EXECUTIVE SUMMARY
The principal aim of this project was to test the models of the Cambrian fault structure in western Tasmania which were developed in P291 and to refine these models where appropriate. This was always going to be difficult given the complexity of Cambrian tectonics. It was not sufficient to identify a structure as Cambrian in age but rather to see the Cambrian as five separate structural events. The extenslonal faulting which has been clearly identified near VHMS deposits is a very short lived event which is restricted to part of the Middle Cambrian (?Undillan). The extension project P291A was proposed to test the model at appropriate locations and test extensions of the model wherever possible.
Project 1: The Mt Cripps Fault as a Middle Cambrian transfer structure
Detail provenance and facies studies indicate the Mt Cripps Fault was present during the deposition of the Southwell Subgroup but no evidence was found for activity on this fault during the deposition of the Animal Creek Greywacke. The Mt Cripps Fault was a relatively small feature during the late stage of the Yolande Cycle with an offset less than the thickness of the Southwell Subgroup, but sufficient to lift the southern block into a subaerial position.
Project 2: Cambrian structure of the western basin margin Pieman-Rosebery-Dundas
There are fundamental differences between Middle and Late Cambrian strata close to the western limits of the Dundas trough in the area near Rosebery which require a complex basin margin geometry. The transfer fault system proposed in P291 is compatible with these geometric requirements but
other solutions are possible.
Project 3: Henty Fault/Red Hills-Selina transfer system
The southern extension of a Cambrian fault in the Moxons Saddle area and an E-W transfer zone south of Red Hills were present during Tyndall Group deposition and subsequent erosion from a subaerial northeastern fault block. A normal, west dipping fault orientation is preferred. The Moxon Fault was inverted, probably in the Late Cambrian and, in the Devonian.
Project 4: Linda Zone-Firewood Siding Fault
A near complete section of Middle to Upper Cambrian strata is exposed north of the Firewood Siding Fault. This succession records a prolonged period of below wave base sedimentation sourced primarily from basement. During the upper part of the Middle Cambrian, a change in basin geometry is heralded by the influx of medium to coarse-grained detritus and the development of a slope fan system.
No structural or stratigraphic evidence was found that the Firewood Siding Fault existed during the Middle Cambrian. The primary evidence remains that this structure existed as a Late Cambrian transfer zone during the N-S folding.
Project 5: Lithostratigraphic correlations
The Cambrian rocks of western Tasmania were subdivided into three cycles based on pre-Tyndall Group (Yolande Cycle), Tyndall and post-Tyndall Group (Denison Cycle) sequences, using biostratigraphic constraints where possible. The Tyndall Cycle, although occupying only a very short time span (1-2 Myr?) at the end of the Middle Cambrian acts as a marker unit that allows this subdivision to be extended across the Cambrian rocks of western Tasmania. A map of this distribution was developed.
Cycle 1 opens with deep water sedimentation and dacitic volcanism. Towards the top of Cycle 1 the basin becomes more complex with increased basaltic volcanism and active extensional faulting. This major change in basin geometry occurs at the stratigraphic interval that hosts the major massive sulphide deposits, perhaps reflecting a thermal maximum that also correlates with maximum basaltic
volcanism. Cycle 2 was initiated in many areas by explosive basaltic volcanism with a mild but distinct
tholeiitic character. This widespread volcanic event
formed at many small centres unrelated to the major
centres of Cycle 1 volcanism. Cycle 3 is dominated by clastic sedimentation and is synchronous with east-west compression and basin inversion.
The heavy mineral assemblages in sandstones provided source signatures for all the basement units of western Tasmania. These sources contributed to units at all stratigraphic levels and, while their relative importance varied, no new ?exotic source appeared during the depositional history.
Project 6: Professor Range to Tyndall Range section
A restorable section was drawn along 535000mN. The strong out-of-section movement at the Henty Bridge prevents a meaningful reconstruction across this zone, so the restoration is in two sections. This section matches the style of sections to the south, with only moderate total shortening of 14 km. The level of erosion is such that a very good separation of this shortening between Cambrian and Devonian deformation with Late Cambrian folding producing a third of the total shortening in the section. The Rosebery section remains a major problem because of the large shortening required by the Rosebery and Mt Black thrusts for which no equivalent has been found on other sections.
Project 7: 2D Geophysical modelling
Four structural sections across the Mt Read Volcanics were tested by 2D modelling of the gravity and magnetic fields. The structural models are consistent with the geophysical data within the constraints of the publicly available physical properties database. Regionally averaged physical properties determined along the line of each section are required to improve the stringency of this test of the structural models.
Project 8: Cleavages associated with the Rosebery Fault
There is a close spatial relationship between the Rosebery Fault and a late N-striking cleavage (S2). An earlier cleavage was not visible in the most intense zones of S2 cleavage development but overprinting was found on the edge of the zone of S2 development. The earlier cleavage has a composite origin including both a NNW striking Devonian cleavage and a N striking cleavage of Devonian or Cambrian age. We could not find any way to resolve whether the N striking cleavage was Cambrian in age.
Project 9: Chemical fingerprints for Cambrian Faults
Growth faults are dynamic structures in a fluid history sense. Where these faults have been examined in detail, their isotopic composition is very variable, and this is accounted for by a complex retainment pattern of the isotopic signature of multiple fluid batches passing through each fault. Only sulfur isotope values >18 permil and 18 permil have been recorded from the Moxons Fault and the
western boundary fault of the Dundas trough in the Pieman Gorge supporting the growth fault interpretation.
The nature of Middle and Late Cambrian deformation is complex in both time and place. The whole of the Cambrian was a period of very active tectonics in western Tasmania. This can be characterised by a number of stages. A period of extension in the Middle Cambrian reached a thermal and structural maximum in the Undillan and Boomerangian stages of the Middle Cambrian. The major phase of VHMS mineralisation occurred at this time. The transition from a simple deep water basin associated with dacite
dominated volcanism to a complex basin with large ranges in water depth and substantial andesitic to basaltic volcanism occurs during the Undillan. Cambrian extensional faults (Henty-Moxons-Cripps zone, Pieman-Rosebery-Husskisson zone) are recognisable at this time but not before. The basin was inverted in the Late Cambrian with active erosion of the older volcanics.
While the Late Cambrian and Devonian deformation have obscured much of the basin geometry during the extensional phase, a few of the larger structures can still be recognised. There is no evidence for wholesale dismemberment of the basin geometry.
The Lihir gold deposit, Papua New Guinea, is the world’s largest alkalic low-sulfidation epithermal gold deposit in terms of contained gold (50 Moz). The deposit formed over the past million years and records a progression from porphyry- to epithermal-style hydrothermal activity. The early porphyry stage was characterized by biotite- anhydrite-pyrite ± K-feldspar ± magnetite alteration and weak gold ± copper mineralization and produced abundant anhydrite ± carbonate veins and anhydrite ± biotite-cemented breccias. These features collectively characterize the deep-seated anhydrite zone at Lihir. Several hundred thousand years ago, one or more catastrophic mass-wasting events unroofed the porphyry system after porphyry-stage hydrothermal activity ceased. Mass wasting may have been facilitated in part by dissolution of porphyry-stage anhydrite veins. Epithermal mineralization occurred after sector collapse, resulting in phreatic and hydraulic brecciation and veining, widespread adularia-pyrite ± carbonate alteration, and formation of mineralized zones at Lienetz, Minifie, Kapit, Kapit NE, Coastal, and Borefields. A NE- to ENE-striking fault array localized several of these orebodies. The pyrite-rich veins and pyrite-cemented breccias that formed during epithermal-stage hydrothermal activity define the sulfide zone at Lihir. This zone mostly contains refractory gold in pyrite, with minor free gold and precious metal tellurides hosted in late-stage quartz veins. A period of diatreme volcanism disrupted the Luise amphitheater during the latter stages of epithermal mineralization. The diatreme breccia complex truncated several of the epithermal ore zones and was crosscut locally by late-stage epithermal veins. Recent geothermal activity produced a steam-heated clay alteration blanket that has overprinted the refractory sulfide-rich epithermal assemblage near the present-day land surface. Gold was remobilized downward from the steam-heated zone into the sulfide zone during argillic and advanced argillic alteration, producing thin gold-rich rims around pyrite grains. This process produced a high-grade tabular enrichment zone immediately beneath the base of the clay blanket.
Sedimentary rock-hosted stratiform copper deposits form by movement of oxidized, copper-bearing fluids across a reduction front that results in the precipitation of copper sulfides. Large-scale production of such oxidized fluids, as well as the formation of mobile hydrocarbons (oil) has probably been common since the formation of the first red beds in the Paleoproterozoic, and deposits of this type occur in rocks from the Paleoproterozoic to the Tertiary. However, supergiant deposits are currently recognized in only three basins: the Paleoproterozoic Kodaro-Udokan basin of Siberia, the Neoproterozoic Katangan basin of south-central Africa, and the Permian Zechstein basin of northern Europe. The paucity of data regarding the Udokan deposit makes understanding this system difficult in terms of Earth history events. Both the Neoproterozoic and the Permian were times of supercontinent breakup with major landmasses at low latitudes. This global tectonic framework favored the formation of failed rifts that subsequently became significant intracratonic basins with basal, synrift red-bed sequences overlain by marine and/or lacustrine sediments and, in some basins located at low latitudes, by thick evaporitic strata. The intracratonic setting of these basins allowed the development of a hydrologically closed basinal architecture in which highly oxidized and saline, moderate-temperature basinal brines were produced that were capable of supplying reduction-controlled sulfide precipitation over very long time periods (tens to hundreds of millions of years). The length of time available for the mineralizing process may be the key factor necessary to form supergiant deposits. However, examination of the absolute ages for the Kupferschiefer (Zechstein basin) and Katangan deposits allows speculation that other factors may also have been important. Both the Neoproterozoic and Permian were times of major glacial events. Glaciation may also be conducive for the formation of supergiant sediment-hosted stratiform copper deposits. Glacial periods correspond to magnesium- and sulfate-rich oceans that could have been responsible for additional sulfur in basinal brines developed during evaporite formation and would then be available during the long mineralization process.
The Central African Copperbelt, spanning the Zambian-Congolese border, is the world’s largest sedimentary copper province. Ores are hosted largely in evaporitic strata of the Neoproterozoic Katangan Basin, a product of Rodinia dispersal, and latest Neoproterozoic to Ordovician (Pan African) orogenesis. Existing structural models for the evolution of the Katangan Basin invoke high magnitude thrust transport and associated dismemberment of the middle and upper Katangan Supergroup stratigraphy, and ores hosted therein. We test these models and present new data that place important constraints on the pre-orogenic configuration of the basin, and the controls on ore location. Our results indicate that macroscale extensional basin architecture remained relatively little modified by orogenesis, with classical stratiform copper ores positioned about the condensed fringe of a central depocentre maximum. The complex structural geometries that characterize the Congolese arm of the copperbelt, in particular, were largely inherited from a systematically orientated array of extension-related halokinetic structures: diapirs, salt walls, salt allochthons, and withdrawal sub-basins. The arrangement of these intra- and supra-salt structural elements were in turn inherited from a syn-rift structural architecture developed at the initiation of basin growth. The position of classical Mines Subgroup-hosted ores appears strongly influenced both by the geometry of the syn-rift compartment, and the localization of overlying salt welds, the latter providing cross-stratal permeability that directed deep-seated fluids to intra-salt redox interfaces. High exploration potential is considered to exist for non-classical stratiform ores positioned at various stratigraphic levels beyond the original limits of salt.
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