Final Reports (Stage One Projects)

Project I1 - Western Succession Basin Architecture and Ore Systems (March 2002 - March 2005)

Introduction

The I1 project was undertaken to understand better the 3D crustal architecture and mineral systems of the Mount Isa Western Succession. Several major mineral deposits occur within this geological province.

The emphasis of Project I1 was on basin evolution and interpretation of the regional tectonostratigraphic and structural framework that gave rise to mineralisation and controlled the depositional process.

High Impact Outcomes
Summary Research Outcomes 

A 3D model of basin architecture integrates sequence stratigraphy, structure, fault kinematics, alteration patterns (mineral index maps), mineral occurrences and fluid pathways. This model is one of the core achievements of the I1 project and allows a level of 3D visualisation of Western Succession basin architecture previously unavailable.

3D models are unforgiving in that their construction leaves little room for compromise or indecision regarding the orientation, attitude and kinematic history of major structures active at the time of basin formation and sedimentation.

Map thicknesses and over 30 000 metres of measured section were employed to constrain basin shape and thereby identify the key structures that controlled sedimentary deposition at any one time. More than two dozen serial structural cross sections were constructed and tested using the potential field data (gravity and aeromagnetics) to further constrain basin architecture.

This analysis indicated that basin geometry from 1800 to ca.1740 Ma was initially controlled by ENE-WSW extension and the formation of NNW-trending growth faults in an intra-continental rift setting; both syn-rift (Myally Subgroup) and post-rift (uppermost Quilalar Formation) sedimentary sequences are recognised. The Leichhardt River Fault Trough and Mount Gordon Arch came into being at this time and thereafter dominated basin architecture in the southern part of the Western Succession (Leichhardt Superbasin). Basement rocks lie at shallow depths below this superbasin as evidenced by inversion of the potential field data.

Sedimentation in the Leichhardt Superbasin took place in eastward deepening half-graben and was dominated by fluviatile to marginal marine shallow water sequences ranging from quartzite through red beds into carbonates (lowermost Quilalar Formation) that have the potential to host Irish-style Zn-Pb deposits. The underlying Eastern Creek Volcanics thicken westward into east-dipping normal faults that form the western margin of the Leichhardt River Fault Trough. Modern day analogues for the Leichhardt Superbasin include the North American Rio Grande and East African rift systems.

Commencing around 1730 Ma or shortly thereafter (1710 Ma in the Leichhardt River Fault Trough), the direction of extension changed from ENE-WSW to NE-SW leading to a rejuvenation of rifting, bimodal volcanism and associated granite intrusion, and development of a new superbasin (Calvert/Isa Superbasin); half-graben and normal faults formed contemporaneously with this phase of basin evolution trend east-west to WNW-ESE. The cause of this upsurge in rifting and related magmatic activity is unclear but most likely involved a thermal perturbation in the mantle and/or lower crust that was not only long lived but culminated in the intrusion and extensional unroofing of 1670 Ma granites in the Sybella Batholith. Uplift and erosion accompanying these events produced a major unconformity at the base of the Bigie Formation although, contrary to some earlier suggestions, there is no evidence for an episode of basin inversion preceding deposition of this unit.

Basin inversion did not occur for at least a further 90-100 My when the whole region was subjected to compression during D1 north-south shortening with accompanying south-directed folding and thrusting. This event affected syn-rift clastic rocks of the Surprise Creek Formation as well as post-rift (sag) carbonate-dominated sequences of the McNamara/Isa Group. The latter contains most of the major stratiform (SEDEX) Zn-Pb deposits in the Western Succession (e.g. Mount Isa, George Fisher, Lady Loretta), almost all of which have been linked to syn-sedimentary faults that acted as fluid conduits. On this basis the hitherto under-explored Surprise Creek Formation should be equally prospective for Zn-Pb mineralisation because it is similarly locally carbonaceous and was deposited in half-graben bounded by syn-sedimentary normal faults. An "episodic rift-sag" rather than a "strike-slip" model for Zn-Pb mineralisation in the McNamara/Isa Group is more consistent with the structural relations. By ca.1670 Ma, the geodynamic setting changed from rifting to passive margin with development of a northeast-facing ramp.

The age of D1 deformation is poorly constrained but may have been as early as 1640 Ma when there was a dramatic change in the north Australian plate vector as represented by a hairpin bend in apparent polar wander path. Uplift and emplacement of basement rocks in the Murphy Inlier to the north of the Lawn Hill Platform may have accompanied D1 thrusting thereby producing sufficient topographic relief to drive fluid flow and create the conditions necessary for MVT deposits. No deposits of this style have yet been identified in the area studied in the I1 project. HYC in the McArthur Basin is of 1640 Ma age but has long been interpreted as a syn-sedimentary or late diagenetic deposit related to fluid movement along the adjacent Emu Fault, a major strike-slip structure.

Even though no mineral deposits of unequivocal D1 age have been recognised, thrusting and folding led to reduced heat flow, low-grade metamorphism at temperatures up to ca.300ºC, and deep burial of the older stratigraphic units, including the Eastern Creek Volcanics, beneath younger rocks that acted as impermeable seals to fluid movement. Prolonged deep burial created conditions favourable to the leaching of metals (particularly Cu from the Eastern Creek Volcanics) although it was not until ca.1590 Ma with the onset of D2 deformation and thrusting in particular that the impermeable seals were breached and upward escape of fluids became possible. Cu mineralisation observed on D2 shear surfaces and thrust planes is compatible with such an interpretation although breaching of D1 seals may also have occurred during subsequent D3 right-lateral wrench faulting (e.g. Mount Gordon Fault Zone).

Phengitic mica distribution as revealed by ASTER satellite imagery and PIMA analysis closely matches the northwest to southeast increase in metamorphic grade determined from thermobarometric data (Kubler index and white mica b0 cell dimensions) and reaches maximum concentration in areas of known Zn-Pb-(Cu) mineralisation (Mount Isa, Hilton). Phengite distribution may serve as a signature for fossil hydrothermal systems, defining areas of increased fluid activity (upwelling?). Areas of silicification identified in the ASTER data may similarly be linked to fossil fluid systems; they are often spatially associated with east-west-trending faults formed contemporaneously with deposition of the Surprise Creek Formation but which were subjected to reactivation during both D1 and D2 deformation.

Numerical modelling of these faults and related basin architecture shows that for even small amounts of shortening these east-west-trending structures serve as zones of dilation and focussed fluid flow. Additional evidence in support of fluid flow along these structures derives from radiometric data showing that several of them are associated with anomalously high K, presumably reflecting wall rock reaction with potassic fluids. The presence of high-K fluids is also supported by widespread K-feldspar-quartz-hematite veining of D3 age although the relationship to Zn-Pb and Cu mineralisation is unresolved.

List of Partners/Sponsors

Geoscience Australia
QLD Department of Natural Resources & Water
The University of Melbourne
CSIRO EM
Monash University
James Cook University

The team gratefully acknowledges the important contribution of local explorers into the project:

* Please note that references, appendices etc are not published in this Final Report Summary. These can be found in the Final Report available via the links below.

Full Report PDF49.6Mb

For Further Information Contact

Project Leader
George Gibson
Geoscience Australia
Tel: +61 2 6249 9727
Email: George.Gibson@ga.gov.au

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Logos of core participants
Monash University University of Western Australia University of Melbourne James Cook University Geoscience Australia Victorian Institute of Earth and Planetary Sciences CSIRO Centre for Exploration Targeting AMIRA