AusGeo News  June 2006  Issue No. 82

Expanding our knowledge of Mt Isa to a third dimension

George Gibson, Paul Henson, Avon McIntyre and Narelle Neumann

Australian researchers have been working together to interpret the geology and prospectivity of the Mount Isa Western Succession.

Since March 2002, a collaborative project to improve our understanding of the 3D crustal architecture and mineral systems of the Mount Isa Western Succession has engaged researchers from Geoscience Australia, universities, industry and other government agencies. The project has been carried out under the auspices of the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC).

The project gave high priority to building of a 3D structural model of the Mount Isa region, incorporating stratigraphy, major unconformity surfaces, fault geometries, basin shape and mineral deposit locations. Complementary investigations into the geodynamic setting, origin and timing of events that may have influenced or controlled fluid flow in the region helped the researchers achieve:

The project built on previous Geoscience Australia successes with NABRE (North Australian Basins Resource Evaluation) and external partners, such as the AMIRA P552 Fluid Flow Modelling project. It involved collaboration with the Queensland Department of Natural Resources, James Cook University, Melbourne University, the University of Western Australia, CSIRO, and two exploration companies currently active in the region, Xstrata Copper and Zinifex. The University of Newcastle provided further geological input and analytical data on a contractual basis.

Following a 12-month confidentiality period, the results of the project became publicly available from 30 April 2006 and are currently being prepared for publication.

As might be expected of such a richly endowed mineral province, the Mount Isa region in northern Australia (figure 1) has long been the subject of intense geological research and exploration activity. There is a diverse range of opinion about its regional structure, tectonic evolution and metalliferous potential. This makes mineral exploration more difficult and fraught with uncertainty, because there is rarely the time or resources to fully assess the wide range of interpretations.

Despite such uncertainties, tectonic setting and its role in controlling crustal architecture and the pattern of fluid flow are widely perceived to be important factors in the formation of ore bodies and their metallogeny. The difficulty for exploration companies is that the Western Succession, like so many other Proterozoic terranes in northern Australia (including the Eastern Succession), has been substantially modified by later deformation and post-depositional processes. The original tectonic setting and basin architecture are no longer obvious, and there is ongoing uncertainty about the age of mineralisation, its relationship to major structures, and whether these structures first became active before or after mineral deposition.

Fig 1. Study area in relation to major tectonic elements in the Mount Isa region.

Marrying basin analysis with regional structural studies was deemed the most effective means of differentiating between pre-, syn- and post-depositional fault movements, although this is seldom enough to narrow the range of potential exploration targets. Details of possible source rocks and traps are also required.

More importantly, the researchers thought it desirable to have all elements of the mineralising system brought together in a single 3D representation, permitting better visualisation of the problem and more effective discrimination between competing geological interpretations. The 3D environment provides a much less forgiving test bed for subsurface geological interpretation than was ever possible from traditional 2D geological maps and cross sections.
Successful exploration now demands a more integrated approach to the problem of target selection, requiring an understanding of the entire mineralising system. This has been the guiding philosophy and visionary goal behind the current research program:

… through a better understanding of the geological controls on existing sediment-hosted Pb–Zn–Cu deposits in the Western Succession derive the key geological parameters that will increase the predictive ability of the exploration industry to locate blind or as yet undiscovered mineral deposits beneath cover elsewhere in northern Australia.

Figure 1. Study area in relation to major tectonic elements in the Mount Isa region. (Larger image [GIF 324.0kb])

Fig 2. Gravity response upwardly continued from 66 to 220 kilometres to produce the ‘Barramundi worm’ as determined by Hobbs et al (2000).

Project scope

The previous Geoscience Australia NABRE study in the Western Succession largely concentrated on elucidation of basin architecture in the Calvert (1730–1670 Ma) and Isa (1670–1595 Ma) superbasins. (See the event chart of Neumann et al for a more detailed geochronology of these two structural entities.)

Figure 2. Gravity response upwardly continued from 66 to 220 kilometres to produce the ‘Barramundi worm’ as determined by Hobbs et al (2000). Note major Pb-Zn mineral deposits located along the trace of this feature which is thought to represent a continent-scale boundary between crustal blocks of contrasting density. (Larger image [JPG 587.1kb])

These studies placed less emphasis on the older underlying Leichhardt Superbasin (1790–1740 Ma) even though it was recognised that major faults dating from that period had the potential to exercise an important control on the geometry and location of future, successor basins. The new study was consequently directed at improved understanding of the architecture of the older Leichhardt Superbasin and how this interacted with younger basins to produce the distribution of sedimentary facies and magmatic rocks preserved in the Western Succession today.

Particularly important in this context was an attempt to recognise rock sequences that may have served as potential source and trap rocks for metals. No less important was a study of fault geometry and distribution, and how these may have influenced fluid flow and the migration of mineralising fluids through the rock pile.

Only part of the Western Succession was investigated in detail, with the bulk of the research being conducted in the northern part of the Leichhardt River Fault Trough and the southern part of the Lawn Hill Platform (figure 1). As such, the study area straddled the Mt Gordon Fault Zone, a prominent geophysical and geological feature (figure 1), as well as a significant length of the ‘Barramundi worm’ (figure 2). The latter had already been identified in upwardly continued regional gravity datasets (Hobbs et al 2000), and appeared to serve as an important locus for several mineral deposits at Century and Lady Loretta (figure 1). The Century Pb–Zn deposit formed part of a PhD project carried out at James Cook University under the banner of the pmd*CRC.

Fig 3. Isopach map for Myally-age sediments combined with mineral occurrences and major basin-bounding faults in the Mount Isa region.

Objectives, organisation and deliverables

To achieve its visionary goal, the project was set up to deliver:

Figure 3. Isopach map for Myally-age sediments (1770-1780 Ma) combined with mineral occurrences (red = Cu; green = Pb-Zn) and major basin-bounding faults in the Mount Isa region. (Larger image [GIF 321.7kb])

To meet its objectives, the project was managed as a series of modules with specific outcomes and deliverables. These included:

For more information phone George Gibson on +61 2 6249 9727 (email george.gibson@ga.gov.au)

References

  1. Neumann NL, Southgate PN, Gibson GM & McIntyre A. In press. New SHRIMP geochronology for the Western Fold Belt of the Mount Isa Inlier: developing a 1800–1650 Ma event framework. Australian Journal of Earth Sciences.
  2. Southgate PN (guest editor). 2000. NABRE results: thematic issue: Carpentaria–Mt Isa belt: basement framework, chronostratigraphy and geodynamic evolution. Australian Journal of Earth Sciences 47:337–657.

Related website/articles

New data on rock ages from Mt Isa Inlier AusGeo News 78

Mineral mapping with ASTER AusGeo News 82

Unless otherwise noted, all Geoscience Australia material on this website is licensed under the Creative Commons Attribution 3.0 Australia Licence.