Three-dimensional magnetotelluric imaging of lithospheric-scale mineral systems from source to deposit
An ARC funded Linkage project - LP0774891
Scott Bilben and Barry De Wet (BHP Billiton)
Lisa Vella and Mike Cawood (Teck Cominco)
Many large mineral deposits are intimately associated with crust (and possibly upper mantle) penetrating structures that either provide a direct link between deep source regions and the near surface, or allow for the advection of magma, heat and fluids that drive near surface mineralising systems. This is particularly true of magmatic Ni-Cu deposits (eg. Norilsk, Voisey’s Bay), porphyry Cu-Au-Mo (eg. Chuquicamata) and Iron oxide Cu-Au-U (eg. Olympic Dam) where there is direct evidence of the involvement of mantle derived magmas and/or fluids. However, the nature of the lithosphere-scale mineral systems that are required to produce such deposits can generally only be inferred because there is a limited capability to view them in three dimensions. Surface geology is inherently two dimensional in the horizontal plane, and most geophysical surveys are conducted along transects, providing limited depth information but not the full, three-dimensional volumetric information necessary to develop a detailed understanding of the spatial aspects of mineralizing systems. Magnetotellurics has been shown to image crustal conductive volumes associated with mineral deposits at crustal and possibly mantle depths. The research questions addressed by this project are:
- Are these conductive volumes a predictive generic link?
- If so, can MT be used to assist discovery of large ore deposits?
- What does a lithospheric-scale mineral system look like in three
dimensions and where are the ore bodies in this system?
- How is the mineral system influenced by major structures and
This project will produce a lithospheric-scale, three-dimensional image of the ~1590 million year old mineral system responsible for formation of the giant Olympic Dam deposit, located in the eastern Gawler Craton of South Australia. Using 3D magnetotellurics the electrical resistivity structure of the eastern Gawler Craton will be mapped. This will provide important constraints on lithospheric architecture (eg. location of major faults and crustal boundaries) and the geometry of ancient fluid pathways (which appear conductive due to the presence of graphite). This will be the first time that a mineral system has been imaged in 3D from the upper mantle to the surface and will have far-reaching implications for mineral exploration.
This project commenced in June 2007 with the appointment of Doctor Katherine Selway. Data was collected between August and September, 2007 from 28 stations from the Carrapateena area (8 stations using broadband instruments and 20 stations using long-period instruments). These data are being processed at present and will be combined with reprocessed data from previously collected surveys with the aim of producing preliminary inversions in early 2008.
This project is a collaboration between The University of Adelaide, BHP Billiton and Teck Cominco Australia. Fieldwork will be conducted in collaboration with, and will rely on logistic support from BHP Billiton and Teck Cominco - including use of their safety network, field offices and logistic support. BHP Billiton and Teck Cominco will also provide proprietary data including detailed geophysical surveys (magnetics, gravity, electro-magnetics, induced polarization), petrophysical data, drill hole logs and access to drill core. These data would not normally be available to the research community and are a major primary resource for this project. The detailed geophysical surveys represent complementary datasets to MT and in combination will provide the most comprehensive constraints on the lithospheric architecture of the eastern Gawler Craton. Access to drill core and petrophysical data will be a vital part of the APA (I) project, which will attempt to constrain the source of the conductivity/resistivity anomalies in the Stuart Shelf crust. The estimated dollar value of these data to the project (conservatively) at $300,000. BHP Billiton and Teck Cominco personnel will bring to the project their expertise in the processing and interpretation of these data.
BHP Billiton and Teck Cominco have each committed $40,000 per annum for the three-year duration of the project and the ARC funding brings the total budget of the project to $591,000.