ASEG Extended Abstracts - 2nd Australasian Exploration Geoscience Conference: Data to Discovery, 2019

This contribution presents a method for efficiently classifying geophysical anomalies and identifying regions and features that share characteristics of many known iron-oxide-copper-gold (IOCG) deposits of the Gawler Craton, and can therefore be used in drill target prioritization. Residual Bouguer gravity and reduced-to-pole total magnetic intensity grids over the Gawler Craton were transformed, generating polygon datasets representing populations of locally anomalous gravity and magnetic intensity. Taken as simple anomaly polygons, there are a very large number of features across the Gawler Craton (>39,000 TMI and >10,000 gravity). Superimposing mineral deposits over these features shows a clear spatial correlation between IOCG deposits and occurrences, and anomalies (>90% of deposits within 1,000 m of an anomaly), but leaves thousands of anomalies of varying magnitudes that cannot all be related to IOCG mineralization. Eliminating TMI and gravity anomalies with a separation of more than 1,000 m reduced the search space to ~20,000 TMI features and ~8,500 gravity features. Limiting the search to a statistically derived gravity threshold ≥0.4 mGal gravity anomalies, the exploration space is reduced to 798 gravity features with coincident TMI features within the Olympic Copper-Gold Province. The Anselin Local Morans I method was used to delineate geographic regions based upon spatial clustering of high magnitude anomalies. The spatial distribution and clustering characteristics of the gravity anomalies provide additional information and can be related to differing basement geology and deposit style. Terranes where lithologies and Cu-Au occurrences are commonly magnetite-rich show clustered high-magnitude gravity anomalies, and correlated spatially with the Mount Woods and Moonta domains within the eastern Gawler Craton. Importantly, it was found that the central, and currently most endowed, the Olympic Domain, was distinct in that it was dominated by spatial outliers (discrete high-magnitude density features).

These results could be used as a starting point in developing IOCG exploration strategies, due to the high number of additional untested, spatially coincident gravity and magnetic anomalies that warrant further investigation.

The blind trial studies conducted for the 2006 3rd International Symposium on the Effects of Surface Geology (Grenoble, France) and the 2015 Inter-comparison of Methods for Site Parameter and Velocity Profile Characterization (InterPACIFIC) Workshop (Turin, Italy) evaluated the utility of microtremor array methods for characterizing seismic site conditions. These studies used a multiplicity of arrays but left an open question as to whether (and under what) conditions might sparse (low-cost) arrays be technically sufficient for the task. Similar questions arise when designing arrays for use in mapping cover thickness or buried high-velocity layers in mineral exploration.

In this study, the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials used microtremor array data from four sites with geology ranging from deep alluvial valleys to an alpine valley. Data were incrementally released to approximately a dozen analysts in four phases: (1) 2-station linear arrays; (2) sparse triangular arrays; (3) complex nested triangular or circular arrays; (4) all available geological control including drillhole data. While data from one site consisted of recordings from 3-component sensors, the other three sites consisted of data from vertical-component sensors only. The sites covered a range of noise source distributions, ranging from one site with a highly directional microtremor wave field, to others with distributed or omni-directional wave fields.

Here, we review the results based on the different processing algorithms (e.g. beam-forming, spatial autocorrelation, seismic interferometry) as applied by the analysts to the incrementally released data, and then compare the effectiveness between the differing wave-field distributions. The results of the study will aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of earthquake hazard site-effects and cover thickness studies in mineral exploration.

Detailed airborne electromagnetic (VTEM), gravity and passive seismic HVSR surveying, followed up by RC and diamond drilling, has identified an isolated circular and geologically young meteorite impact crater filled with sediments and located next to the town of Coolgardie in Western Australia. It was discovered in 2017 (Meyers, 2017), is 800 m in diameter and estimated to be >140 m deep based on passive seismic results, because only angled drilling was carried out for gold exploration along the western side of the crater. The crater has been filled with impact debris and sedimentary deposits to form a flat modern topographic feature. The pre-impact bedrock is Archean greenstone, which hosts numerous gold occurrences and mines surrounding the crater. The crater is filled with a polymict impact breccia of mixed greenstone rocks from below and surrounds near its base. A thin (<2 m) tuffaceous looking ejecta layer occurs at the boundary between the impact breccia and underlying in situ brecciated greenstone bedrock (high Mg basalt and ultramafic lava). This transition zone is also imaged as two “bedrock” layers in passive seismic HVSR cross-sections. The impact breccia is overlain by a very thick deposit of anoxic peat and carbonised wood fragments containing framboidal pyrite, as well as slump blocks of greenstone rocks, starting from over 120 m deep to a depth of 14 m from surface, where this organic layer is capped by a fine clay deposit, which was likely transported into the crater after its raised rim was breached by erosion. The crater and its fill deposits cannot explain the entire circular gravity anomaly low of -5mGal, and the brecciated in situ greenstone bedrock around and below the crater is modelled to contribute to the gravity anomaly low. It is estimated that the meteorite projectile was about 40 m in diameter, came from an easterly trajectory, and impacted Coolgardie between the Miocene to Pleistocene, with age dating and Ir analysis of the impact layer and overlying plant material soon to be carried out. A high-resolution ground magnetic survey using 10 m line spacing was carried out to try and detect large iron meteorite fragments, but only a weak and diffuse anomaly pattern occurs at the crater centre, indicating that the meteorite was highly fragmented on impact and/or later demagnetised by weathering. A vertical diamond drillhole down the centre of the crater is required to properly study this relatively young impact structure and analyse the preserved plant material filling the crater over a vertical column of 100 m or more, continuously recording local environmental changes over a considerable time period.

A high-resolution seismic survey was carried out across the metropolitan reach of the Swan River (Perth, Western Australia) to investigate its Late Quaternary sub-surficial geomorphology. Shallow imaging data, integrated with sediment cores, pre-existing literature (including dating) and LiDAR images, revealed three main units, forming a complex system of buried paleochannels, which developed during the Late Quaternary glacial sea level lowstands, and infilled during interglacial highstands.

The deepest unit was interpreted as comprising estuarine to fluvial sediments of the Perth Formation, deposited during the Last Interglacial (~130-80 thousand years before present) in a wide paleo-valley that cut the basement.

The sedimentary sequence of the overlaying middle unit belongs to the Swan River Formation, which consists of heterogenic fluvial to lacustrine sediments, deposited during the Last Glacial lowstand (~80-18 thousand years before present).

The shallowest unit comprises Holocene fluvial and estuarine sediments, up to ten-thousand-year-old.

This research represents the first environmental high-resolution acoustic investigation of the Swan River estuary. The findings have improved the understanding of the Late Quaternary Swan River development, providing a useful tool for modelling river onset and evolution, following sea level transgressions.

The challenges in drilling a high pressure and high temperature (HPHT) well are unique to every opportunity. The key to successfully drilling an HPHT well is to utilise the appropriate people, technology and design at the appropriate time. To do this it is critical to know the questions that need solving. Knowing and answering these questions cannot be effectively achieved without accessing subject matter experts in several disciplines including drilling, petrophysics and data acquisition and integrating into a coherent plan.

This paper discusses the internal field and gradient tensor components for a permeable ellipsoidal body in a magnetic medium with an applied non-uniform magnetic field. The formulae can also be applied to the field and gradients measured in an ellipsoidal cavity within a magnetic medium. A long, narrow cylindrical cavity, representing a borehole that penetrates a magnetic body, is a particularly useful special case. I give explicit expressions for the internal gradient tensor within a permeable triaxial ellipsoid placed in a uniform gradient. Expressions are given for the shielding of applied gradients of all orders for the special case of a spherical body and for the amplification of applied gradients of all orders in a spherical cavity within a magnetic medium.

Magnetic sensors cannot measure the internal field and gradient components of a magnetic body directly, as the measuring instrument must be placed within a cavity, such as a borehole, within the magnetic material. This modifies the measured field and gradients. I give expressions for correcting measured gradient tensor components in a borehole within a magnetic body.

Cover thickness estimation is critical to mineral exploration effectiveness in covered terrains. Geophysical methods are able to detect physical properties contrasts in different earth materials without seeing them. We present the application of the audio-magnetotelluric (AMT) technique to cover thickness estimation using deterministic and stochastic inversion approaches. The deterministic Occam’s inversion method solves the regularised problem by searching for the smoothest model that fits the data within certain tolerances. The stochastic algorithm uses trans-dimensional Markov chain Monte Carlo techniques to generate an ensemble of millions of conductivity-depth models that adequately fit the data given the assigned noise levels. Statistics are derived from the posterior probability distribution of the conductivity at depth. This approach gives more pronounced layer boundaries that allows more straightforward interpretation of resistivity structure.

We have applied the AMT method to cover thickness estimation for two regional drilling projects, i.e. Southern Thomson Orogen and Coompana Province. The application of the method was validated using drillhole results. Cover thickness estimates compared favourably with drillhole results, demonstrating that the method is a valuable tool to an Explorers’ Toolkit of techniques to reduce the risk in searching for new mineral deposits in covered terrains.

Triassic deposition on the central North West Shelf (CNWS) mainly took place in a large depocentre comprising the Beagle Sub-basin and the Exmouth Plateau, and a small depocentre located within the Rowley Sub-basin. Thermal subsidence prevailed but syn-depositional extension is especially evident in the NE Barcoo Sub-basin. Sources of sediment delivery to the region included two re-entrants that extended into the Canning Basin. During time Interval 1 (Induan–mid Anisian) a basal marine mudstone transgressed the region and fluvio-deltaic systems were confined inboard. Local deep incision at the start of Interval 2 (mid Anisian–mid-Landinian) was followed by extensive fluvio-deltaic deposition. During Intervals 3 (mid-Ladinian–early Rhaetian) and 4 (Rhaetian), the fluvio-deltaic system progressively back-stepped and was replaced in outboard areas by deposition of carbonate-rich platform sediments. The discrete Rowley Sub-basin depocentre and associated Steel Dragon Structure may be igneous complexes generated by rifting processes on the adjacent Argo landmass. Palaeogeographic maps that track the evolution of the CNWS inform aspects of Triassic prospectivity, such as the geographic extent of the new fluvio-deltaic petroleum system in the Bedout Sub-basin, the distribution of a carbonate build-up play, and the significance of syn-depositional extension for source rock accumulation.

In this study we examine the effect of the boundary conditions on the laboratory measurements of the elastic properties of a fluid-saturated sedimentary rock at low frequencies. In laboratory experiments associated with studying fluid effects on elastic properties of a porous rock sample, the tested sample cannot be completely sealed due to the presence of the fluid lines connected to its pore space. These lines form a pore-fluid storage which can affect the results of the elastic moduli measurements of fluid-saturated rocks. We developed a modified version of the Gassmann model which can estimate the bulk moduli of fully saturated rocks in dependence on the capacity of the pore-fluid storage. Here, we compare the predictions of the modified Gassmann model with the moduli measured on an n-decane-saturated limestone sample with the volume of the pore-fluid storage changing from 2 ml to 260 ml. The experimental results were obtained using a low-frequency apparatus based on the forced-oscillation method at a frequency of 0.1 Hz. We demonstrate that the predictions of the modified Gassmann model are in good agreement with the experimental data.

Chevron has developed a method for merging, viewing, and interpreting multiple 3D seismic surveys in stratal slice domain. The products of this method are referred to as Regional Stratal Slice Volumes (RSSVs). Within the Northern Carnarvon Basin (NCB), NWS, Australia, local RSSVs have been built using a patchwork of 3D seismic data sets of various vintages and data quality covering an area of approximately 68,000 km². RSSVs have been built within the Triassic post-rift and Jurassic-Cretaceous syn-rift and post rift intervals. The Triassic section is comprised of a 4 plus km-thick interval of alluvial to shallow-marine deposits of the Late Triassic Mungaroo /Brigadier Formations and Early Jurassic North Rankin Formation, whereas the younger Jurassic-Cretaceous succession is comprised of a tectonically influenced Jurassic interval overlain by syn-rift to post rift Barrow Group.

Proprietary and vendor stratal slice imaging was performed in each 3D survey, spliced together, correlated with adjacent surveys, and combined with supporting structural, cultural and well-based data. This has created a temporal series of unbroken, regionally-extensive, chrono-stratigraphic seismic images which, when viewed successively, capture the evolving geomorphology and palaeogeography of the NCB.

RSSVs provide a predictive stratigraphic framework for the NCB. Triassic and Jurassic-Cretaceous RSSV analysis has resulted in an enhanced understanding of changing depositional environments at a regional scale providing insights into the regional distribution of intra-formational seals, presence of potential source rocks, and the distribution of fluvial, shoreline, and marine deposits. This work has been used to support local prospect and field scale evaluations and has influenced Chevron’s acreage position and strategy in the NCB.

Soils are widely used as a geochemical sample medium. These samples are sieved to <250 μm or <180 μm to remove larger gravels and coarse sand and later pulverized for analysis, but no other refinement is commonly used. Why is this the case? In transported cover, the soils that host/adsorb the mobile element signature are the smallest size fractions, so we should consider concentrating the clay size fraction (<2 μm) as an improved sample medium. Industry is unlikely to fully adopt these changes without proof of the benefit of refining the size fraction so a series of experiments were conducted to demonstrate the value of using <2 μm fractions for exploration geochemistry. Twenty-seven composite reference soils were collected in the vicinity of known mineral deposits (including background areas) that reflect the common soil types of Western Australia. These soils were used in replicate testing to assess differences in particle size, sample weight, dispersants and how this relates to the geochemistry with intent to optimise the extraction and speed of ultra-fine fraction recovery. Following the refinement of the technique, we commercialised and then applied the new UltraFine+ workflow to numerous orientation site studies, including reprocessed archived regional soil samples from the Geological Survey of Western Australia. Orientation results were promising. Analysing fine fractions (<2 μm) generated reproducible, reliable results, with bigger concentrations. Key benefits were the removal of nugget effects (for Au) and the challenges with detection limits in materials that are dominantly quartz sand. Importantly, the study revealed a marked decrease in censored results for Au (63% to 10% below detection limit) and less variation with sampling depth in soil profiles. The UltraFine+ workflow demonstrates the additional value from (re-) assaying regional soil and sediment samples to generate new targets and improve regional geochemical maps.

SPECTREM AIR has been successfully acquiring data in Australian terrain since 2017 and has shown substantially better depth of penetration as well as increased sensitivity in shallow sounding with the SPECTREM^PLUS as compared to earlier systems. The higher-powered transmitter (amongst other improvements) has shown significant improvements in signal to noise ratio of the system.

The combination of higher transmitted power, wide system bandwidth and advanced data processing gives SPECTREM^PLUS the unique advantage of being able to map at depths of up to 600m (dependent on overburden conductivity), without compromising the simultaneous resolution of shallow features, thus making the system an ideal tool for exploring under cover in Australian terrain where difficult geological settings are present.

The application of recent technological advances to the acquisition of petrophysical datasets has significantly increased the amount of quality, systematic physical rock property measurements that can be captured efficiently and cost effectively. These large datasets provide a more comprehensive understanding of petrophysical values and, as importantly, their statistical distribution – particularly when encompassing a diverse set of rock types and stratigraphy. To date, these types of comprehensive datasets are limited in the mineral exploration space, in both number and their spatial distribution.

This paper presents a case study from the Stavely Project area in western Victoria outlining the workflow from acquisition, compilation and analysis, to the application of a large, pre-competitive petrophysical dataset. These data, including gamma density, magnetic susceptibility, P-wave velocity, electrical resistivity and natural gamma, were acquired by scanning of diamond core from 21 stratigraphic and mineral exploration drill holes by Multi-Sensor Core Logger equipment, providing a combined final dataset of 216,699 petrophysical values. The precompetitive data and subsequent analyses are available for download.

In developing a statistical analysis of the petrophysical values, the datasets were segregated into stratigraphic, lithological and spatial sub-populations, therefore giving a geological context for the data, and providing baseline values for regional geophysical interpretations, forward modelling and inversions, and the identification of regions of anomalous petrophysical values potentially associated with hydrothermal mineralisation. Crosscorrelation of various petrophysical properties highlights important relationships within the data, thus maximising its value. In western Victoria these data have been important in the delineation of prospective volcanic belts of the Stavely Arc beneath cover, and the development of a 3D model displaying the regional geometries and distributions of the belts at depth.

In this study a mafic underplate along the margin of the Proterozoic east Albany–Fraser Orogen and Archean Yilgarn Craton is imaged using 3D gravity forward modelling. The 3D model was constructed in GeoModeller and tested by 3D gravity forward modelling, an approach selected due to the distinct, regional-scale gravity anomalies observed along the margin. Important constraints on the model include an interpreted bedrock geology map, Moho depth points from a passive seismic survey and three deep crustal reflection seismic lines. The Moho model shows a zone of thickened crust extending, parallel to the Albany-Fraser Orogen, along the length of the orogen. 3D gravity forward modelling demonstrates that dense material is required in this zone of thickened crust and also demonstrates that this dense material is most likely in the lower crust, coincident with a large non-reflective zone imaged in reflection seismic data. This zone is interpreted to represent a mafic underplate that formed in the lower crust of the Yilgarn Craton during the Proterozoic. Some of the possible tectonic settings for the emplacement of this voluminous mafic underplate include Paleo-Mesoproterozoic extension that occurred along the margin or during the Mesoproterozoic Albany–Fraser Orogeny.

This study uses shallow passive seismic HVSR (horizontal-to-vertical spectral ratio) technique to determine the depth and extent of a Cenozoic paleochannel composed dominantly of sand and clay incised into the Proterozoic granitic basement of the Capricorn Orogen. The paleochannel contains sand-dominated intervals that host water that is presently being explored by Hastings Metals. There is some drilling data available but only a few drill cores intersect the basement. Improved understanding of the paleochannel geometry will assist with water exploration.

The measured resonant frequency is related to shear wave velocity (Vs) and layer thickness. Passive seismic measurements at drill hole SWMB007 allow us to define a Vs for the regolith package overlying the basement. This Vs value is applied to 53 passive seismic measurements along Traverse 7 and the thickness of the paleochannel has been imaged in normalised H/V amplitude images.

Along Traverse 7, HVSR data image a symmetrical paleochannel with a maximum depth of ~115 m. The geometry of the paleochannel imaged is broadly similar to the geometry obtained from 2.5D AEM inversion. However, the paleochannel has a greater maximum depth in AEM and some internal features of the paleochannel also differ.

The Nova-Bollinger Ni-Cu sulfide ore deposits are hosted within a suite of mafic-ultramafic intrusive cumulate bodies. The Lower Intrusion is a thin semi-conformable chonolith comprising unlayered mafic to ultramafic orthocumulates, hosting the bulk of the mineralisation. The much thicker overlying Upper Intrusion is bowl-shaped and modally layered with alternating peridotite and norite mesocumulate layers. A distinctive rock-type containing both orthopyroxene and olivine as cumulus phases is a characteristic of the Lower Intrusion. The intrusions differ in olivine and spinel chemistry, and in the volume of sulfides. Sector zoning in Cr content of pyroxenes is observed in the Lower Intrusion, and in the lower marginal zone of the Upper, and is attributed to crystallisation under supercooled conditions. Symplectite orthopyroxene-spinel-amphibole coronas at olivine-plagioclase contacts are attributed to near-solidus peritectic reaction between olivine, plagioclase and liquid during high pressure emplacement (>6 Kb), consistent with high Al contents in igneous pyroxenes and estimates of the peak regional metamorphism. The Upper and Lower Intrusion rocks represent cumulates from a similar parental magma, derived via multiple magma pulses, variably fractionated and undergoing sulfide saturation prior to emplacement into the deep crust at pressures of 6-10 Kb during the peak of regional metamorphism under extremely low cooling rates. The ores show a remarkable assemblage of textures indicative of emplacement into hot, soft country rocks at a large-scale melting-infiltration front. Sulfide infiltration was accompanied by partial melting of the country rock producing felsic leucosomes, some of them strongly enriched in garnet, showing close spatial association with sulfide infiltrations and veins. Coarse grained pentlandite – chalcopyrite – pyrrhotite “loop textures” are characteristic of all ore types, down to the scale of the infiltrating sulfides within the gneisses, and are regarded to be diagnostically magmatic textures generated by sulfide liquid fractionation and growth of high-T pentlandite by peritectic reaction between fractionated sulfide melt and early crystallised MSS.

High resolution borehole images were used to study the internal architecture of mass transport deposits (MTD units) in a well drilled in offshore deepwater Borneo. The main objective was to utilize information from bedding orientations within MTDs to infer the direction of sediment transport which can in turn imply the distribution of reservoir facies. In this work, we first subdivided the deepwater package into turbidite and MTD deposits. Then, we utilized the dip attributes of bedding features in the deposits interpreted as moderately deformed slumps to identify the folding within the MTD packages. We utilized the geometry of these folds to infer the paleoslope trend which is NE-SW with overall MTD transport direction almost perpendicular to that, i.e. northeast. This trend direction corresponds to fairways along which reservoir facies sands have been transported and deposited in deepwater depocenters. Therefore, using this approach, we can provide vital clues to one of the most persistent exploration challenges in the energy industry, which is to identify main sediment transport directions in deepwater environments.

A mine development in the high Andes of north-western Argentina requires a supply of fresh water for various uses, the largest being mineral processing. Complementary survey techniques in a gravel basin have provided vastly different and sometimes contradictory results. Mapping of the basin geometry proceeded via downhole resistivity logging, moving loop ground EM, and galvanic resistivity. The expected layering of low resistivity gravel fill over high resistivity basement rock, which was demonstrated by prior downhole logging, was turned on its head by surface electrical and EM surveys. Furthermore, subsequent borehole resistivity logs, which do not reach bedrock, support the increase of conductivity with depth, while water sampling indicates this is not due to greater salinity. Surface nuclear magnetic resonance was trialled for the direct detection of the upper part of the water table. The majority of these data were contaminated by mysterious noise sources in an area hundreds of kilometres from the nearest atmospheric activity, and fifty kilometres from the closest settlement. False negative readings appear to be common. The most recent drilling, demonstrating much deeper gravel than previously drilled or imagined, may explain an inability to image the bottom, but the reason for the deep conductivity remains a mystery.

The Chrysos PhotonAssay™ Max is an instrument utilising high-energy X-rays but reimaged for the modern world with full robotics and ability to keep up with the high demands of mining. Since Pierre Gy’s introduction of sampling theory into the gold mining industry, many researchers have struggled with limitation of the available techniques, like fire assay and cyanide leach, and their impact on the fundamental errors in relating the assay value to the drill hole or deposit. Gold provides some unique sampling challenges over other commodity elements as it is malleable and is difficult to homogenise by pulverisation.

By analysing a sample size 10 times that of a traditional fire assay, the sample variance is often decreased providing immediate benefit to the geologists, as predicted by sampling theory. By reducing the sample preparation down to the 3 mm crush, there is also an immediate benefit in cost and time savings. Statistical tests show that the analysis on the 3 mm product size by PhotonAssay™ is similar to the traditional analysis by fire assay, suggesting the two techniques can be used interchangeably during a drilling program. This paper describes some of the tests we have done to give the industry confidence that this technique provides immediate benefit and can be used to compare to historical fire assay data.