Regional Geophysical Datasets Research Articles

A revised Proterozoic tectono-stratigraphy of the South Nicholson region, Northern Territory, Australia—insights from SHRIMP U–Pb detrital zircon geochronology

New SHRIMP U–Pb detrital zircon geochronology on Mesoproterozoic and Paleoproterozoic siliciclastic rocks from the South Nicholson region, in concert with recently acquired complementary regional geophysical datasets, have enabled comprehensive revision of the regional Proterozoic tectono-stratigraphy. The identification of analogous detrital zircon spectra of units deposited in half-graben hanging walls of major east-northeast-trending extensional faults, offers compelling evidence for regional tectono-stratigraphic correlation. This study sampled units from the hanging walls of the Benmara, Bauhinia, and Maloney-Mitchiebo (east-northeast-trending) faults, which identified immature proximal lithofacies hosting a small yet persistent population of ca 1660–1640 Ma aged zircon and lack Mesoproterozoic detritus. The timing of this event is consistent with deposition coincident with crustal extension during the extensional River Event at ca 1640 Ma, an event previously identified from the Lawn Hill Platform in western Queensland. This finding suggests the hanging wall sequences are chrono-stratigraphically equivalent to the highly prospective sedimentary rocks of the Isa Superbasin, host to world-class sediment-hosted base metals deposits across western Queensland and northeastern Northern Territory. Subsequent inversion of the extensional faults resulted in development of south-verging thrusts, and exhumation of late Paleoproterozoic hanging wall siliciclastic rocks through overlying Mesoproterozoic South Nicholson Group rocks as fault propagated roll-over anticlines. These geochronology data and interpretations necessitate revision of the existing stratigraphy and the renaming of a number of stratigraphic units in the South Nicholson region. Accordingly, the distribution of the highly prospective late Paleoproterozoic units of the McArthur Basin, Lawn Hill Platform and Mount Isa Province is greatly expanded across the South Nicholson region. These findings imply that the underexplored South Nicholson region is a highly prospective greenfield area for energy and mineral resources. KEY POINTS SHRIMP U–Pb detrital zircon geochronology provides a basis for revision of the Proterozoic stratigraphy in the South Nicholson region. Correlation of sedimentary units across the region is based on closely matching zircon age spectra, lithofacies and tectono-sedimentary setting. Revision of the Proterozoic stratigraphy increases the prospectivity for sediment-hosted base metal resources across the South Nicholson region.

Probabilistic Assessment of Antarctic Thermomechanical Structure: Impacts on Ice Sheet Stability

AbstractUncertainty in present‐day glacial isostatic adjustment (GIA) rates represents at least 44% of the total gravity‐based ice mass balance signal over Antarctica. Meanwhile, physical couplings between solid Earth, sea level and ice dynamics enhance the dependency of the spatiotemporally varying GIA signal on three‐dimensional variations in mantle rheology. Improved knowledge of thermomechanical mantle structure is therefore required to refine estimates of current and projected ice mass balance. Here, we present a Bayesian inverse method for self‐consistently mapping shear‐wave velocities from high‐resolution adjoint tomography into thermomechanical structure using calibrated parameterisations of anelasticity at seismic frequency. We constrain the model using regional geophysical data sets containing information on upper mantle temperature, attenuation and viscosity structure. Our treatment allows formal quantification of parameter covariances, and naturally permits propagation of material parameter uncertainties into thermomechanical structure estimates. We find that uncertainty in steady‐state viscosity structure at 150 km depth can be reduced by 4–5 orders of magnitude compared with a forward‐modeling approach neglecting covariance between viscoelastic parameters. By accounting for the dependence of apparent viscosity on loading timescale, we find good agreement between our estimates of mantle viscosity beneath West Antarctica, and those derived from satellite GPS. Direct access to temperature structure allows us to estimate lateral variations in lithosphere‐asthenosphere boundary (LAB) depth, geothermal heat flow (GHF), and associated uncertainties. We find evidence for shallow LAB depths (63 ± 13 km), and high GHF (76 ± 7 mW m−2) beneath West Antarctica that, combined with low asthenospheric viscosities, indicate a highly dynamic response to ice mass loss.

Wedge tectonics in South China: constraints from new seismic data

Collisional orogens form when tectonic forces amalgamte fragments of Earth’s continental lithosphere. The sutures between individual fragments, or terranes, are potential sites of weakness that facilitate subsequent continental breakup. Therefore, the lithospheric architecture of collisional orogens provides key information for evaluating the long-term evolution of the continental interior: for example, the South China Block (SCB), where the tectonic history is severely obscured by extensive surface deformation, magmatism, and metamorphism. Using new passive-source seismic models, we show a contrasting seismic architecture across the SCB, with three prominent crustal dipping structures across the Jiangnan Orogen. Combined with constraints from multi-disciplinary regional geophysical datasets, these pronounced dipping patterns are interpreted as relict wedge-like lithospheric deformation zones initiated in the fossil collisions that assembled the Yangtze Block and the SCB. The overall trend of these tectonic wedges implies successive crustal growth along paleo-continental margins and is indicative of northward subduction and docking of accretional terranes. In contrast, no such dipping structures are preserved in the Cathaysia Block, indicating a weak and reorganized lithosphere. The variations in the deformation responses across the SCB reflect the long-term modifications of the lithosphere caused by prolonged collision and extension events throughout the tectonic history of the SCB. Our results demonstrate the critical roles that suture zones played in the successive growth and evolution of the continental lithosphere.

Integrating a Minerals Systems Approach with Machine Learning: A Case Study of ‘Modern Minerals Exploration’ in the Mt Woods Inlier – northern Gawler Craton, South Australia

SummaryThere has been extensive exploration in the Mt Woods inlier in the northern Gawler Craton, South Australia since the 1960s, particularly for iron oxide-copper-gold (IOCG) mineralisation since the 1975 super giant Olympic Dam IOCG-U discovery by Western Mining Corporation ~150 km to the SE.IOCG discoveries in the Gawler Craton have been largely made based on targeted discrete magnetic and/or gravity anomalies since the advent of regional geophysical datasets. Only one significant economic discovery has been made in the Mt Woods inlier – the Prominent Hill metasediment-hosted IOCG deposit in 2001 by Minotaur Resources, with no subsequent economic mineralisation encountered elsewhere in the inlier.In an effort to accelerate the discovery of a new economic deposit, OZ Minerals opened up more than 2TB of their private geoscience data to external participants as part of the ‘Explorer Challenge’ competition in early 2019. Once registered, participants had 3 months to use the data provided, along with what was already in the public domain, to attempt to predict where the next mineral deposit in the Mt Woods inlier might be.Integrating a minerals systems approach with machine learning (ML) has allowed the development of a series of conceptual models for targeting a range of deposit types and commodities. Exploration success based on this modern targeting approach will eventually lead to new suitable exploration strategies to discover the next generation of economic mineral deposits, particularly those concealed under cover.

Isostasy as a tool to validate interpretations of regional geophysical datasets – application to the mid-Norwegian continental margin

Abstract Isostasy is a well understood concept, yet rarely applied to its full capacity in regional interpretations of crustal structures. In this study, we utilize a recent density model for the entire NE Atlantic, based on refraction seismic data and gravity inversion, to calculate isostatically balanced bathymetry along the mid-Norwegian margin. Since gravity and isostatically balanced elevation are independent observables but both depend on the underlying density model, consistencies and discrepancies point towards model deficits, erroneously interpreted or poorly understood areas. Four areas of large isostatic residuals are identified. Along the outer Vøring Margin, a mass deficit points to more extensive high-density bodies or a shallower Moho than currently mapped. Farther seaward, along the Vøring Marginal High, a mass excess indicates inaccurate mapping of the continent–ocean boundary and surrounding structures. A number of eclogitic bodies along the proximal mid-Norwegian margin have been described in recent publications and their presence is now also confirmed by isostatic calculations. Major elevation and gravity residuals along the transition between the Vøring and Møre margins signify that the structure of this region is poorly understood and modifications to the mapped continent–ocean transition may be required.

The dynamics of a very large intra-plate continental basaltic volcanic province, the Newer Volcanics Province, SE Australia, and implications for other provinces

Abstract The Newer Volcanics Province of SE Australia is a very large continental basaltic province, with an area of >23 000 km 2 , a dense rock equivalent volume of <900 km 3 and >400 monogenetic volcanoes; it has been active since c. 8 Ma. Lava fields, shields, scoria cones are common, and there are >40 maars and volcanic complexes. Maars occur dominantly in the south where magmas erupted through Tertiary sedimentary aquifers, whereas in the north, over Palaeozoic crust, there are few. Complex interactions of the magma volatile content, magma ascent rates, conduit characteristics and the availability and depth of aquifers caused diverse eruption styles. Volcanoes commonly occur close to major crustal faults, which acted as magma conduits. There is no simple age pattern of volcanism across the province. Volcanism was probably triggered by transtensional decompression in the crust where fault sets intersect, affecting hot, hydrated mantle that had welled up through edge-driven convection where the base of the lithosphere thins abruptly at the edge of the continent. Rock compositions range from picritic to basaltic andesitic. Some volcanoes are polymagmatic. Regional geophysical datasets have clarified the regional characteristics of the province, whereas detailed ground magnetic and gravity surveys resulted in new insights into the subsurface structure of maar-diatremes.

Structural geophysics: Geological principles applied to geophysical data Minerals keynote paper

The fundamental precept of structural analysis is that descriptions of geometry lead to interpretations of kinematics and the dynamics of geological systems. Controls of the formation of structures at all scales depend on the starting material heterogeneity, and the anisotropy and time dependence mechanical response of the materials. Processes that control mechanical response at the small scale (~1cm) may be quite different to those at the regional scale (>1km), the flow fields and structures that develop can be similar and therefore can be interpreted in similar ways. This approach is second nature in structural geology but is also easily applicable to analysis of regional geophysical datasets such as aeromagnetic and gravity data. Structural geophysics is a discipline defined by Jessell and Valenta, (1996), where structural methodology and interpretation could be applied to regional geophysical datasets to unravel 3D architecture and overprinting relationships. Geophysical datasets are increasingly used in conjunction with structural analysis, and as a stand-alone tool, to resolve regional crustal architecture and geological evolutions, particularly in remote regions and areas of limited outcrop, or where there is some ambiguity in how structure can be determined in the third dimension. Importantly, aeromagnetic data is also effective interpret regional kinematics of major shear zones and high strain zones at different scales. Applying structural kinematic analysis to regional geophysical datasets informs a broad range of disciplines including, but not limited to structural analysis, plate reconstruction, tectonic analysis, and prediction of mineral systems in exploration. In the context of resource exploration 3D and kinematic analysis allows you to predict Earth structuring that controls mineralisation, but more importantly allows prediction of favourable sites of dilation and fluid flow. In this presentation will highlight many of the principles of 3D geometry and kinematic analysis and illustrate how they improve our understanding of geological architecture and dynamic processes of the Earth.

Defining major structures and their depth extent under cover in the southern Thomson Orogen, New South Wales

Regional geophysical datasets are critical to the task of uncovering the basement geology of the southern Thomson Orogen in far western New South Wales. As part of a National Collaborative Framework project, aeromagnetic, gravity and seismic data have being processed and interpreted to construct the structural framework. Subdivision into structural domains has been validated and constrained by geological information, relying on observations and measurements from sparse drill holes and outcrops. Boundaries between structural domains are complex and poorly understood. This study aimed to recognise major faults and, where possible, define their displacements, depth extent, and understand their dynamics and timing. Analysis of available company and government seismic surveys provided details for some of the major fault systems such as the Olepoloko Fault, Culgoa Lineament, and also for many newly recognised fault trends The seismic interpretations were reconciled with deep sourced aeromagnetic and gravity gradients that were enhanced by multiscale edge analysis. The structural framework will underpin geochronology and mineral systems studies as the Southern Thomson Orogen project continues.

Volcanological and Structural Controls on Mineralization at the Mount Keith and Cliffs Komatiite-Associated Nickel Sulfide Deposits, Agnew-Wiluna Belt, Western Australia—Implications for Ore Genesis and Targeting

The Mount Keith region of the Agnew-Wiluna belt in Western Australia is perhaps best known for hosting the giant komatiite-associated disseminated Ni sulfide deposit known as MKD5. It also hosts the sizeable Cliffs massive Ni sulfide deposit, which is the eighth largest in the Archean Yilgarn block. A concerted program of regional-scale three-dimensional model construction has integrated a plethora of geological, geochemical, and geophysical data and led to a much better understanding of the tectonic and volcanic architecture of this ancient greenstone belt. The disseminated MKD5 orebody occupies a curvilinear thickening in the olivine adcumulate core of the host Mount Keith ultramafic unit. The orebody plunges shallowly to the south from surface, over a distance of 3.5 km, before bending sharply to plunge steeply to the north-northwest at depth. The thickest part of the orebody and the best Ni grades are associated with the bend in the overall ore trend. This also marks the approximate intersection point of two growth faults that appear to control the thickness variations in the adcumulate core and the plunge variations in the ore trend. The olivine adcumulate core is mantled by olivine orthocumulate rocks, which are best developed above and below the flanks of the orebody, and a fractionated sequence of pyroxenitic and gabbroic rocks occurs at the top of the Mount Keith ultramafic unit above the inside of the bend in the ore trend. These observations suggest a strong volcanological control on the position of the MKD5 orebody, and the local curvilinear thickening in the adcumulate core is interpreted as a fluid pathway, the locus of which was in turn controlled by the intersection of two broadly orthogonal growth faults. The Cliffs ultramafic unit consists of a lower olivine ortho- to mesocumulate unit, which varies in thickness along strike, overlain by a sequence of thin, spinifex-textured komatiite flow units intercalated with variably sulfidic tuffaceous shales. The Cliffs massive Ni sulfide deposit occurs at the base of a relative thickening (to ~100 m) in the lower olivine ortho- to mesocumulate unit. The orebody is typically less than 6 m thick, 500 m wide and plunges gently south from surface over a distance of 1.5 km, terminating in a structurally complex zone across which there are marked differences in the internal stratigraphy and thickness of the Cliffs ultramafic unit. The orebody has relatively low tenor (maximum ~8% Ni, typically ~5% Ni) and the tenor is strongly zoned with extremely low tenors (1–3% Ni) developed on the flanks. A barren exhalative massive Fe sulfide unit is typically present at the base of the Cliffs ultramafic unit on the flanks of the orebody and probably acted as a source of sulfur. The tenor zonation pattern suggests that the melting and entrainment of the underlying exhalative sulfide horizon occurred more vigorously in the center of the lava pathway. The structurally complex zone at the southern end of the Cliffs orebody is interpreted as a possible vent location; this would explain the relatively low tenor of the orebody and the tenor zonation (through limited mixing and entrainment of melted exhalative iron sulfide melt with the komatiite lava), the poorly incised nature of the lava pathway and the abrupt change in internal komatiite stratigraphy across this zone. There is good evidence that the southern terminations of both the MKD5 and the Cliffs orebodies are marked by growth faults. At a belt scale these points correspond roughly with the intersection of NNE- and NW-trending lineaments. These lineaments are on the order of 5 to 10 km in strike length and are visible in regional geophysical datasets (both gravity and magnetics). They are likely to mark the position of accretionary structures that were active at the time of volcanism and that probably controlled the volcanic architecture of the komatiite flow field and the location of Ni sulfide accumulations within komatiite lava pathways. Repeated reactivation of these early crustal-scale structures during subsequent orogenesis resulted in their upward propagation as sets of late brittle structures. Of broader significance to the understanding of the generation of komatiite-associated nickel sulfide deposits is the conclusion that many such deposits may be developed proximal to vent rather than in a distal setting, as has been proposed for the Kambalda camp.

The Application of AEM as a Regional Mapping and Targeting Tool: A Mineral Explorer’s Perspective

Much of the highly prospective Paterson Province of Western Australia lies under sand cover, which has previously inhibited mineral exploration in much of the region. The Yeneena Project, held by WA - based company Encounter Resources, lies in this region and is actively being explored using geophysics to identify undercover targets. AEM has proved a particularly effective geophysical technique in mapping unexposed regional structures and geological units. Surveys generated by both Geoscience Australia and Encounter has directly led to the identification of new targets and significant investment in detailed ground geophysics and drilling. Crucially, these activities have led to the discovery of copper sulphides and prospective mineral systems within the Yeneena Project, demonstrating the significant impact that regional geophysical datasets can have on the exploration potential of an area.

Revised Eburnean geodynamic evolution of the gold-rich southern Ashanti Belt, Ghana, with new field and geophysical evidence of pre-Tarkwaian deformations

Integration of regional geophysical datasets and detailed field observations provide new insights into the paleoproterozoic structural evolution of southwestern Ghana. The study area is dominated by three metavolcanic and metasedimentary packages known as the Sefwi Group, the Kumasi Group (Birimian) and the Tarkwa Group (Tarkwaian) that were intruded by abundant TTG granitoids during the Eoeburnean and Eburnean phases of an event termed the “Eburnean Orogeny”. This study identifies an Eoeburnean (pre-Tarkwaian) deformation event (D1) that produced significant deformation in the Sefwi Group metavolcanics. D1 is associated with N-S shortening manifested as regional scale folding in the southern Ashanti Belt. D1 synorogenic granitoids were intruded between 2187Ma and 2158Ma under greenschist metamorphic condition. Syn-D1 gold mineralisation associated with quartz veining could be the original source of Tarkwaian paleo-placers and/or remobilised gold concentrations along major shear zones.D2 represents an extensional phase associated with the Kumasi Group sedimentation (2154–2125Ma) which could be related to activation of major structures such as the Ashanti Fault as low angle detachments that controlled the deposition of the Kumasi Group and the opening of the Kumasi and Akyem Basin. The Tarkwa Group (2107–2097Ma) unconformably overlies the Birimian Supergroups and was deposited in response to D3 shortening. D3 resulted in the inversion of syn-D2 detachments faults within the Ashanti Belt. NW-SE D3 shortening produced regional scale folding within the Birimian and the Tarkwaian metasediments. D4 deformation corresponds with sinistral reactivation of D3 thrust faults, and is locally associated with macro-scale folding at Obuasi and Wassa gold mines. By the end of D4, the regional scale architecture was built and was only slightly modified by the two last events. D5 postdates the Eburnean metamorphic peak and corresponds to open recumbent folds associated with a subhorizontal crenulation cleavage. D6 is present as a subvertical crenulation cleavage and reverse faults associated with NE-SW shortening.

Palaeocene–Recent plate boundaries in the NE Atlantic and the formation of the Jan Mayen microcontinent

Abstract: Breakup and sea-floor spreading between Greenland and Eurasia established a series of new plate boundaries in the North Atlantic region since the Late Palaeocene. A conventional kinematic model from pre-breakup to the present day assumes that Eurasia and Greenland moved apart as a two-plate system. However, new regional geophysical datasets and quantitative kinematic parameters indicate that this system underwent several adjustments since its inception and suggest that additional short-lived plate boundaries existed in the NE Atlantic. Among the consequences of numerous plate boundary relocations is the formation of a highly extended or even fragmented Jan Mayen microcontinent and subsequent deformation of its margins and surrounding regions. The major Oligocene plate boundary reorganization (and microcontinent formation) might have been precluded by various ridge propagations and/or short-lived triple junctions NE and possibly SW of the Jan Mayen microcontinent from the inception of sea-floor spreading (54 Ma) to C18 (40 Ma). Our model implies a series of failed ridges offshore the Faeroe Islands, a northern propagation of the Aegir Ridge NE of the Jan Mayen microcontinent, and a series of triple junctions and/or propagators in the southern Greenland Basin.

Possible Diatremes, Hay Region, NSW

The New Frontiers exploration initiative of the NSW government has implemented the analysis of regional geophysical datasets for the Ana Branch, Pooncarie, Booligal, Balranald, Hay and Deniliquin 1:250 000 map sheet areas. The aspiration is to encourage exploration in regional areas by extrapolating the geology beneath covered areas using regional aeromagnetic, gravity, radiometric, landsat, seismic and borehole stratigraphy datasets.

The Proterozoic Mount Isa Fault Zone, northeastern Australia: is it really a ca. 1.9 Ga terrane-bounding suture?

In marked contrast to Palaeoproterozoic Laurentia, the location of sutures and boundaries of discrete crustal fragments amalgamated during Palaeoproterozoic formation of the North Australian Craton remain highly speculative. Interpretations of suture locations have relied heavily on the analysis of regional geophysical datasets because of sparse exposure of rocks of the appropriate age. The Mount Isa Fault Zone has been interpreted as one such Palaeoproterozoic terrane-bounding suture. Furthermore, the coincidence of this fault zone with major shale-hosted massive sulphide Pb–Zn–Ag orebodies has led to speculations that trans-lithospheric faults may be an important ingredient for the development of this deposit type. This study has integrated geophysical and geochemical data to test the statute of the Mount Isa Fault as a terrane-bounding suture. Forward modelling of gravity data shows that basement rocks on either side of the Mount Isa Fault have similar densities. These interpretations are consistent with geochemical observations and Sm–Nd data that suggest that basement lithologies on either side of the Mount Isa Fault are geochemically and isotopically indistinguishable from each other, and that the Mount Isa Fault is unlikely to represent a suture zone that separates different Palaeoproterozoic terranes. Our data indicate that the crustal blocks on both sides of the Mount Isa Fault Zone must have been in within close proximity of each other since the Palaeoproterozoic, and that the Western Fold Belt was part of the (ancestral) North Australian Craton well before the ∼1.89–1.87 Ga Barramundi Orogeny. It appears that deep crustal variations in density may be related to the boundary between a shallowly west-dipping high-density mafic to ultramafic plate and low-density basement rocks. This interpretation in turn impacts on crustal-scale models for the development of shale-hosted massive sulphide Pb–Zn mineralisation, which do not require trans-lithospheric faults to tap deep-seated metal reservoirs and/or mantle plumbing systems. The approach applied herein demonstrates the value of multi-disciplinary investigations to the critical assessment of long-lived Proterozoic fault systems which, in the absence of methodical analysis, are commonly assumed to represent terrane-bounding sutures.

Eastern Victoria: a New Exploration Frontier?

High-quality airborne magnetic and radiometric data provide the cornerstone for the geological understanding of the relatively unexplored eastern highlands of Victoria. The Geological Survey of Victoria has obtained over 180 000 line kilometres of new airborne data in eastern Victoria. This data collection is a major component of the Victorian Initiative for Minerals and Petroleum (VIMP).Eastern Victoria has an excellent geological section across the eastern Lachlan Fold Belt. The Early Silurian Benambran event deformed Ordovician turbidites. Two cycles of rifting during the Silurian and Early Devonian resulted in transtensional rift-like grabens. Silicic volcanic units and marine sediments filled these rifts. The Late Silurian Bindian deformation resulted in major northwest and northeast fault trends which are apparent in the geophysical data. Granites with varied magnetic and radiometric responses have intruded the area, with several reaching the surface to produce calderas.The area is rich in potential mineral targets. These include porphyry style copper-gold mineralisation, dyke-associated tin-gold mineralisation, volcanic-associated massive sulphide mineralisation, epithermal gold mineralisation, and carbonate-hosted Irish style lead-zinc mineralisation.The new geophysical data allow improved mapping of the geological units of eastern Victoria, interpretation of the regional structural features, and identification of exploration targets. The regional geophysical datasets provide an excellent basis for mineral exploration in the under-explored eastern Victoria.

The Big Picture for Exploration in the Eastern Goldfields Province, Western Australia, defined by Regional Magnetic and Gravity Data

Geological understanding of the Eastern Goldfields has been limited in the past by poor outcrop owing to regolith and surficial cover. The economic importance of the greenstone belts relative to gneiss and granitoids, has led to geological maps and models concentrating on the geology and geological evolution of the greenstone belts. The regional aeromagnetic and gravity data are used to derive models for the whole area, and therefore also to subdivide the non-greenstone rock. lnterpretation of these regional geophysical data sets indicates the presence of features which are tens to hundreds of kilometres long. They include gneiss-granite domains, gneiss-granite domes, greenstone terranes and large bodies of granite. Recent more detailed surveys acquired under the National Geoscience Mapping Accord provide better resolution of the internal structure of these domains and enhance characteristic anomaly patterns used to discriminate between domains. Greenstone-belt deformation has been influenced by the shape of adjacent gneiss-granite domains, granites and lineaments. The geophysical models developed provide a valuable framework in which to assemble a wide variety of geological, geochemical, and geophysical information. It is through the generation of comprehensive regional models of the geology that effective exploration of areas of poor outcrop will be developed.