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

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. ll1 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. lV 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 Supergroup. 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 Vl 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
ormal 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...