Scale reduction magnetotelluric survey in the East Tennant region, Northern Australia

Jiang, Dr Wenping1, Duan, Jingming1, Schofield, Anthony1, Brodie, Dr Ross1, Clark, Andrew1

1Geoscience Australia, canberra, Australia

Geoscience Australia has undertaken a series of integrated studies to identify prospective regions of mineral potential using new geological, geophysical and geochemical data from the Exploring for the Future (EFTF) program, together with legacy datasets. Data from the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) have been used as first-order reconnaissance survey to resolve large-scale lithospheric architectures for mapping areas of mineral potential in northern Australia. The resistivity model derived from the newly-acquired AusLAMP data has mapped deep lithospheric-scale conductivity anomalies in highly endowed mineralised regions and in greenfield regions where mineralisation was not previously recognised. For example, the model reveals a conductivity anomaly extending from the Tennant Region to the Murphy Province, representing a potential fertile source region for mineral systems. This conductive feature coincides with a broadly northeast-southwest-trending corridor marked by a series of large-scale structures identified from preliminary interpretation of seismic reflection and potential field data. This under-explored region, referred to as East Tennant, is, therefore, considered to have significant mineral potential.

We undertook a higher-resolution infill magnetotellurics survey to investigate whether the deep conductivity anomaly is linked to the near surface by crustal-scale fluid pathways. Broadband MT (BBMT) and audio-MT (AMT) data were acquired at 131 stations with station spacing of ~2 km to ~15 km in an area of approximately 90 km x 100 km. The 3D resistivity model revealed two prominent conductors in the resistive host whose combined responses result in the lithospheric-scale conductivity anomaly mapped in the AusLAMP model. The resistivity contrasts coincide with major structures preliminarily interpreted from seismic reflection and potential field data. Most importantly, the conductive structures extend from the lower crust to the near surface. This observation strongly suggests that the major faults in this region are deep-penetrating structures that potentially acted as pathways for transporting metalliferous fluids to the upper crust where they could form mineral deposits. This result indicates high mineral prospectivity for iron oxide copper–gold deposits in the vicinity of these major faults. We then used AMT data to estimate cover thickness to assist with drill targeting for the stratigraphic drilling program which, in turn, will test the models and improve our understanding of basement geology, cover sequences and mineral potential. This study demonstrates that integration of geophysical data from multiscale surveys is an effective approach to scale reduction during mineral exploration in covered terranes with limited geological knowledge.

This abstract is published with the permission of the CEO, Geoscience Australia


Biography

Dr Wenping Jiang is a senior geophysicist working in Mineral Systems Branch in Geoscience Australia.

dh2loop 1.0: An open-source python library for automated processing and classification geological logs

Joshi, Ranee1,2, Madaiah, Kavitha1,2, Jessell, Mark1,2 and Lindsay, Mark1,2

1Centre of Exploration Targeting, School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley 6009 Western Australia,2Mineral Exploration Cooperative Research Centre (MinEx CRC), School of Earth Sciences, University of Western Australia, 35 Stirling Highway, Crawley 6009 Western Australia

Exploration and mining companies rely on geological drill core logs to target and obtain initial information on the composition and size of mineralized zones and/or a potential ore deposit. The drilling data is also used as inputs to 3D geological modelling to allow better visualization and understanding of the geology in a local and/or mine scale. With the amount of legacy drilling data available in geological surveys, extraction and processing of these data will allow for better shallow subsurface constraints for semi-regional and regional 3D geological models. These models will be helpful in designing mapping programs and more efficiently targeting sustainable new discoveries.

In this contribution, we focus on the processing and classification of lithological logs from the Geological Survey of Western Australia Mineral Exploration Reports Database in the Yalgoo-Singleton Greenstone Belt (YSGB) region. We refer to lithological logs as the component of a geological log that contains the dominant rock type in a specific downhole interval. Inevitably, lithological drill core logging is subjective and plagued with uncertainty, particularly as at a regional scale it is likely to have been conducted by tens to hundreds geologists, all of whom would have their own personal biases. It can also be difficult to recognize lithology with confidence and to establish subtle variations or boundaries in apparently homogeneous sequences. Given that we are dealing with geological legacy datasets, a large amount of important data are recorded in an unstructured textural form using varying geological drill core logging forms and formats depending on the company, logging geologist, investigation method, investigated materials and/or drilling campaign.

To resolve these challenges and unlock the vast information store in legacy drilling datasets, we developed dh2loop (github.org/Loop3D/dh2loop), an open-source python library that provides the functionality to extract and standardize drill hole data and export it into readily importable interval tables (collar, survey, lithology). dh2loop addresses the subjective nature and variability in nomenclature of lithological descriptions within and across different drilling campaigns by integrating published dictionaries, glossaries and/or thesaurus that were built to improve resolution of poorly defined or highly subjective terminology and idiosyncratic logging methods. Furthermore, lithological data is classified into multi-level groupings that can be used to systematically upscale and downscale drill hole data inputs for multiscale 3D geological modelling. dh2loop also provides drill hole desurveying and log correlation functions so that results can be plotted in 3D for analysis and comparison. dh2loop formats legacy data bridging the gap between utilization and maximization of legacy drill hole data and drill hole analysis functionalities available in existing python libraries (lasio, welly, striplog).

Keywords: drill core logging, legacy data, subsampling, Yalgoo-Singleton Greenstone Belt

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 2020/41.


Biography

Ranee is a PhD candidate in the Centre for Exploration Targeting working with Mark Jessell, Mark Lindsay, Nicolas Thebaud and Tim Ivanic. She works on developing subsampling workflows to be able to automate multiscale 3D geological modelling. This project is a collaboration between the Loop and MinEx CRC consortium.

Geophysical Data Optimisation for Modelling: Data collection in a value-of-information framework

Lindsay, Mark1,*, Pirot, Guillaume1, Jessell, Mark1,*, Giraud, Jeremie1, Scalzo, Richard2,*, Cripps, Edward3,*, Aitken, Alan1

1Centre for Exploration Targeting, School of Earth Sciences, The University of Western Australia, Perth, Australia, 2School of Mathematics and Statistics, The University of Sydney, Sydney, Australia, 3Department of Mathematics and Statistics, The University of Western Australia, Perth, Australia

* ARC Centre for Dare Analytics for Resources and Environment

Crustal 3D models provide an understanding of the tectonic history of a region and its mineral endowment. As mineral resources are now mostly discovered under sedimentary cover, geophysical data are necessary to guide exploration. Recent developments in modelling 3D uncertainty with optimisation techniques are combined to guide data acquisition to image mineral systems and identify prospective regions. Most mineral systems are difficult to image with individual geophysical techniques so it is important to understand which data combinations are most effective for each system component (architecture, fertility, depositional trap, geodynamic throttle, preservation). In the course of these model-driven studies, there are often competing choices to be made around which data should be collected in order to reduce geological uncertainty. The “GDOM” project – “Geophysical Data Optimisation for Modelling” – seeks to determine what and how much geophysical data is worth collecting, the best processing methods and application in the most economically efficient manner. The intention here is to guide government policy and industry data collection practices. The workflow aims to inform how geophysical datasets can be best used to constrain geoscientific concepts and models to reduce uncertainty and more reliably answer geological questions. Recent advances in 3D model analysis helps to place focus where required and be used within a value-of-information (VoI) proposition to help us decide where and what data to collect. Two important parameters used in a VoI calculation are the estimate of “gain” from data collection and the probability of that gain. “Gain” may be the increase in value through deposit discovery, or finding prospective mineralisation amongst a portfolio of prospects and results. Both gain and the its dependence on critical system parameters are also uncertain, and have a large influence on the VoI analysis and estimating the risk involved in an exploration project. The cost of data collection is likewise critical information for decision-makers, especially if hierarchical scenarios where additional data reduces the cost of drill targeting are considered.  We propose a method that places these parameters into a hierarchical Bayesian framework to give us a clearer understanding on the uncertainties around VoI analyses, and helps us to determine the relative utility of collecting different geophysical data.

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/xx.


Biography

Mark is a geoscientist and Senior Research Fellow at the University of Western Australia, specialising in integrated geoscientific and 3D modelling and understanding the value of geoscientific information. He also has research interests that include investigating complex mineral systems and their representations.

Using gases for mineral exploration through cover: Importance of microbial activity

Plet, Chloe1, Siegel, Coralie1, Noble, Ryan1

1CSIRO Mineral Resources, Kensington WA 6151, Australia

In Australia, the presence of a thick transported cover on ~70 % of the continent has hindered the discovery of new world-class mineral deposits. To overcome this challenge, novel approaches are developed. Gases, with their high mobility, are thought to hold great potential as pathfinders for mineral exploration through cover. Previous investigations of gases detected at the soil surface have shown varying degrees of success in revealing the presence of buried mineralisation.

Here we investigate gases produced during laboratory weathering of sulfides. The experiments were run under sterile and non-sterile conditions. Carbon dioxide (CO2) and carbon disulphide (CS2) were the most abundant gases detected in all experiments. Non-sterile experiments produced more abundant gases than their sterile equivalents highlighting the importance of microbes in the weathering of sulfides.

In addition, the results of sterile experiments were compared to equilibrium thermodynamic predictions. In all experiments, as predicted, CO2 was the most abundant gas detected. However, some sulfur gases predicted by thermodynamic modelling were not detected in the laboratory (e.g. S2, H2S, CH3SH and C2H6S). Moreover, the most abundant sulfur gas predicted, carbonyl sulfide (COS) was only detected at trace levels. These results indicate that the experiments did not reach equilibrium.

Further soil gases experiments should include characterisation of the microbial communities. These would permit to gain a better understanding of the processes by which microbes impact the gases and improve the reliability of both techniques in the search for buried mineral deposits.


Biography

Chloe Plet obtained her PhD in organic and isotope geochemistry at Curtin University in 2017. In 2018 she joined CSIRO Mineral Resources, where her work largely focuses on investigating the potential of soil gases as a medium for mineral exploration.

New strato-tectonic model and geochemical tool for revitalised IOCG targeting

Anderson, John1

1Austrike Resources Pty Ltd, Glenalta, Australia

The combination of a new strato-tectonic model for the Olympic Metallogenic Event (OME) and zircon-based geochemical tool is a significant step-change for ore vectoring for a spectrum of IOCG and coeval deposit styles in the Gawler Craton.

Minerals exploration by Aberfoyle Resources, MIM Exploration and Investigator Resources resulted in the recognition of the Nankivel advanced argillic cap in the nineties and resulting discovery of the 42Moz Paris silver deposit in 2011. Paris is interpreted as an intermediate sulphidation epithermal associated with Nankivel and Helen copper gold silver magnetite skarn, all within a 100km2 sericite pyrite lithocap. The mineralisation is hosted in part by 1620Ma subduction monzodiorites with the Paris-Nankivel mineralisation dated within 2Ma of the 1590Ma Olympic Dam mineralisation.

The revised model proposes the Paris-Nankivel epithermal-porphyry belt is formed at the same time as Olympic Dam on the margins of a super caldera filled with upper Gawler Range Volcanics (GRV). Prior subduction tectonics produced precursor epithermal-porphyry conditions on the southern shoulder of the caldera, whereas IOCGs formed on the northern and eastern margins with haematite- or magnetite-dominated systems respectively forming on the shoulder or in hotter more reduced conditions under the GRV blanket within the caldera.

The epithermal/porphyry and IOCG belts are both fluorine-anomalous and connected by a conductive magneto-telluric (MT) corridor possibly representing a fossil transfer fault and metal source along the mantle interface.

Variations of a mid-GRV stratigraphic marker of the caldera collapse and OME are interpreted as the Bitalli Rhyolite at Paris, Nankivel palaeosurface and as lateral ferruginous sediments and volcanogenic conglomerate facies that collapsed into the IOCG systems preserved under the Stuart Shelf. Drill logs indicate the overlying Pandurra Formation is transitional in places with the mid-GRV marker, so the cover role of that unit needs reassessment with implications for deposit preservation and exploration.

The Zircon Alteration Index (ZAI = 40 – Zr/Hf) is a robust search tool that is universally applicable to the OME spectrum of deposit styles and hosts. Compared with other pathfinder applications, ZAI is a simpler, more reliable and far-seeing proximity tool requiring less assay samples in less drill holes for future targeting. The exclusive association of Hf with Zr in zircon enables wholerock analysis of the Zr/Hf ratio to measure the amount of hydrothermal overprinting of the inherited volcanic or detrital zircon in a host rock and hence proximity to a mineral target. Downhole variations of ZAI and comparisons with lithologies enables lateral or vertical target vectors to be often added to the proximity measure.

The ZAI tool and strato-tectonic vectoring have been validated for the Stuart Shelf IOCG systems with a comprehensive study of 35 holes with the requisite Zr and Hf analyses in varying proximities to known IOCG deposits. Preliminary target ranges are assigned to the ZAI values. Combined with the approach of structural and MT targeting, ZAI identified ten drilled prospects recommended for reconsideration by the tenement holders. The mineral systems targeting approach is also recommended to select areas elsewhere in the Gawler Craton for ZAI target analysis.


Biography

Geologist by training and mineral explorer seeking research developments with applications to discovery. Formerly Aberfoyle, MIM Exploration and founding Managing Director of Investigator Resources responsible for several mineral discoveries and subsequent mines. Current Chair of Advisory Board for the Institute of Mineral and Energy Resources at the University of Adelaide.

Cr-Zoning in pyroxene as a prospectivity indicator for magmatic Ni-Cu sulfide deposits

Schoneveld, Louise1; Barnes, Steve1, HV Makkonen5, M Le Vaillant1, D Paterson6, V Taranovic1, K-Y Wang2,3, Y-J Mao4

1CSIRO, Kensington, Australia, 2Chinese Academy of Sciences, China, 3University of Chinese Academy of Sciences, China, 4Chinese Academy of Sciences, China, 5Boliden FinnEx, Finland, 6Australian Synchrotron, ANSTO, Clayton, Australia

Small intrusions dominated by olivine- and pyroxene-rich cumulates are common in a variety of settings around the world, but only a very small proportion contain economically exploitable sulfides. We aim to provide a new tool for distinguishing these fertile intrusions from sparse exploration drilling.

Cumulate and poikilitic pyroxenes in strongly mineralised intrusions have complex grain-scale Cr zonation. We separate the zonations into three distinct types: 1) abrupt zoning; 2) sector zoning 3) oscillatory zoning. This combination of zoning patterns is likely to indicate high magma flux and fluctuating cooling rate that accompanies wall rock assimilation in the dynamic conduits where sulphide liquid forms and accumulates. As the diffusion of chromium is extremely slow, these zoning patterns can last thousands of years within hot terranes. 

We have investigated pyroxene-bearing samples from small intrusions containing magmatic sulphide deposits including the Noril’sk-Talnakh camp in Siberia, the Kotalahti Nickel Belt in Finland, Ntaka Hill in Tanzania, Nova-Bollinger in the Albany-Fraser Orogen and Savannah in the Halls Creek Orogen of Australia, Jinchuan in central China, Xiarihamu in Tibet and Huangshanxi in the east Tianshan Ni province of NW China. To compare, we analysed samples from the weakly mineralised or barren intrusions in four of these regions along with four mafic intrusions that are not associated with any economic sulfide mineralisation.

Cumulus orthopyroxene with a combination of abrupt zoning, sector zoning and resorbed olivine inclusions has so far only been detected in mineralised intrusions. Desktop XRF mapping instruments easily image this distinctive zoning pattern in large pyroxenes,which provides a useful fertility indicator for exploration of new magmatic Ni-Cu-(PGE) deposits.


Biography

Louise obtained her bachelors and honours in geology from James Cook University, Townsville and in 2018 graduated with a PhD from the Australian National University (ANU).  Currently she is focussed on investigating the trace element signatures in minerals as possible indicators for economic mineralisation.

About the GSA

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