Final Reports (Stage One Projects)

Project Y2 - 3D geological models of the eastern Yilgarn Craton

Introduction

The eastern Yilgarn Craton (EYC) is one of the most intensely mineralised areas of continental crust containing numerous world-class gold and nickel deposits. Within the Yilgarn, however, there are clearly province to sub-province scale geodynamic processes that were important in localising gold-rich mineral systems.

Spatial prediction depends on the architecture and dynamics of ore systems, and it is paramount to have a good knowledge of the complex processes at a variety of scales. Understanding of these processes at the province and sub-province scale helps first-order evaluation and ranking of one terrain over another.

High Impact Outcomes
Research Summary

The principal goal of the Y2 project was to determine the 3D architecture and its evolution through time to enable prediction of where within the terrain the location of major gold deposits is likely to occur.

In order to achieve this six key deliverables were designed:

  1. An integrated geological and geophysical 3D map (model) for three specific regions within the EYC. These regions were the Kalgoorlie-Ora Banda region, the Leonora-Laverton region, and the Norseman-Wiluna region, the first two being nested within the later model;
  2. Interpreted seismic sections for the 2001 seismic recorded in the Leonora-Laverton region;
  3. Tomographic model of the Kalgoorlie-Ora Banda region which links the region's velocity structure from the surface, through the base of the crust to the base of lithosphere with the integrated geological and geophysical 3D geological models;
  4. Map of chalcophile elements across selected portions of the EYC;
  5. Assessment of the utility of the new 3D data versus the 2D data at a regional scale, and;
  6. Prospectivity analysis of the derived 3D models.
Summary of research Outcomes

The following subheadings describe the new findings. Further details may be found throughout the report, appendices, and the 3D map/GIS.

Geodynamics

The broadband data provide a unique view of the crust and mantle lithosphere. The main conclusion concerns the interpretation of the fast S-wave velocity layer at 100-120 km in the lithosphere. The layer is likely to represent the residue from High-Ca granite magmatism which ceased around 2660 Ma that was delaminated from the base of the crust.

The main contractional event across the EYC involved ~east-west shortening during D2. An orogenic surge model has been proposed to account for the diachroneity of the D2 deformation, Late Basins and the emplacement of the Mafic granites and termination of the High-Ca granites. Much of the architecture and large-scale faults were developed and active during this event.

Early versions of the model were tested numerically and the results implied that the model is mechanically viable. Numerous analogues of have been discussed in support of the model. Alternative models of remnant ocean basins and strike-slip basins were found to be inapplicable to the EYC geodynamic setting.

Architecture

There are clear differences in the endowment of the various terrains of the EYC. A conclusion from the analysis of the receiver function data is that there are architectural differences in terms of the velocity structure (i.e., lithology) between the terrains.

The velocity anomaly under the Kalgoorlie terrain has been interpreted as a function of greater volumes of Low-Ca granites, which are temporally associated with a major thermal change in granite type across the Yilgarn Craton at the same time as gold was deposited.

The Kurrawang Basin and Zuleika Fault region is known for significant gold deposits. New findings suggest that the basin is not a simple syncline, rather it comprises two synclines separated by a largely faulted anticline.

The Ida Fault, although largely unmineralised, is a fundamental boundary of the Yilgarn Craton. The tracing of this boundary beneath a younger thrust fault has implications for geodynamics through the structural history and any late-stage gold mineral systems.

The Kalgoorlie terrain's architecture is built on a 'Golden Corridor' that stretches from Kambalda possibly to Plutonic, with locations under thin cover providing new opportunities for exploration (i.e, north of Wiluna).

Drivers and pathways

The question regarding fluid flow drivers and pathways was not answered specifically. The orogenic surge model was proposed to explain a number of tectono-thermal and temporal features of the orogen. The model implied that the whole crust was involved and that deformation in the mid crust (imbrication) may have driven fluids into a structurally pre-prepared architecture.

Fluids, their sources and reservoirs

The question of fluid sources and reservoirs was explored by considering the nature of the strong areas of reflectivity in the seismic data, and whether these regions represented alteration from fluid flow. Numerical modelling determined that there are regions of stagnant fluid in the viscous lower crust. It showedThe following subheadings describe the new findings in each of the five questions. Further details may be found throughout the report, appendices, and the 3D map/GIS.

Metal deposition

One of the most compelling discoveries of the project was the relationship between domes and the location of large mineral deposits. There is an empirical relationship, both globally and in the EYC, between domes and mineralisation. The domes occur at three levels in the crust and are stacked and linked by breaching faults, resulting in an ever increasingly focussed fluid being channelled upwards.

The domes are best mapped by seismic reflection data, however 3D gravity inversions have also successfully mapped them regionally, allowing a rapid and inexpensive method for detecting them.

These domes are superimposed on the Golden Corridor, providing a highly efficient pathway and trap. Fluids are focussed upwards into the apices of the domes, which may act as a reservoir that is tapped on breaching by late faults.

Future directions

The Y2 Project will drive the next phase of the pmd *CRC research in the Yilgarn. - the Y4 Project.

A clear gap in the work of the Y2 and Y3 projects is one of scale. The Y2 project has been constructing 3D maps analogous to 1:250 000 scale and smaller (Norseman-Wiluna), and 1:100 000 scale (Kalgoorlie-Kambalda).

In contrast, the Y3 project has been spatially limited in their work, restricted to small areas of a few deposits. Taking the data and concepts up to the next scale (camp and district) is the next challenge.

The Y4 project will endeavour to achieve this.

The Partners

In collaboration with:

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

Full Report PDF30.2Mb

Appendices

For Further Information Contact:

Project Leader
Dr Richard Blewett
Geoscience Australia
Tel: 02 6249 9713
Email: Richard.Blewett@ga.gov.au

The Executive Summary is Confidential to sponsors until December 2005.

Back to Summaries Menu

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