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

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