Gravity inversion constrained by probabilistic magnetotellurics models: methodology and application

Giraud, Dr Jeremie Eugene Cyril1,2, Seillé, Dr Hoël3, Ciolczyk,Damien4, Grose, Dr Lachlan5, Visser, Dr Gerhard3, Lindsay, Dr Mark1,2, Jessell, Dr Mark1,2, Ogarko, Dr Vitaliy6

1Centre For Exploration Targeting, School Of Earth Sciences, University of Western Australia, Crawley, Australia, 2Mineral Exploration Cooperative Research Centre, School Of Earth Sciences, University of Western Australia, Crawley, Australia, 3CSIRO Deep Earth Imaging Future Science Platform, Kensington, Australia, 4Université de Strasbourg, School and Observatory of Earth Sciences (EOST), Strasbourg, France, 5School of Earth, Atmosphere and Environment, Monash University, Clayton, Australia, 6The International Centre for Radio Astronomy Research, The University of Western Australia, Crawley, Australia

In this contribution we introduce an inversion workflow where we integrate magnetotelluric (MT) data, which are primarily sensitive to horizontal resistivity interfaces, together with gravity  measurements, which are well suited for the recovery of lateral density variations. We connect these data using petrophysical prior information and geological principles. The approach we
present relies on a flexible, cooperative workflow where the different datasets are modelled using standalone algorithms. The workflow consists of the following three steps.

First, we perform the inversion of MT data in a 1D probabilistic fashion. For each MT site, the results include an ensemble of plausible 1D models, from which the probability of having an
interface between rock units of varying electrical conductivity is calculated. Second, these probabilities are interpolated to the whole study area using the implicit geological simulator LoopStructural. During this process, geological rules and principles (stratigraphy, superposition and cross-cutting relationships) are used to ensure that the models resulting from such probabilities are geologically plausible. Finally, domains corresponding to different rock units are derived using interfaces’ probabilities and are used in combination with petrophysical information to define the range of density contrast values allowed at every location in the model. These ranges are used to constrain deterministic gravity inversion using the alternating direction method of multipliers as implemented in the Tomofast-x engine.

We first summarise the methodology and present the proof-of-concept using a realistic synthetic model built using geological data from the Mansfield area (Victoria, Australia). Results demonstrate that the proposed workflow can effectively leverage the complementarity between geophysical methods relying on different physical phenomena such as MT and gravity and improve imaging. We then show preliminary results modelling existing real-world data that crosses the Eucla Basin (Western Australia), which is a region that is receiving increasing attention for its undercover mineral potential and where prior geological knowledge suggests complementarity between MT and gravity.

We acknowledge the support of the MinEx CRC and the Loop: Enabling Stochastic 3D Geological Modelling (LP170100985) consortia, and of the CSIRO Deep Earth Imaging Future Science Platform. The work has been supported, in part, by the Mineral Exploration Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Programme. This is MinEx CRC Document 2020/xx


Biography

Jeremie Giraud is a research fellow at the Centre for Exploration Targeting. His research efforts focus on the integration of geophysics and geology and on multi-physics integration.

How to computationally include regional interpretations into the seismic imaging process

Rashidifard, Mahtab1,3, Giraud, Dr Jérémie1,3, Ogarko, Dr Vitaliy1,2, Lindsay, A/Prof Mark1,3, Jessell, Prof Mark1,3

1Centre for Exploration Targeting, University Of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia, 2International Centre for Radio Astronomy Research (ICRAR), University Of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia, 3Mineral Exploration Cooperative Research Centre, School of Earth Sciences, University of Western Australia, 35 Stirling Highway, WA Crawley 6009, Perth,, Australia

Understanding the regional evolution of the Earth and subsurface processes is a key for mineral exploration. Crucial to this understanding is accounting for geoscience knowledge obtained from
integrated interpretation of geological and geophysical data. Reflection seismic data, although sparsely distributed due to the high cost of acquisition, is the only data that gives a high resolution image of the crust to reveal deep subsurface structures and the architectural complexity that may vector attention to minerally prospective regions. Without an iterative depth migration and a depth conversion step, seismic images remain in time domain and are totally data-driven. However, for reconstructing the architecture and history of the area it is necessary to have these images in depth. To obtain the depth of seismic events, depth correlations need to be applied to them from other sources of information. The limitation here is that existing depth conversion methods rely on borehole information which is rarely available for deep crust studies.

We introduce a new methodology which allows inclusion of deep regional interpretations from potential field and geological sources of information in depth migration of seismic data. This fast algorithm aims at reducing the ambiguity in time-to-depth conversion and reduce the time spent on depth migration of seismic data by numerically including geologists’ interpretations.
A modelling-ray approach, not dependent on borehole information and accounting for lateral variations in velocity models, is used for switching from a time-migrated to depth-converted domain. An imageray approach is used for the reverse process to move from a depth-migrated to a time-converted domain. On the other hand, primary geological models are directly used in potential field inversion algorithms without re-parametrization of the model using an existing generalized level-set algorithm. Integrating solutions of the Eikonal equation in the form of Hamilton-Jacobi for both potential-field levelset inversion and seismic migration leads to a simultaneous modelling through which seismic, potential field, and geological data mitigate each other’s limitations. An investigation of the proposed methodology and a proof-of-concept using a fairly advanced realistic synthetic dataset are presented.

The proposed workflow is a novel approach for questioning the geological meaning of the sparsely distributed seismic sections and to integrate them in a 3D volume following the regional geological data and potential field results. The primary outcome of this study is a step toward adding equal weight to geological data not only as primary models but also as additional quantitative information within geophysical modelling and seismic imaging algorithms.

Our results lead to a significant improvement in the final model consistency with available sparsely distributed data sets. As a result, seismic migration is influenced by complementary information from potential field inversion results while simultaneously respecting sparse seismic sections in the 3D model obtained from regional interpretation and potential field inversion.

We acknowledge the support of the MinEx CRC and the Loop: Enabling Stochastic 3D Geological Modelling (LP170100985) consortia. The work has been supported by the Mineral Exploration
Cooperative Research Centre whose activities are funded by the Australian Government’s Cooperative Research Centre Programme. This is MinEx CRC Document 2020/44.


Biography

I completed my undergraduate and masters in petroleum exploration engineering. I started my Ph.D. at UWA (CET) as a minexCRC student focusing on data fusion methodologies for integrated inversion of geological and geophysical data.  This presentation is part of my PhD project for integrating seismic and gravity with different coverage.  

Mapping the near surface architecture of the Amadeus Basin using magnetic data: Petrophysical properties and geophysics pitfalls

Austin, James1, Schmid, Susanne 2 and Foss, Clive1

1Potential Fields Geophysics, CSIRO Mineral Resources, Lindfield, NSW 2070, 2Multidimensional Geoscience, CSIRO Mineral Resources, Kensington, WA 6151

The Amadeus Basin in central Australia is prospective for stratiform base metal deposits and hydrocarbons. The Basin displays subtle magnetic anomalies that trace strata for considerable distance, highlighting complex folding patterns. Magnetic modelling techniques can be utilised on these stratiform anomalies to extrapolate the near-surface structure of the basin. Generally magnetic anomalies are assumed to have predominantly induced magnetisation, and with this assumption dip can be reasonably estimated using magnetic data alone. However, where the magnetisation is not purely induced (i.e., includes remanent magnetisation) the mathematical trade-off between the dip and magnetisation of bodies means that the dip of a body cannot be known unless the magnetisation is also known. Normally it would be optimal to measure the magnetisation, but this is not always possible or feasible, e.g., due to land access issues. In this study, we investigate the relationships between dip and magnetisation using an approach that would generally be considered a little backward. Rather than constrain structure using petrophysics, we use structural geology to constrain petrophysics. Three study areas were chosen to investigate numerous stratigraphic horizons in three major study areas, the Waterhouse Range, Glen Helen Station and Ross River areas. Modelling results suggest that a paucity of layers retain predominantly induced magnetisation, remanence is dominant in some, but both induced and remanent magnetisation are typically present. Remanence is mainly associated with relatively oxidised units that contain only hematite (e.g. Arumbera Sandstone), and comparisons with known apparent polar wander paths suggest that these magnetisations pre-date major folding in the basin. In some cases, magnetic anomalism reflects redox zonation within units, e.g. the Pertatataka Formation near Glen Helen, where discrete magnetic layers coincide with thin grey (reduced, magnetite-rich) horizons interbedded with more prevalent red (oxidised, hematite-rich) horizons, which are only very weakly magnetised. We also found that where magnetised units are relatively thin and occur near the surface, their magnetic response is sharp. However, in coincident aeromagnetic data, adjacent anomalies commonly overlap to form a single anomaly, thus misrepresenting the magnetic field, and mis-mapping the dip of the magnetic horizons. This study highlights some major pitfalls in attempting to map structure using magnetics. Near surface sedimentary units tend to be variably oxidised, and their petrophysical properties are inconsistent along strike. Their total magnetisation is commonly comprised of a significant component of remanent magnetisation, and therefore due to the mathematical trade-off between dip and magnetisation direction, industry standard inversions will commonly mis-map surface structure. Remanent magnetisation pre-dates major folding in many cases therefore, opposite limbs of the same fold can have completely different magnetic signatures. Our ability to target mineral systems in sedimentary systems is contingent on our ability to map the structure of such systems. This study demonstrates that petrophysical knowledge is a pre-requisite constraint for successfully informing structural and tectonic studies using geophysics.


Biography

James Austin is specialised in structural geology and potential fields geophysics, but he dabbles in many aspects of geology. His research is  focused on understanding  relationships between crustal processes and geophysical fields. His recent work is focused on the development of integrated technologies for mapping mineral systems.

Reconstructing the Soldiers Cap Group – Kuridala Group basin: Implications for BHT and IOCG mineralisation

Connors, Karen1

1Sustainable Minerals Institute, The University of Queensland, Brisbane, Australia

The vast basin hosting the 1700-1650 Ma Soldiers Cap and Kuridala groups (SCG-KG), eastern Mount Isa Province, NW Queensland, extends >300 km east to include the ca 1700-1610 Ma Etheridge Group, Georgetown Inlier. The present-day extent and thickness represents only part of the original depocentre following inversion, uplift and erosion (1610-1500 Ma). Whilst the importance of extension has long been recognised, pervasive compression, voluminous 1535-1490 Ma granites, and limited seismic integration, has prevented elucidation of the extensional architecture and its influence on inversion and mineral systems. Integrated interpretation of seismic and potential field data, and review of geochronology has provided a new understanding of the extensional architecture, potential age range and thickness of the basin, and the tectonic evolution.

The preserved thickness, extent, age range, erosion estimates, and the crustal architecture provide first-order constraints on basin reconstruction. The SCG-KG basin overlies several crustal-scale boundaries, including the Gidyea Suture where the thinned eastern margin of the poorly reflective, Mount Isa crust is thrust over the thinner Numil crust. The Numil comprises a series of moderate to low-angle fault blocks, many only 5-15 km thick, and typically has pervasive, dipping reflections.

While outcrop mapping suggests the SCG group is 2-5 km thick, seismic data indicates 3-5 sec TWT, implying 12-15 km preserved thickness. Mapping indicates localised isoclinal folding, nappes and structural repetition within some highly-deformed zones. Although regional structural repetition can’t be ruled out, the seismic data suggests many areas are dominated by limited repetition and thickening on inverted, normal faults.

The minimum age for the SCG-KG is generally accepted as 1650 Ma. However, the 1650-1610 Ma units of the Tommy Creek Domain and Etheridge Group are likely to have been widespread across the basin. In addition, zircon populations from drainages along the eastern outcrop margin and SCG show peaks in juvenile mafic magmatism at 1630-1625 Ma, as well as 1667 Ma. Although the upper SCG-KG unit (Toole Creek Volcanics (TCV)) is attributed to thermal relaxation from ca 1670 Ma, coeval felsic and mafic magmatism at 1655 and 1625 Ma, and the large volume (20-30%) of high-Fe mafic sills within the TCV suggest extension continued or was episodic.

Prior to inversion and erosion, the 12-15km SCG-KG basin was thicker as well as wider than the present ~350 km. While the stretching factor and total extension are unknown, the thin low-angle fault blocks of the Numil, are consistent with highly-thinned to hyperextended crust (i.e. 10 km thickness or less), and exhumation of lower crust or mantle may have occurred. The resulting high geothermal gradient has implications for BHT mineralisation, and raises questions regarding controls on metal deposition.

The extensional fault system and preliminary reconstruction provide insights into the extensional evolution and controls on later inversion. The structural framework and its links to the underlying basement blocks and crustal-scale structures that form the first-order conduits of the plumbing system provide insights for both syn-sedimentary BHT mineral systems and later IOCG deposits.


Biography

Karen has had a varied career in mineral and petroleum exploration. She specialises in integrated interpretation of seismic with potential field data to understand 3D crustal architecture, structural inheritance and influence of basement on basin evolution, 3D modelling, and controls on mineral systems.

Review of Australian Mesoproterozoic basins: Geology and resource potential

Anderson, Jade1, Carr, Lidena1, Henson, Paul1, Carson, Chris1

1Geoscience Australia, Canberra, Australia

Australian cratons underwent substantial tectonism and cratonic reorganisation during the Mesoproterozoic, coinciding globally with the transition from Nuna to Rodinia (e.g. Li et al., 2008; Pisarevsky et al., 2014). The full extent and nature of this tectonism remains contentious (e.g. Bagas, 2004; Betts and Giles, 2006; Cawood and Korsch, 2008; Maidment, 2017).

During the Mesoproterozoic several sedimentary basin systems were deposited, and are now variably preserved, in the Northern Territory, Queensland, Western Australia, South Australia and Tasmania; providing an invaluable indirect record of the evolving Australian lithosphere and tectonic processes. Most of these basins were deposited on or at the margins of Archean to Paleoproterozoic cratons (North Australian Craton, West Australian Craton and South Australian Craton; e.g. see Myers et al. 1996; Cawood and Korsch, 2008 for spatial geography and constituents of these cratons). The remnants of these basins vary from weakly-deformed, relatively continuous units, such as the Roper Group of the McArthur Basin in the Northern Territory, to basins that were subsequently deformed and metamorphosed under high grade conditions, such as the Arid Basin of the Albany Fraser Orogen in Western Australia.

Individual basins are typically studied in isolation or in subsets, for which available geological datasets are commonly disparate with markedly different levels of knowledge. Mineral and energy resources have been identified in some of these basins; including oil and gas resources hosted in the Roper Group in the Beetaloo Sub-basin; manganese deposits in the Collier Basin and Manganese Group (Western Australia); and polymetallic, stratabound, hydrothermal mineralisation in the late Paleoproterozoic to early Mesoproterozoic Edmund Basin (Western Australia). Typically, these more overtly prospective basins, or groups, have been studied in greater detail than other Mesoproterozoic basins or groups.

This study provides a holistic overview of Australian Mesoproterozoic sedimentary basin systems, integrating geological, geochronological, and publically available resource data. As part of this collated approach, we also discuss potential inter-basin correlations for Mesoproterozoic-aged successions in Australia. This study aims to assist future work targeted at improving the geological understanding of these Mesoproterozoic sedimentary provinces and their resource prospectivity.

References

Bagas, L., 2004. Proterozoic evolution and tectonic setting of the northwest Paterson Orogen, Western Australia. Precambrian Research 128(3-4), 475-496.

Betts, P. G. and Giles, D., 2006. The 1800-1100 Ma tectonic evolution of Australia. Precambrian Research 144(1), 92-125.

Cawood, P. A. and Korsch, R. J., 2008. Assembling Australia: Proterozoic building of a continent. Precambrian Research 166(1-4), 1-38.

Li, Z. X., Bogdanova, S. V., Collins, A. S., Davidson, A., De Waele, B., Ernst, R. E., Fitzsimons, I. C. W., Fuck, R. A., Gladkochub, D. P., Jacobs, J., Karlstrom, K. E., Lu, S., Natapov, L. M., Pease, V., Pisarevsky, S. A., Thrane, K. and Vernikovsky, V., 2008. Assembly, configuration, and break-up history of Rodinia: A synthesis. Precambrian Research 160(1-2), 179-210.

Maidment, D. W., 2017. Geochronology from the Rudall Province, Western Australia: implications for the amalgamation of the West and North Australian Cratons. Geological Survey of Western Australia, Perth, 95 pp.

Myers, J. S., Shaw, R. D. and Tyler, I. M., 1996. Tectonic evolution of Proterozoic Australia. Tectonics 15(6), 1431-1446.

Pisarevsky, S. A., Elming, S.-Å., Pesonen, L. J. and Li, Z.-X., 2014. Mesoproterozoic paleogeography: Supercontinent and beyond. Precambrian Research 244, 207-225.


Biography

Jade Anderson completed a PhD at the University of Adelaide in the areas of metamorphic geology, geochronology and Proterozoic Australia tectonics. She currently works as a Geoscientist in Basin Systems at Geoscience Australia.

The tectonostratigraphic evolution of the South Nicholson region, Northern Territory and Queensland: key discoveries from the Exploring for the Future and implications for resource exploration

Carson, Chris1, Henson, Paul1, Lidena, Carr1, Southby, Chris1 and Anderson, Jade1.

1Geoscience Australia, Canberra, Australia

Proterozoic rocks of the South Nicholson region, which straddle the NT and QLD border, are juxtaposed between the Proterozoic Mount Isa Province to the east and the southern McArthur Basin to the northwest. The McArthur Basin and Mount Isa Province are comparatively well-studied and prospective for energy and mineral resources. In contrast, rocks of the South Nicholson region are mostly undercover and, as such, there is incomplete understanding of their geological evolution, relationship with adjacent geological provinces and resource potential. To address this gap, two deep crustal seismic reflection surveys, the South Nicholson and Barkly surveys (completed in 2017 and 2019, respectively), were conducted across the South Nicholson region by Geoscience Australia, under the federally funded Exploring for the Future (EFTF) initiative, in collaboration with the Northern Territory Geological Survey, the Geological Survey of Queensland and AuScope (e.g. Carr et al., 2019, 2020). While the Barkly seismic data are still being interpreted, these seismic datasets, together with other complementary regional studies, provides an improved understanding of the geological evolution and resource potential across this poorly understood region.

Both seismic surveys targeted both suspected undercover sedimentary basins and known crustal structures to resolve regional subsurface fault geometry. A key finding from the South Nicholson seismic survey is the discovery of a large concealed sedimentary sag basin that is up to 8 km deep, around 120 km wide and 190 km from north to south, called the Carrara Sub-basin (e.g. Carr et al., 2019). The sub-basin is interpreted to contain Mesoproterozoic to late Paleoproterozoic rocks equivalent to those outcropping in the Lawn Hill Platform and Mount Isa Province. The eastern end of the one of the lines (17GA-SN1), connects with a legacy seismic line that intersects the world class Pb-Zn Century deposit on the Lawn Hill Platform, the late Paleoproterozoic host rocks of which can be traced into the Carrara Sub-basin.

The South Nicholson profiles also reveal a series of ENE–trending, north-dipping half grabens which evolved during two episodes of crustal extension, at ca. 1725 Ma and ca. 1640 Ma, broadly coinciding with structural and basin forming events identified from the Lawn Hill Platform and the Mount Isa Province. Inversion of the half-graben bounding faults, resulting in south–verging thrusts, probably commenced during N-S crustal contraction characteristic of the early Isan Orogeny at ca. 1600-1580 Ma to at least the Paleozoic Alice Springs Orogeny (ca. 400-300 Ma).

Furthermore, our comprehensive regional geochronology program proposes extensive revision of regional stratigraphic relationships. Some successions, previously mapped as Mesoproterozoic South Nicholson Group may instead represent late Paleoproterozoic successions, that form part of the highly prospective Isa Superbasin (and the broadly stratigraphic equivalent McArthur Group in the McArthur Basin), which hosts numerous viable base metal deposits and is prospective for energy commodities (e.g. Jarrett et al., 2020; MacFarlane et al., 2020). Our findings significantly expand the extent of highly prospective late Paleoproterozoic stratigraphy across the South Nicholson region, which, possibly, extends an as yet unknown distance west beneath the Georgina and Carpentaria basins.

Carr, L.K., et al., 2019. Exploring for the Future: South Nicholson Basin Geological summary and seismic data interpretation. Record 2019/21. Geoscience Australia, Canberra. http://dx.doi.org/10.11636/Record.2019.021

Carr, L.K., et al., 2020. South Nicholson seismic interpretation. In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/132029

Jarrett A.J.M., et al., 2020. A multidisciplinary approach to improving energy prospectivity in the South Nicholson region. In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/134164

MacFarlane, S. et al., 2020. A regional perspective of the Paleo- and Mesoproterozoic petroleum systems of northern Australia In: Czarnota, K., et al., (eds.) Exploring for the Future Extended Abstracts, Geoscience Australia, http://dx.doi.org/10.11636/133716


Biography

Chris has worked in Antarctica, Canadian Arctic, Alaska, New Caledonia and northern and central Australia, specialising in metamorphic petrology, geochronology and structural geology. Joining Geoscience Australia in 2006 he dabbled in SHRIMP geochronology and, in 2017, joined the Onshore Energy program, working in the South Nicholson region of the NT.

Interpreting and validating trans-lithospheric faults in the Central Andes to investigate their control on the localisation of giant porphyry copper deposits

Farrar, Alexander1,2, Cracknell, Dr Matthew1, Cooke, Professor David1, Hronsky, Dr Jon3,4, Piquer, Dr Jose5

1Universiity Of Tasmania, Hobart, Australia, 2First Quantum Minerals, Santiago, Chile, 3Western Mining Services, Perth, Australia, 4University of Western Australia, Perth, Australia, 5Universidad Austral de Chile, Valdivia, Chile

The central Andes (between latitudes of 14°S and 35°S) accounts for approximately 40% of the world’s annual copper production and is the most important copper province on the planet. However, since 1998, just one giant porphyry copper greenfield discovery has been made in the central Andes. Post-mineralisation cover consisting of transported gravels and young volcanics make up at least 50% of the surficial outcrop of the central Andes and of the 60 or so known giant Cu ± Au ± Mo deposits, only three giant ore deposits have been discovered beneath these surficial materials in the greenfields domain. Therefore it is likely that many concealed, undiscovered giant porphyry copper deposits are waiting to be discovered. However, no proven effective exploration process exists that enables explorers to consistently achieve economic greenfield discoveries through cover.

Giant porphyry copper deposits tend to cluster in discrete geographic “camps” of a similar age. This indicates that exceptional transient geologic processes have affected localised regions of the crust prior to and during the age of mineralisation and that the formation of giant porphyry deposits is non-random. Key predictive geological features of giant porphyry Cu deposits are the structural pathways (basement faults) that focus fluid and magma flow from the mantle to upper crust. Nevertheless, these so called trans-lithospheric faults (TLFs) are notoriously difficult to identify in the field due to their subtle surficial characteristics, complex multi-stage reactivation history and continental-scale. As a result, the notion of TLFs has, until recently, been treated with scepticism by many in the geologic community.

This research focuses on identifying and mapping the continental-scale trans-lithospheric structural architecture of the central Andes through the integration and interpretation of multiple geoscience datasets supported by field observations. Datasets used in this analysis include geophysical inputs such as airborne magnetics, regional gravity, magneto-tellurics and seismic epicentres as well as geologic reconstructions through time from the Proterozoic to present, which map out inherited basement architecture as well as regions of rapid crustal thickening or thinning. Fieldwork undertaken in the regions of the interpreted TLFs demonstrates that on the surface they are expressed as linear zones of brittle faulting, tens of kilometres wide and hundreds of kilometres long, consisting of thousands of individual fault planes. This is interpreted to reflect the upper crustal propagation of the underlying zone of basement weakness through younger sequences in the geologically active convergent plate margin.

A map of the TLFs across the central Andes shows that the TLFs have a fractal distribution with N, NW and NE strikes. A prominent relationship exists with the location of known giant porphyry deposit camps occurring where two or more TLFs meet. Such regions are inferred to have been loci of deep-seated strain-anomalies which have localised dilation and increased permeability, during transient changes to the regional stress field. Regions adjacent to the intersection of two or more TLFs that overlap with the magmatic arc during metallogenic epochs (themselves transient geodynamic anomalies) are deemed to represent valid exploration targets in this model.


Biography

Began in the mining industry in 2007 with Xstrata in Mt Isa. In 2009 joined First Quantum Minerals based in DRC and Zambia. From 2013-2020 based in South America with First Quantum running greenfields project generation. Currently undertaking a PhD at University of Tasmania concerning fundamental emplacement controls on porphyries.

Unravelling the “late” evolution of the Gawler Craton: high T/P metamorphism, tectonism and magmatism of the Yorke Peninsula, South Australia

Bockmann, Mitchell1,2, Hand, Professor Martin1,2, Morrissey, Dr Laura3,1, Payne,Dr Justin4,3,1, Teale, Graham5, Conor, Associate Professor Colin4, Dutch, Dr Rian6,2

1Departments of Earth Science, University Of Adelaide, Adelaide, Australia, 2Mineral Exploration Cooperative Research Centre, University of Adelaide, Adelaide, Australia, 3Mineral Exploration Cooperative Research Centre, Future Industries Institute, University of South Australia, Adelaide, Australia, 4UniSA STEM, University of South Australia, Adelaide, Australia, 5Teale and Associates Pty Ltd, Prospect, Australia, 6Department for Energy and Mining, Geological Survey of South Australia, Adelaide, Australia

The early Mesoproterozoic is a geologically active time in the Gawler Craton, recording widespread magmatism, deformation, metamorphism and mineralisation. Much of this activity occurs within the time period of 1600–1575 Ma, during the Hiltaba tectonothermal event and associated the worldclass Iron-oxide–Copper–Gold (IOCG) mineralisation, which has focussed attention on this timeline. This event has often been considered the timing of ‘cratonisation’ as there was perceived to be little tectonic activity that post-dates this timeline. However, sporadic evidence across the Gawler Craton for metamorphism, deformation and minor magmatism post-dating this major event has indicated tectonic activity extends beyond this age. This study further highlights the importance and extent of post-1575 Ma activity in defining the modern-day structural and metamorphic architecture of the Gawler Craton.

The Yorke Peninsula in the southern Gawler Craton is a highly prospective region for IOCG mineralisation, as it hosts the historically significant Moonta and Wallaroo mines and more recently discovered Hillside deposit. Despite extensive evidence for early Mesoproterozoic hydrothermal fluid activity and great potential for mineralisation, the Yorke Peninsula is incredibly understudied with modern analytical techniques.

Here we present evidence for high T/P metamorphism from the Yorke Peninsula at c. 1555 Ma, with peak metamorphic constraints of c. 3.5 kbar, 660°C and c. 4.2 kbar, 700°C from two samples taken approximately 35km apart. In addition, monazite U–Pb geochronology also provides evidence for  shear zone activation at this time, along with possible evidence for re-activation of the Pine Point Fault as young as 1500 Ma; significantly post-dating mineralisation at the Hillside deposit. Apatite U–Pb cooling ages from these rocks provide relatively young ages between 1460–1400 Ma, indicating that these rocks remained at elevated temperatures for an extended period following the metamorphic peak, supporting a long-lived thermal driver for metamorphism. The record of this post-Hiltaba event is also manifest in published monazite and zircon ages from the Barossa Complex on the Fleurieu Peninsula, signifying that this event impacted the entire south-eastern Gawler Craton. The metamorphic conditions and prolonged time at depth indicated by relatively young apatite U–Pb cooling ages from the Yorke Peninsula are consistent with thinned continental crust, implying that the south-eastern Gawler Craton was in an extensional setting after the Hiltaba Event. Post-Hiltaba activity is distinct from metamorphism and deformation associated with the Hiltaba Event, which is also recorded within the south-eastern Gawler Craton, but typically with lower thermal gradients. The metamorphism reported in this study has long been assumed to be linked to the Hiltaba Event, along with much of the Mesoproterozoic magmatism, deformation and mineralisation on the Yorke Peninsula. This study reveals that the c. 1515 Ma Spilsby Suite is not the only expression of post-Hiltaba activity on the Yorke Peninsula and demonstrates that the thermal and tectonic footprint of post-Hiltaba events is much greater than previously interpreted, suggesting that some of the mineralisation hosted within the region (e.g. Moonta-Wallaroo) may also be postHiltaba, with evidence for potential thermal drivers, fluid sources and deformation in operation after this time.

Biography

A third year MinEx CRC PhD student from the University of Adelaide, studying the tectonic setting of early-Mesoproterozoic mineral systems in the Gawler Craton, South Australia

Mineral Systems of the Capricorn Orogen through time

Occhipinti, Dr Sandra , Metelka, Dr Vaclav1, Lindsay, Dr Mark2, Aitken,Dr Alan2

1CSIRO, Kensington, Australia; 2Centre of Exploration Targeting, University of Western Australia, Crawley, Australia

Opening up greenfields regions for minerals exploration programs is best facilitated through the understanding of regional minerals prospectivity. The Capricorn Orogen is a greenfields-dominated region, for which a multicommodity mineral systems analysis has been completed forming the basis for new prospectivity analysis and mapping. Known mineral occurrences or deposits in the region formed between the Paleoproterozoic and Neoproterozoic. Mineralisation can be related to basin development and orogenesis in the region, in turn related to periods of supercontinent assembly and breakup. These were manifested in the region through the contractional 2005–1950 Ma Glenburgh, 1830–1780 Ma Capricorn, and c. 1030–950 Ma Edmundian and 920–850 Ma Kuparr orogenies. These periods of orogenesis were preceded and interspersed with periods of subsidence, perhaps including the 1680–1620 Mangaroon Orogeny, which led to the development of volcanosedimentary and sedimentary basins throughout the region. Prospectivity models were generated for several commodity groups of various ages and ore genesis mechanisms, including combinations of Ni, Cu, PGEs, V, Ti, Au, Pb, Zn, channel Fe and U. The work has found a link between key mineral systems and a spatial relationship between disparate styles of mineral deposits in the region. Crustal-scale tectonic architecture was analysed by allying a 2D map view geological-geophysical interpretation with 2.5D magnetic and gravity joint inversions of selected profiles, a 3D Moho model, and by inference from 2D and 3D magnetotelluric models, 2D reflection seismic images and 3D passive seismic models from the region.  This work clearly illustrates that different ‘zones’ of the Capricorn Orogen are prospective for different commodity groups due to the tectonic environment in which they developed. Major crustal-scale deformational zones intrinsically control the location of known ore deposits in the area, and are inferred to be sites of fluid migration associated with ore deposition. Of these, some are considered to be of Archean origin, whereas others are thought to have first developed during the early Paleoproterozoic. In both cases, many structures have been re-activated through time, influencing the formation of basins over them and perhaps the formation of ore deposits.


Biography

Sandra Occhipinti is a geologist with over 20 years experience in regional mapping, geophysical interpretation, mineral systems analysis and structural geology. She is the research director of the Discovery Program, CSIRO Mineral Resources

Lithospheric-scale magnetotellurics over the Eastern Goldfields Superterrane, Yilgarn Craton

Selway, Kate1, Dentith, Michael2, Gessner, Klaus3

1Department of Earth and Environmental Sciences, Macquarie University, Australia; 2Centre for Exploration Targeting, School of Earth Sciences, The University of Western Australia, Crawley, WA 6009, Australia; 3Geological Survey of Western Australia, East Perth, WA 6004, Australia

The Eastern Goldfields Superterrane, in the Yilgarn Craton, Western Australia, is one of the most highly mineralised regions on Earth, hosting world-class orogenic gold and nickel-sulfide deposits. Mineral systems models for both of these deposit types suggest that lithospheric-scale processes are involved in their formation. Therefore, lithospheric-scale geophysical imaging is a promising tool to improve understanding of the formation of the deposits and to aid future exploration.

Long-period magnetotelluric (MT) data were collected over an approximately 250 km x 200 km area covering the western part of the Eastern Goldfields Superterrane and the eastern Youanmi Terrane. The survey region covers the Kalgoorlie and St Ives gold camps and the Kambalda nickel camp, as well as the Ida Fault, a prominent isotopic boundary between the older Nd model ages of the Youanmi Terrane and the younger Nd model ages of the Eastern Goldfields Superterrane. A 3D conductivity model was produced from the data, with good resolution to depths of 150 to 200 km.

Results show that the lithospheric mantle from depths of approximately 100 to 150 km is more conductive (~10 to 100 ohm m) beneath the Youanmi Terrane than the Eastern Goldfields Superterrane (>100 ohm m). Crustal conductivity is more heterogeneous but most of the strongly conductive regions (<100 ohm m) are located in the Eastern Goldfields Superterrane. The resolution of the model in the near-surface is insufficient to make a detailed comparison with the locations of known deposits, but most upper crustal conductors are spatially correlated with regional-scale faults, which are inferred to be important in the formation of orogenic gold deposits.

Anomalously conductive zones in tectonically stable regions often indicate past metasomatism, either through the hydration of nominally anhydrous minerals or the growth of conductive mineral phases such as amphibole or phlogopite.

Quantitative interpretation of the MT model shows that the mantle conductors in the Youanmi Terrane are too conductive to be explained purely by hydrated peridotite and imply the presence of hydrous metasomatic minerals. The observed patterns of lithospheric conductivity suggest a more complex relationship between mantle metasomatism and gold and nickel mineral systems than expected from previous studies.


Biography

Kate, Mike and Klaus are all interested in the multi-disciplinary application of geophysical, geological and geochemical data to understanding tectonic evolution and mineral systems.

About the GSA

The Geological Society of Australia was established as a non-profit organisation in 1952 to promote, advance and support Earth sciences in Australia.

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