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.

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.

As a broadly based professional society that aims to represent all Earth Science disciplines, the GSA attracts a wide diversity of members working in a similarly broad range of industries.