.


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