# Structure and mineralisation of western Tasmania. CODES/AMIRA Project P.291A. Final report, March 1997

Berry, RF, Meffre, S, Jenner, G, Fulton, R, Selley, D, White, MJ, Corbett, KD, Bull, SW, Davidson, GJ, Kitto, PA and Roach, M 1997 , Structure and mineralisation of western Tasmania. CODES/AMIRA Project P.291A. Final report, March 1997.

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## Abstract

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 (S$$_2$$). An earlier cleavage was not visible in the most intense zones of S$$_2$$ cleavage development but overprinting was found on the edge of the zone of S$$_2$$ 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 <-5 permil are diagnostic of Cambrian fluids in the Mount Read Volcanic Belt, because of overlaps with the isotopic range of Devonian magmatic and metarmorphogenic fluids. Values less than -5 permil have only been identified in background oxidised Cambrian aquifers, particularly associated with albitisation. It is likely that this fluid facies was widespread, but poorly preserved, in early Cambrian faults. Cambrian signatures >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.

Item Type: Report (Technical Report) Berry, RF and Meffre, S and Jenner, G and Fulton, R and Selley, D and White, MJ and Corbett, KD and Bull, SW and Davidson, GJ and Kitto, PA and Roach, M CODES/AMIRA View statistics for this item