BCGS Technical Talk – February 28, 2018
Speaker: Mike Dentith, Centre for Exploration Targeting, The University of Western Australia
Title: The Implications of the Mineral Systems Concept for Geophysical Exploration: A Perspective
Date/Time: Wednesday, February 28, 2018 @ 5:00pm PST
Location: 4th Floor Conference Room, Room 451, 409 Granville St. (UK Building at Granville and Hastings), Vancouver
About the Author:
Prof Mike Dentith is Professor of Geophysics at The University of Western Australia and a research theme leader in the Centre for Exploration Targeting (CET). He has 25 years experience researching, teaching and consulting in mineral exploration geophysics. He is editor of two case study ‘geophysical signatures’ publications on Australian mineral deposits and co-author of the textbook ‘Mineral Exploration Geophysics for Geoscientists’ published by Cambridge University Press.
Mike Dentith, Centre for Exploration Targeting, The University of Western Australia
A mineral system is defined as “… all geological factors that control the generation and preservation of mineral deposits”. Most authors describe a mineral system as comprising a source of metals and/or ligands, a pathway along which fluids transport the metals to a location where they are concentrated (physical trap) and precipitated (chemical trap).
The mineral system concept has two main implications for geophysical exploration practices: the definition of additional types of targets at the district and larger scale (metal source region, fluid flow path, fluid reservoir), and the need to provide information over the full extent of the mineral system, i.e. larger areas and in particular to greater depths than is currently normal exploration practice. At the same time the mineral system concept draws attention to the need for a better understanding of the petrophysical consequences of fluid-related alteration.
Where metal-bearing fluids are sourced in the lower crust or mantle it is possible that the processes that create the fluids, or the preferential removal of certain components of the rocks, cause changes to the physical properties of the rocks. For example, depleted mantle may be different from primitive mantle, as may be mantle that has been re-fertilised via metasomatism. Targeting the fluid flow path(s) is also a possibility, but may critically depend on the nature of the fluid flow. If fluid flow is concentrated along a relatively small number of major faults then these comprise very difficult targets given they are expected to be relatively narrow and at significant depths. Since they are expected to be shallower, the fluid flow pathways post-deposition of metals are a potential target. If fluid flow is distributed (which may equate to focussed flow at a scale that cannot be resolved) in the lower crust then the associated alteration is another possible target. However, the most attractive target which emerges from the mineral systems concept, as described by McCuaig and Hronsky (2014), is the postulated reservoir that contains high pressure metal-bearing fluid which is subsequently rapidly emptied causing concentrated fluid flow and metal deposition. With dimensions measured in kilometres at depths of a few kilometres these are much more attractive targets than fluid flow paths. Such reservoirs are potential targets at a camp/district scale that are needed to fill the ‘gap’ between prospect-scale targets (mineralisation, alteration) and regional/terrane-scale targets (major linears, suture zones).
Successful identification of the kinds of targets described above requires the petrophysical properties of alteration to be understood. As noted by Witherly (2014) there is little known about this. An exception is serpentinisation. The available data, nearly all collected as part of academic studies of ophiolites and ocean crust, demonstrates how important a process this is. The difference in density and magnetic properties between fresh mafic/ultramafic rocks and fully serpentinised equivalents encompasses the entire range of the common rock types. Serpentinisation also affects electrical properties, albeit probably to a lesser degree, and potentially also electrical polarisation (due to the creation of magnetite) and dielectric properties due to the involvement of water in serpentinisation reactions. The petrophysical consequences of other forms of common deposit-related alteration are virtually unstudied in a systematic fashion, although it is intuitively obvious that processes such as silicification will significantly affect electrical properties and talc-carbonate alteration will significantly affect density.
The large size and depth extent of most proposed mineral systems, compared to that of a mineral deposit, requires deep penetrating geophysical methods to detect features of exploration significance. This has led to increased numbers of deep penetrating geophysical surveys such as magnetotelluric (MT) and passive seismic surveys, often funded by Government’s seeking to encourage exploration within their jurisdictions. Deep seismic reflection surveys are too expensive to be widely used in this way. MT surveys have the great advantage of being comparatively cheap and with the widespread availability of 3D inversion codes and the super computers required to run them the resulting sub-surface conductivity distributions are much more ‘interpretable’ due to reduction in artefacts and more accurate representation of actual source geometries. The weakness of the method remains its poor resolution and the very limited understanding of causes of conductivity variations in the deep crust and to a lesser extent the mantle. Passive seismic methods, i.e. those that use natural sources of seismic energy, are also comparatively cheap but require deployment of equipment for periods of months to a few years. Ambient noise and teleseismic body-wave tomography can map major crustal and mantle boundaries. The final product is a data volume, usually of seismic wave speed, and these can make a useful contribution to minerals system analysis at the largest scales. Another potentially significant development in passive seismic techniques is in receiver-function based methods. Traditionally used to determine crustal thickness and Moho character (sharp, diffuse) the development of common-conversion-point based processing has allowed closely spaced (few kms) recordings to be combined with the resulting product resembling a low-resolution seismic reflection section.
Deep seismic and MT surveys can in principle, detect broad zones of alteration associated with metal-bearing fluid sources, pathways and reservoirs indicative of the presence of a mineral system. A programme of trial surveys, comprising MT, receiver function and wide-angle seismic surveys across selected deposit camps/mineralised terrains and also unmineralised areas in Western Australia is on-going. A parallel line of research aims to better understand the petrophysical consequences of fluid-related alteration processes and hence predict the geophysical responses of the various components of a mineral system.