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Sedimentology, geochemistry, vocanology and basin evolution of the Soldiers Cap Group, Eastern succession, Mt Isa, Inlier, Northwest Queensland, Australia


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Hatton, OJ 2004 , 'Sedimentology, geochemistry, vocanology and basin evolution of the Soldiers Cap Group, Eastern succession, Mt Isa, Inlier, Northwest Queensland, Australia', PhD thesis, University of Tasmania.

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The Soldiers Cap Group is a polydeformed, complexly altered and highly
metamorphosed volcanosedimentary terrane which represents the easternmost
extent of outcropping lithologies of the informal Maronan Supergroup, in the
Mesoproterozoic Eastern Succession of the Mt Isa Inlier, northwestern Queensland,
Australia. The Maronan Supergroup is a regionally important sequence hosting a
range of base and precious metals deposits. This study presents the results of a
detailed program of field mapping and sampling, whole rock and isotopic
geochemistry, undertaken to 'look through' the deformation, alteration and
metamorphism to determine the primary sedimentological, volcanological and
geochemical processes, and define detrital source areas and orientations of the host
basin of the upper Soldiers Cap Group. Although the Soldiers Cap Group is
iuterpreted as extending well south and west of the Williams Batholith, this thesis is
only based upon the area of extensive outcrop to the north and east of the batholith.
A revision of the lithostratigraphy of the upper Soldiers Cap Group along genetic
lines has resulted in the definition of 3 major and 5 minor sub-units within the Mt
Noma Quartzite. Sedimentological structures preserved in quartzites, psammites and
psammopelites include swaley- and hummocky-bedforms, climbing ripples,
hummocky cross stratification, rip-up clasts, trough cross-bedding, extremely
thickly- bedded quartzites, and cycles of massive quartzite. When these are placed
into the new lithostratigraphic framework for the upper Soldiers Cap Group, the
sedimentary processes of deposition are defined as storm generated currents above
storm wave-base inducing oscillatory flow which then continued below storm wavebase
as sand rich turbidity currents (tempestites). Once thought to be a deeper water
turbidite sequence, this new interpretation places the environment of deposition for
the upper Soldiers Cap Group at a narrow, storm-dominated shelf. A probable
shallowing upward succession is suggested by a marked change from the underlying
turbidites with classical Bouma sequences of the Llewellyn Creek Formation to the
debris flow and turbidity current dominated basal Mt Norna Quartzite. Similarly in
the mid Mt Norna Quartzite there is a discrete change in the sedimentological
environment from turbidity current to tempestite dominated units. The
sedimentology of the overlying Toole Creek Volcanics was not as well studied due to poor outcrop. However, the presence of similar sedimentary structures in the
Toole Creek Volcanics suggests the continuation throughout the upper part of the
sequence, of the processes occurring in the mid Soldiers Cap Group.
Despite the lower amphibolite facies metamorphic grade and numerous regional
alteration events, metabasalts and metadolerites of the upper Soldiers Cap Group
have numerous well preserved contact relationships with adjacent metasedimentary
lithologies. These include stratigraphically well constrained, brecciated (peperites),
planar and diffuse contacts, all of which are indicative of synsedimentary sill
emplacement. Within parts of the upper Soldiers Cap Group, distinctive volcanic
structures including pillowed flows and in-situ hyaloclastites represent a sequence of
synsedimentary flows. The geochemical signature of these mafic rocks defines them
as ferrobasalts (;;o: 12wt% FeO,) with continental tholeiite affinities derived from
partial melting of an N-MORB source, with an unquantified degree of crustal
contamination. Fe-enrichment in the Soldiers Cap Group is always associated with
increased Ti, P and Zr. Mafic units with a well defined Fe-Ti-P fractionation trend,
interpreted as representing crystallization of Ti-magnetite and apatite, occur in the
Weatherly Creek Syncline. Lower Fe-Ti-P units occur in the Pumpkin Gully
Syncline where early fractionation ofFe-Ti-P phases was suppressed. Units with
intermediate Fe-Ti-P values in the Toole Creek Syncline require the mixing of
magmas from at least two magma chambers with differing fractionation histories.
Ferrobasalt sequences are generally produced in discrete, high level chambers
separated from the main magma source in active tectonic environments, where
composition is controlled by the rate of magma resupply, redox conditions (j02), size
of the magma chamber and cooling rates: In the case of the Soldiers Cap Group the
more Fe-enriched sequence of the Weatherly Creek Syncline represents a closed
magma chamber, with limited magma resupply where .f02 controlled magnetite
fractionation. Conversely, the Pumpkin Gully Syncline sequence represents a
smaller, more open, magma chamber in which resupply was an important process.
Selected immobile and rare earth elements were used to define a geochemical
'fingerprint' for the Soldiers Cap Group metatholeiites that is distinct from other Mt
Is a Inlier basalts and could be of use in future exploration.Detailed geological mapping, combined with petrographic and whole-rock
geochemical studies of two mineralised and barren, unusually apatite+Mn-garnet
rich, stratigraphically well-constrained, laterally extensive iron formations, allowed a
confident interpretation of their genesis. Concordant contact relationships with
surrounding units, rare erosional contacts, an uncommon P-Mn rich mineralogy and
predeformational quartz-epidote±carbonate(±garnet) alteration haloes, all indicate
that they are the product of primary, hydrothermally derived, chemical
sedimentation. Multivariate statistics were used to define groups of elements with
strong interelement correlations, which were then refined by bivariate plots. This
method defined mixed hydrothermal detrital and minor hydrogeneous components.
The hydrothermal component comprised two stages, a relatively simple Fe-Si stage
and a more complex P-Mn-Eu-Ca bearing stage. Detrital input was largely from
extrabasinal mafic sources located presently to the east of the Soldiers Cap Grpup.
Minor hydrogeneous uptake of (Pb+Zn)-Mn also occurred. Positive En anomalism
and distinct chondrite normalised REE patterns represent reduced, acidic, Fe-P-Mn
rich hydrothermal fluids with temperatures of ;:o:250°C. To account for these features,
large scale convection cells in overpressured sections of the sedimentary pile are
proposed. These were driven by the intrusion of fractionating Fe-rich magma
chambers into the lower parts of the basin. These generally metal-poor aquifers were
breached by renewed movement on extensional faults resulting in fluid expulsion
along these structures to the sediment -water interface, where they mixed with cold
oxid.ised seawater in restricted, relatively quiescent seafloor depressions. The
anomalous P-Fe-Mn rich geochemical signature similar to that of exhalites
associated with base-metals mineralization globally (Sullivan, Canada and Broken
Hill Australia) has important implications for future exploration within the Maron an
U-Pb dating of detrital zircons from three samples covering -4km of stratigraphic
thickness from the basal Mt Noma Quartzite to the mid Toole Creek Volcanics, by
the recently developed Laser Ablation Inductively Coupled Plasma Mass
Spectrometry (LA-ICP MS) method, provided insights into possible source terranes
for the Soldiers Cap Group. Pb207/Pb206 probability plots define several zircon
populations. The dominant age inheritance is represented by a ca.l750 Ma peak
consistent with a source from Mt Isa Inlier Cover Sequence 2 volcanosedimentary
units. Possible contributing sequences include the 1760-1720 Ma Mary Kathleen Group, the ca.l740 Ma Mt Fort Constantine Volcanics, and units of the similarly
aged Ballara Quartzite, Marraba Volcanics and Mitakoodi Quartzite. Other detrital
populations include a ca.l970 Ma population interpreted as representing zircons
from the earliest rifting events in the Mt Isa Inlier, a ca.l860 Ma peak consistent with
Barramundi Basement equivalents in the Eastern Succession, a ca.l790 Ma aged
population likely sourced from the Argylla Formation, ca.l680 Ma 'Maronan
Supergroup' aged zircons, and a ca.1580 Ma population representing metamorphic
overgrowths. The ca.1790 Ma population, present in other samples, is absent from
the upper Mt Norna Quartzite suggesting changes in extra basinal erosion patterns.
Age populations present in all samples include a ca.2450 Ma Archaean peak which
has a poorly constrained source within the region and likely represents significant
recycling of grains. These age dates suggest that much of the material for the
Soldiers Cap Group could be derived from Eastern Succession units and that the
Maronan Supergroup was likely formed proximal to the eastern margin of the ancient
Mt Isa Inlier.
Basin scale structural controls in comparable volcanosedimentary sequences are
commonly unraveled using isopachs or balanced cross section methods. Strong
deformation and the current subvertical orientation of the stratigraphy largely
prevented this in the Soldiers Cap Group. The approach taken in this study was to
combine sedimentology with palinspastically reconstructed palaeoflow markers,
volcanology, detrital zircon data and interpretations of iron formation genesis to
determine the orientation and nature of the host basin to the Soldiers Cap Group at
Mt Noma Quartzite-Toole Creek Volcanics time. Based upon mapped thickness and
lithostratigraphic changes, map scale faults in the Weatherly Creek Syncline area
(Lomas Creek Faults, Mt Noma Fault) are now interpreted as synsedimentary faults.
Reconstructed palaeocurrent markers interpreted to represent shore"normal flow and
detrital zircon U-Pb provenance data, concur, indicating a broadly south-southeast
deepening basin. The Lomas Creek Faults are now interpreted as the result of
renewed extension in the upper part of a classical 'sag-phase' of an intracontinental
rift. Faulting was possibly initially controlled by a larger scale fault, represented by
the modem Cloncurry Overthrust. To account for mapped thickness changes in the
mid-Mt Norna Quartzite, extension was likely transferred to an as yet unidentified
fault to the southeast of the study area. Comparisons to other Proterozoic terranes worldwide show that there are at least two
close geological analogues for the Soldiers Cap Group. These are the Aldridge
Formation, Belt-Purcell Basin, B.C. Canada, host to the massive Sullivan Pb-Zn
deposit and the Etheridge Group, Georgetown Inlier, northeast Queensland,
Australia. Analogous features include several basin-wide extension events associated
with hydrothermal activity and linked to voluminous Fe-tholeiite magmatism,
chemical sedimentation, input of coarser sediment, and synsedimentary faulting.
These three basins may represent an as yet unrecognized variant on the accepted
genetic evolution of the sag-phase of intracontinental rift basins. This analogy has
important implications for future exploration, with a 'hybrid' empirical model
drawing on both BHT and SED EX models being proposed for use in the Soldiers
Cap Group.

Item Type: Thesis - PhD
Authors/Creators:Hatton, OJ
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