Crustal Architecture Beneath Olympic Dam from Magnetotelluric Sounding
An ARC Linkage grant 'Developing a Tectonic Framework for the Gawler Craton' subproject.
Project Investigators
Rob Gill (PhD Candidate)
Associate Professor Graham Heinson
Project Collaborators
Peter Milligan (Geoscience Australia)
Antony White (Flinders University)
Project details
The magnetotelluric (MT) method involves measurement of natural-sources of time-varying magnetic fields that penetrate the earth, and induce electric with secondary magnetic fields. Observations of such fluctuations in horizontal and vertical components of magnetic field and corresponding changes in horizontal and orthogonal electric fields provide constraint on both lateral and vertical changes in electrical resistivity. By measuring MT signals over a bandwidth of periods of 10 to 10 4 s, structural information on resistivity on scale lengths of 5-50 km are obtained.
Two field campaigns in October 2003 and April 2004 using 58 new MT sites produced interesting results. Most sites were located adjacent the Woomera - Roxby Downs road and the Borefield Road, with a site spacing of between 5 and 10 km over 220 km. Additional sites were located towards Andamooka. Impedance sensor data were rotated to orientation of dominant geoelectric strike, approximately 60ยบ west of geomagnetic north, and were generally found to be predominantly 2D. Both the transverse electric (electric field parallel to strike) and transverse magnetic (electric field perpendicular to strike) modes of the MT responses were inverted using the 2D inversion approach of Rodi and Mackie 'Nonlinear conjugate gradients algorithm for 2-D magnetotelluric inversion' Geophysics, vol 66, pp 174-187.
The inversion shows four notable features:
Sedimentary cover sequences have low resistivity (< 20 ohm- m) and show a thickening of Adelaidean sediments to the north. Over Olympic Dam, the sedimentary sequences thin, in agreement with the seismic section.
The Andamooka Fault is imaged as a northwards dipping crustal boundary that separates a region of high resistivity of >1000 ohm- m to the south, with the a more conductive crust to the north (typically <500 ohm- m).
To the north of Olympic Dam, the upper-middle crust to about 20 km is quite resistive (~1000 ohm- m), but the lower crust is much more conductive (<100 ohm- m). A region of low resistivity appears to extend upwards beneath Olympic Dam, possibly due to mineral phases (graphite?) along ductile zones and fluid pathways.
To the north of Olympic Dam, the middle and lower crust appear uniform. In the upper few kilometres beneath Olympic Dam, we image a resistive region that is presumably the batholith. The more conductive region in the mid-lower crust below the batholith is coincident with a seismically transparent region.
Publications
Heinson G. S., Direen N. G., & Gill R. M. 2006. Magnetotelluric evidence for a deep-crustal mineralising system beneath the giant Olympic Dam Iron-Oxide Copper Gold deposit, southern Australia. Geology vol. 34, no. 7, pp. 573-776.
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