Morrissey, Laura1,2,4, Payne, Justin3,2,4, Hand, Martin4,2, Bockmann, Mitchell4,2, Yu, Jie4,2, Reid, Anthony5
1Future Industries Institute, University Of South Australia, Adelaide, Australia, 2Mineral Exploration Cooperative Research Centre, Adelaide, Australia, 3UniSA STEM, University of South Australia, Adelaide, Australia, 4Department of Earth Sciences, University of Adelaide, Adelaide, Australia, 5Department for Energy and Mining, Geological Survey of South Australia, Adelaide, Australia
The Gawler Craton is commonly considered to record a complex tectono-metamorphic and magmatic history from the Archean to the Mesoproterozoic, culminating in voluminous magmatism of the Hiltaba Suite and the Gawler Range Volcanics (GRV) between 1600–1570 Ma. Although there is widespread evidence for tectono-metamorphic and magmatic events younger than 1570 Ma, these commonly receive little attention because they are enigmatic, occur in poorly outcropping parts of the craton and are difficult to correlate regionally. In addition, because the Hiltaba–GRV event was associated with the development of large Iron-Oxide-Copper-Gold (IOCG) and Au deposits it holds the most interest for mineral exploration, with the younger events considered to be of little importance. However, these younger events have potential for both new addition of metals and remobilisation of existing deposits, and therefore understanding their character and expression is vital to create a holistic mineral systems framework for the Gawler Craton.
Reanalysis of legacy drill holes using modern geochronologic and metamorphic techniques has led to the identification of younger metamorphic events between c. 1570–1550 Ma, c. 1530–1520 Ma and 1500–1400 Ma in the northern Gawler Craton. In the Mount Woods region, monazite age populations of c. 1570 Ma and c. 1550 Ma are interpreted to record a phase of high thermal gradient metamorphism, deformation and mafic magmatism that post-dates Hiltaba-aged granitic magmatism in this region. Similar metamorphic ages between c. 1570–1550 Ma elsewhere in the northern Gawler Craton, Yorke Peninsula and Barossa Complex suggests that this event is widespread across the Gawler Craton. In the Peake and Denison Inlier, calcic alteration has been dated at 1530 Ma, and is approximately coeval with migmatisation and metamorphism in the Nawa Domain at c. 1520 Ma. Magmatic rocks with inferred ages of c. 1520 Ma in the northern Gawler contain marialitic cavities, suggesting that they intruded at relatively shallow depths. Post 1500 Ma, the Gawler Craton is thought to record only minor sedimentation such as the intra-continental Pandurra Formation. However, A-type magmatism and high thermal gradient metamorphism occur at c. 1450 Ma in the Nawa Domain, and reactivation of lithospheric-scale shear zones occurs between 1470–1450 Ma. The crustal-scale Karari Shear zone and Cairn Hill Fe-Cu deposit also record monazite growth at c. 1490–1480 Ma.
The high thermal gradients recorded in the metamorphic rocks, combined with a lack of evidence for significant exhumation or dissection of the GRV in the central Gawler Craton, suggests that all these younger events record periods of extension. The locus of extension and shear zone reactivation may have migrated through time, leading to apparently discrete zones of reworking. These extensional events have the potential to drive fluid flow events along reactivated shear zones (derived from overlying sediments such as the Pandurra Formation, or from magmas intruded at depth), as well as add new metals to the crust. This, with the evidence for young metamorphism/alteration at Cairn Hill, suggests that the Hiltaba Event may not be the final mineralising event in the Gawler Craton.
Laura Morrissey completed a PhD in metamorphic petrology and tectonics at the University of Adelaide in 2016. She is now a research fellow at the University of South Australia and is working in the MinEX CRC.