Mineral system imaging from source to deposit using magnetotellurics
Date
2023
Authors
Kay, Ben Vincent
Editors
Advisors
Heinson, Graham
Thiel, Stephan (CSIRO)
Brand, Kate (BoM)
Thiel, Stephan (CSIRO)
Brand, Kate (BoM)
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Thesis
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Abstract
Modern mineral exploration is evolving to address the challenges of probing deeper into the Earth’s crust and locating concealed ore deposits. This transformation involves exploring greater depths and deposits hidden beneath thick post-mineralisation cover. The mineral exploration industry has embraced a mineral systems approach, considering ore deposits as part of larger Earth system processes with a focus on mass and energy interactions in the crust. Empirical methods in mineral exploration are giving way to a conceptual targeting approach, that delves into the fundamental geological processes governing element distribution, offering a more predictive exploration strategy by focusing on the whole mineral system. Mineral systems involve various critical elements operating at different spatial and temporal scales, including geodynamic processes, source regions of metals and fluids, conduits, geological forces, structural mechanisms, and processes at depositional traps. Geophysical methods, particularly electromagnetic (EM) techniques, are playing an increasing significant role in mineral exploration. EM methods offer a comprehensive view of the mineral system, from the lithosphere to the deposit, thanks to the wide range of electrical resistivity values in different rock types. Magnetotellurics (MT), a natural field EM method, has emerged as a valuable tool for imaging lithospheric properties. MT data provide insights into the Earth’s thermal and geochemical composition over geological time by providing insight into rock types, fluid pathways, and structural architecture, aiding in the understanding of subsurface conditions and potential mineral resources. Despite the potential of geophysical methods, scale-related challenges exist in exploration, often leading to undersampled target zones. Overcoming these challenges requires better spatial understanding and greater integration between geological and geophysical approaches. Keeping this perspective in mind, MT surveys were conducted across three distinct regions: (1) In the central Curnamona Province, where the presence of conductive post-mineralization cover has posed challenges to mineral exploration; (2) Across the Adelaide Rift Complex, encompassing historical sites like the Burra hydrothermal Cu deposit and Kapunda sedimentary copper deposit, aimed at gaining a more precise understanding of the fluid signatures within; (3) Over the Vulcan IOCG prospect, located 45 km northeast of Olympic Dam and concealed beneath 750 meters of postmineralization cover. Each of these MT surveys was strategically designed to address exploration challenges associated with greater depths and concealed deposits. They sought to enhance comprehension of mineral systems’ functioning beneath various spatial scales throughout the entire lithosphere. In Chapter 2, three-dimensional resistivity modeling of MT data in the Curnamona Province has identified the Curnamona Conductor, a significant geological feature with extremely low resistivities at shallow depths, extending over 200 km in a north-south orientation. This conductor, dipping westward beneath the Benagerie Suite Volcanics, shares similarities with other Paleoproterozoic conductors in Australia, suggesting they represent graphitic suture zones formed during a period of enhanced carbon burial between 2200 and 1850 Ma. In Chapter 3, in the Adelaide Superbasin, a MT survey and modeling were conducted to investigate the processes involved in entraining and focusing magmatic hydrothermal fluids in an intracontinental tectonic setting. The results reveal a contiguous zone of low electrical resistivity in the lower crust, extending to the surface, aligned with mineral deposits and geological features, suggesting the presence of a magmatically-hosted copper system influenced by low-temperature fluids and subsequent basin-wide fluid recycling. In Chapter 4, the presence of thick and electrically conductive ground cover poses challenges for mineral exploration in deep resource areas. A study of the Vulcan IOCG prospect used a 100-site broadband MT and passive seismic array to image the prospect’s physical properties and structural geometry, revealing distinct domains including stratigraphy in the cover sequences, brecciated hematite with lower resistivity, and a vertically conductive zone likely associated with graphite deposition from CO2-rich fluids generated by lower crustal magmatism and metamorphic processes.
School/Discipline
School of Physics, Chemistry and Earth Sciences
Dissertation Note
Thesis (Ph.D.) -- University of Adelaide, School of Physics, Chemistry and Earth Sciences, 2023
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