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

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Doyle, MG (1997) A Cambro-Ordovician submarine volcanic succession hosting massive sulfide mineralisation: Mount Windsor Subprovince, Queensland. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
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Date Deposited: 21 Jul 2009 02:39
Last Modified: 11 Mar 2016 05:55
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