Speaker event: "What magnetic measurements from satellites tell us about the Earth's crust" with Simon Williams

WHEN: 19th September, from 6 pm

WHERE: Earth Science Lecture theatre (nibbles beforehand from 5:30pm upstairs)

Abstract:

Measurements of variations in Earth’s magnetic field strength have made fundamental contributions to our knowledge of planetary evolution. Mapping of magnetic anomalies across the oceans, primarily by marine research voyages, has played a key role informing our knowledge of past plate motions, as well as the long-term evolution of the geodynamo. However, recent satellite missions are providing data which fills many remaining gaps in terrestrial coverage. This presentation will illustrate novel applications of lithospheric magnetization models derived from satellite data. One set of examples focusses on the oceans – here, long-wavelength magnetic anomalies reveal the nature of sloping magnetization boundaries and allow reinterpretation of past plate tectonic motions and seafloor age maps. The second set of examples focuses on subduction zones defined by seismicity and seismic tomography to understand the nature of magnetization within these domains. Anomalies at many subduction zones are well modeled by magnetization distributed across both the slab and mantle wedge, providing clues to the distribution of temperature and magnetic minerals deep within the Earth.

Simon Williams is an ARC Future Fellow at the Institute of Marine and Antarctic Studies, University of Tasmania. He obtained a PhD in geophysics from the University of Leeds, and worked at the University of Sydney and Northwest University in Xi’an. His research has concentrated both on marine geoscience and global-scale plate tectonics and geodynamics, and includes conducting research voyages  looking for fragments of lost continents submerged deep beneath the oceans. His current research focuses on the Earth’s magnetic field as an invaluable resource for studying the structure and dynamics of our planet.

Save the date - GSA film night 25th July!

The Mount Isa mineral province - architecture, regional metallic isotopes and metal sources

June 13th:  6:00 PM, School of Earth Sciences Lecture Theatre, University of Tasmania. 

The Mount Isa mineral province in northwestern Queensland and it extension into southeastern Northern Territory is one of the richest mineral provinces in the world, with three world-class Zn-Pb deposits and several major Cu (Au) deposits. Although there is some disagreement, most workers interpret that the Zn-Pb deposits formed early during basin development, whereas the Cu (Au) deposits formed much later, during and after the Isan Orogeny. Regional geological, geophysical, geochemical and isotopic data acquired over the last 20 years by Geoscience Australia and its collaborators has been used to: map major architectural features that controlled mineralisation at the regional scale; provide evidence of province-scale metal (Zn, Cu and Co) leaching that likely provided the metals to the ores; suggest that the mineralogy and temperature of alteration had a strong control on zinc leaching; and provide evidence of regional variations in Cu, Fe and Zn isotopes possibly related to regional fluid flow events.

A range of regional datasets (radiogenic isotopes, passive and active seismic, gravity, magnetic and magnetotelluric (MT)) have been used to identify crustal boundaries that have a strong control on the regional location of both the Zn-Pb and Cu (Au) deposits. The Gidyea Suture, as defined by active seismic data, marks the eastern margin of the province and is spatially associated with iron oxide-copper gold (IOCG) deposits of the Cloncurry district. This IOCG district is also spatially associated with a conductive zone defined from MT data. The Rufus Fault and associated faults mark the western margin of the province and are associated with gradients in radiogenic isotope and upward-continued gravity data, a gradient in the depth of the lithosphere-asthenosphere boundary and a resistive zone in MT data. McArthur-type zinc-lead deposits are strongly associated with this major crustal boundary. The Gidyea Suture and Rufus structure probably formed prior to the initiation of the North Australian Basin system, served as buttresses as the basin system evolved, and acted as foci for deformation and fluid low.

Following up on work by CODES in the 1990s, regional geochemical studies have identified significant changes in basinal rocks due to regional and pervasive fluids flow. Although most obvious in mafic volcanic rocks, this alteration has affected all rock types, changing not only the chemical composition of the rocks, but also their isotopic composition. Mafic rocks that range in age from 1790 Ma to 1720 Ma experienced extensive Zn, Cu and Co loss through the mineral province; back-of-the-envelop calculations indicate that the metal released from the mafic rocks is an order of magnitude or more greater than the metal known to be present in the ore deposits. Moreover, consideration of the mineralogy of the altered rocks and comparison with other mineral systems suggest that the mineralogy and temperature of hydrothermal alteration has a strong influence on the leaching of zinc. Our and previous studies by CODES indicate that the most intense zinc leaching is associated with alteration assemblages dominated by K-feldspar with minimal chlorite, whereas altered mafic rock dominated by chlorite has not lost zinc. Comparison of higher temperature alteration, chlorite-rich alteration systems suggests that low temperature chlorite retains zinc, whereas higher temperature chlorite loses zinc, which has a strong effect on the leachability of zinc from source rocks.

Finally, regional copper, iron and zinc isotope analyses of the regional basalts identify regional isotopic anomalies, which likely are related to mineralisation. A 25 km-50 km 65Cu-enriched zone is located to the east of the Mount Isa copper orebody. This anomaly is consistent with Rayleigh-type fractionation during regional alteration and copper leaching. The results indicate the likely size of the mineral system that produced the Mt Isa copper orebody and suggest that copper isotopes are potentially useful during regional- or province-scale exploration. Our results highlight the power of integrating data from a range of sources to develop regional understanding of mineral systems from the province- to the deposit-scale.

Biography:

Dr David Huston has had a long association with CODES. He completed his PhD at UTAS in 199* as Ross Large’s first PhD student. Now David has just retired from Geoscience Australia where he has spent nearly 30 years investigating the metallogenesis of Australia’s mineral deposits. He has worked throughout Australia and other countries on deposits that range in age from Paleoarchean to Tertiary, with experience with many different deposit types. Over the past ten years he has had a special interest between metallogenesis, tectonics and the evolution of Earth’s hydrosphere/atmosphere.