The Yerrida Basin in the south-eastern Capricorn Orogen, WA is a Proterozoic sedimentary basin that is prospective for base metals. A regional TEMPEST airborne electromagnetics (AEM) survey over the Capricorn Orogen, coinciding magnetotelluric (MT) survey and available downhole electrical property measurements provide an opportunity to compare the electrical conductivity of the rocks, and the electromagnetic (EM) responses of near surface and deeper sedimentary packages of the Yerrida Basin. Of particular interest is assessing whether it is possible to obtain reliable structural and stratigraphic information from, and to what depth, such as is required for exploration of basin terrains. The MT data were modelled using a 3D inversion algorithm, and the AEM data were modelling using a 1D inversion algorithm. An integrated interpretation of the two datasets allows surface geology to be mapped and for a conductivity section to be created to several kilometres depth. Electrical property measurements showed that one of the stratigraphic units, the Johnson Cairn Formation, is anomalously conductive and hence is readily mapped allowing faults, folds, thickness variations etc to be inferred from the inversions. The underlying and resistive Juderina Formation also gives rise to EM responses, albeit less strong. The results suggest significant modification of the published geological map of the area. The study demonstrates the usefulness of EM methods for exploring sedimentary terrains, but this is greatly facilitated by the presence of conductive stratigraphic units. While not always common practice in green fields exploration, it is recommended that electrical petrophysical measurements are made before committing to large scale EM surveys to better constrain interpretations.

Airborne electromagnetic (AEM) survey data are typically inverted with one-dimensional (1D) AEM algorithms because 1D approaches are efficient methods for calculating subsurface conductivity and thickness variations. However, 1D AEM inversion algorithms are known to perform poorly in regions where the geology is 2D or 3D. In 2013, a regional-scale (5 km spaced) TEMPEST AEM survey was acquired across the regolith dominated sedimentary and volcano-sedimentary basin terrain of the Capricorn Orogen in Western Australia. In this contribution, we evaluate the efficacy of 1D inversions of this data to map the resistivity and thickness variations of the basin geology. The limitations of 1D inversions of the Capricorn Orogen AEM data were investigated through the synthetic forward modelling and inversion of two different geological examples: (1) the depth of detection and resolution of a (1 omega m) conductor beneath conductive regolith, and (2) the geometry resolution of a dipping contact comparing both conductor-over-resistor and resistor-over-conductor scenarios. The main findings from this study are: (1) that 1D AEM inversions reliably resolve the depth of a conductor beneath conductive cover where it is located at approximately 100-150 m below cover, and (2) that dipping layers with a strike perpendicular to an AEM survey line can be accurately recovered when their apparent dip angle is less than 20 degrees. These results are used to interpret the resistivity and thickness variations resolved along a 1D GA-LEI TEMPEST AEM inversion from the north-eastern Yerrida Basin of the Capricorn Orogen to show that there are limitations to the level of detail which can be interpreted from 1D AEM inversions, and that there are implications for interpreters using AEM methods to map sedimentary basin sequences and structure in deeply weathered terrains.

SummaryMagnetotellurics (MT) is emerging as a key tool for understanding giant mineral systems. For MT to be effective in mineral exploration we require both an improved understanding of the causes of electrical conductivity anomalies and an increased number of surveys over known mineral deposits.We present results from a new, lithospheric-scale 3D MT dataset collected in the southern Yilgarn Craton, Western Australia. This 250x250 km survey covers the western Eastern Goldfields Super Terrane and the world-class Kambalda nickel and Kalgoorlie gold deposits. We interpret the electrical conductivity models using experimental electrical conductivity data to link the MT results to a mineral systems analysis.

Summary The Capricorn Orogen is located in central Western Australia, and comprises a series of Paleoproterozoic volcanic sedimentary basins that are prospective for base metal mineralisation. Airborne electromagnetic techniques can detect base metal deposits, however due to the thick and conductive regolith, sparse outcrop, and a lack of publicly available drill core and geophysical data, exploration for new base metal deposits is difficult within the Capricorn Orogen. We propose using available petrophysical data from the Abra Pb-Zn deposit to create a synthetic resistivity model to forward model the AEM response over Abra. The calculated AEM responses over different lithologies can be used as a proxy for understanding the ability for widely spaced AEM data to detect altered base metal rocks in regolith dominated terrains.

There is still much that needs to be understood about the physical properties of rocks in mineralised geological environments. This knowledge gap becomes more important as the transition to deeper exploration targets under cover occurs, with an associated greater reliance on geophysical exploration methods. The major challenge associated with understanding petrophysical data is not making the measurement, but rather understanding the results. The interpretation of the data is a cross disciplinary problem. Fundamentally it is necessary to understand the rock mineralogy and geochemistry to put the petrophysics in context with the geophysical results. Several case studies are presented where the petrophysics have determined not only which geophysical techniques to apply but whether a geophysical target has indeed been tested. Drill testing EM plate approximations for nickel sulphide and volcanogenic massive suphide (VHMS) ore deposits can benefit from inductive conductivity measurements on core as it can determine whether an EM conductor has been intersected. Chargeability highs associated with porphyry copper mineralisation is indicative of disseminated pyrite in the propylitic and pyrite +/- chalcopyrite +/-bornite in the potassic alteration zones and higher chargeability does not necessarily mean more copper. In most porphyry systems magnetite is coarse-grained, therefore a world class porphyry deposit should not have dominant remanent effects and the only likely source of remanence features in younger terrains are oxidised mafic intrusions and skarns. Furthermore, porosity and specific types of alteration (argillization) display the strongest correlations with resistivity and can be tied to gold distribution in Carlin Type Deposits.

The cratonisation of Western Australia during the Proterozoic overlapped with several key events in the evolution of Earth. These include global oxidation events and glaciations, as well as the assembly, accretionary growth, and breakup of the supercontinents Columbia and Rodinia, culminating in the assembly of Gondwana. Globally, Proterozoic mineral systems evolved in response to the coupled evolution of the atmosphere, hydrosphere, biosphere and lithosphere. Consequently, mineral deposits form preferentially in certain times, but they also require a favourable tectonic setting. For Western Australia a distinct plate-margin mineralisation trend is associated with Columbia, whereas an intraplate mineralisation trend is associated with Rodinia and Gondwana, each with associated deposit types. We compare the current Proterozoic record of ore deposits in Western Australia to the estimated likelihood of ore-deposit formation. Overall likelihood is estimated with a simple matrix-based approach that considers two components: The "global secular likelihood" and the "tectonic setting likelihood". This comparative study shows that at least for the studied ore-deposit types, deposits within Western Australia developed at times, and in tectonic settings compatible with global databases. Nevertheless, several deposit types are either absent or poorly-represented relative to the overall likelihood models. Insufficient exploration may partly explain this, but a genuine lack of deposits is also suggested for some deposit types. This may relate either to systemic inadequacies that inhibited ore-deposit formation, or to poor preservation. The systematic understanding on the record of Western Australia helps to understand mineralisation processes within Western Australia and its past connections in Columbia, Rodinia and Gondwana and aids to identify regions of high exploration potential.

Summary What have conventionally been seen as ‘academic’ geophysical methods used to study the deep crust are now increasingly finding applications in mineral exploration ( Dentith et al., 2018 ). This is largely due to the adoption of the mineral systems concept, which is seen as a basis for future mineral exploration which will increasingly seek deeper targets and under cover. A mineral system comprises all the geological processes that lead to the creation of a mineral deposit. Conventionally, geophysical exploration has sought to map the near-surface geology and/or detect the mineralisation itself and the immediately surrounding geological environment.

Providing a balance between principles and practice, this state-of-the-art overview of geophysical methods takes readers from the basic physical phenomena, through the acquisition and processing of data, to the creation of geological models of the subsurface and data interpretation to find hidden mineral deposits. Detailed descriptions of all the commonly used geophysical methods are given, including gravity, magnetic, radiometric, electrical, electromagnetic and seismic methods. Each technique is described in a consistent way and without complex mathematics. Emphasising extraction of maximum geological information from geophysical data, the book also explains petrophysics, data modelling and common interpretation pitfalls. Packed with full-colour figures, also available online, the text is supported by selected examples from around the world, including all the major deposit types. Designed for advanced undergraduate and graduate courses in minerals geoscience, this is also a valuable reference for professionals in the mining industry wishing to make greater use of geophysical methods. In 2015, Dentith and Mudge won the ASEG Lindsay Ingall Memorial Award for their combined effort in promoting geophysics to the wider community with the publication of this title.

Using geophysical data for recognising targets for testing and accurately mapping the geology are equally dependent on petrophysics, which constitutes a link between the geologist’s largely mineralogical ‘view’ of the Earth and the geophysicist’s physics-based ‘view’. The availability of portable petrophysical instruments and spectral scanners allow co-located multiple geochemical, mineralogical and physical property measurements and allow larger volumes of petrophysical property data to be collected, and in a better geological context, than has been possible in the past. Many rock physical properties are heterogeneous and a large number of data is required. Accurate interpretation of the data requires analysis of the data as populations and in the context of all of lithology, alteration, stratigraphy and spatial location. This requires close integration of the petrophysical data with the geochemical and mineralogical data. A recommended interpretation workflow will be demonstrated using two examples: ultramafic rocks from greenstone terrains and carbonate successions hosting base-metal mineralisation.

An integrated interpretation of the east Kimberley, northern Western Australia was completed to determine mineral prospectivity, and was centred on a portion of a magnetotelluric (MT) survey conducted across the entire Kimberley Craton and surrounding orogens. A structural geophysical interpretation used potential field data, and was constrained by geological field observations, petrophysics, remote sensing and understanding of the tectonic history of the region. Potential field forward modelling located along the same survey traverse as the MT data allowed comparison between the two datasets and their interpretations revealing interesting features suggesting the presence of large-scale structures, the presence of mineralization deep in the crust, and where mineralization may be at or near the surface. The King River Fault is shown from both the MT inversion and potential field modelling as a crustal-scale, west-dipping structure, the footwall of which bounds the western side of a large resistive body. A conductive anomaly is also located on the hanging wall of the King River Fault. Our assessment suggests that graphitic rocks, most likely with some sulphide content, contribute to the strength of this anomaly, and highlights the potential of the east Kimberley to host graphite and base metal deposits.

Understanding how the Australian continent came together requires an understanding of structure in all levels of the lithosphere. Deep seismic reflection profiles across several Proterozoic orogens have revealed entirely buried tectonic elements, termed seismic provinces. Although undoubtedly important, the nature of these seismic provinces is typically not well characterised. The Capricorn Orogen is one such region, where the upper crust is relatively well known from geological and geophysical studies, but much of the deep crust is buried beneath Proterozoic basins. Here we combine geophysical datasets, including active and passive source seismic data and gravity data, to image the density, seismic velocity and compositional structure of the deep crust of the Capricorn Orogen. Crustal structure interpreted from deep seismic reflection studies is re-scaled using velocity information from receiver function studies. This modified geometry is used to construct a density model that satisfies Bouguer gravity data. Finally, after correcting for temperature and pressure dependencies, the velocity and density information is used to generate a compositional model of the orogen. This model indicates a varied structure with at least four distinct blocks between the Yilgarn and Pilbara cratons, bounded by major shear zones. We suggest that this variation is linked to multiple accretion events during the amalgamation of the West Australian Craton.

Downhole acoustic and optical televiewer images, and formation microimager (FMI) logs are important datasets for structural and geotechnical analyses for the mineral and petroleum industries. Within these data, dipping planar structures appear as sinusoids, often in incomplete form and in abundance. Their detection is a labour intensive and hence expensive task and as such is a significant bottleneck in data processing as companies may have hundreds of kilometres of logs to process each year. We present an image analysis system that harnesses the power of automated image analysis and provides an interactive user interface to support the analysis of televiewer images by users with different objectives. Our algorithm rapidly produces repeatable, objective results. We have embedded it in an interactive workflow to complement geologists' intuition and experience in interpreting data to improve efficiency and assist, rather than replace the geologist. The main contributions include a new image quality assessment technique for highlighting image areas most suited to automated structure detection and for detecting boundaries of geological zones, and a novel sinusoid detection algorithm for detecting and selecting sinusoids with given confidence levels. Further tools are provided to perform rapid analysis of and further detection of structures e.g. as limited to specific orientations.