Lu, Yongjun1, Smithies, RH1, Champion, DC2, Wingate, MTD1, Johnson, SP1, Martin, L3, Jeon, H4, Poujol, M5, Zhao, J6, Maas, R7, Creaser, RA8
1Geological Survey of Western Australia, 100 Plain Street, East Perth, WA 6004, 2Geoscience Australia, GPO Box 378, Canberra ACT 2601, 3Centre for Microscopy, Characterisation, and Analysis, University of Western Australia, Perth, WA 6009, 4Swedish Museum of Natural History, Box 50 007, SE-104 05 Stockholm, Sweden, 5Univ Rennes, CNRS, Géosciences Rennes – UMR 6118, 35000 Rennes, France, 6Radiogenic Isotope Facility, School of Earth Sciences, The University of Queensland, Brisbane, QLD 4072, Australia, 7School of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia, 8Dept. Earth & Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada
Multi-isotope maps can characterise lithospheric architecture through time, play an increasingly important role in predictive exploration targeting, and are consequently sought-after datasets by industry. We present the first zircon oxygen isotope map and an updated whole-rock Sm–Nd isotope map of Western Australia. These shed new light on crustal evolution and mineral system distributions. The isotope maps were generated from datasets that are subject to ongoing updates as new data are generated and compiled.
Median zircon δ18O values for about 125 igneous rocks have been spatially visualized so far, and coverage currently extends across the Pilbara and Yilgarn Cratons, the Capricorn, Paterson, and Albany-Fraser Orogens and the Eucla basement (Madura and Coompana Provinces). The Pilbara and Yilgarn Cratons are dominated by mantle-like δ18O values (4.7 – 5.9‰), consistent with reworking of igneous material that had not been exposed at the surface. A c. 3.47 Ga diorite, four c. 3.3 Ga hornblende-bearing granitic rocks, and a c. 2.95 Ga hornblende monzogranite in the Pilbara Craton exhibit weakly elevated zircon δ18O values (5.9 – 6.5‰), which together with trace element enrichment were attributed to hydrous sanukitoids or to derivation from a sanukitoid-enriched source. The Capricorn, Paterson, and Albany–Fraser Orogens and the Eucla basement also contain rocks with elevated δ18O values (6.6 – 9.9‰), suggesting significant reworking of upper crustal material during magma genesis. Zircons with sub-mantle δ18O values (<4.7‰) were found for granitic rocks of c. 3.55 Ga in the Sylvania Inlier, of c. 3.44 Ga in the northern Pilbara Craton, and of c. 3.0 and 2.67 Ga in the South West Terrane, suggesting recycling of crustal material subjected to high-temperature hydrothermal alteration, such as observed in post-Archean rift systems or calderas.
Sm–Nd isotopes for about 1120 felsic igneous rocks provide regionally extensive images of crustal architecture. The map of two-stage depleted mantle model ages (TDM2) highlights the distinction between Archean cratons (TDM2 >2.6 Ga) and Proterozoic orogens (TDM2 <2.2 Ga), and isotopic boundaries correlate well with most existing proposed terrane boundaries. However, the isotopic boundary between the South West Terrane and the Youanmi Terrane appears to be about 100 km west of the previously proposed boundary, but correlates well with magnetic and gravity anomaly zones and the distribution of gold mineralization. The crustal residence map highlights predominantly short residence times (<0.5 Ga) for the Pilbara and Yilgarn Cratons, and much longer crustal residence times (>0.8 Ga) in the Paterson, Albany–Fraser, Pinjarra and Capricorn Orogens, suggesting decreased juvenile crust generation in these orogens.
These maps are directly applicable to metallogeny. For example, most giant gold deposits in WA are located on or near significant isotopic boundaries and tectonic structures. Interestingly, Telfer, Plutonic and giant gold deposits in the Murchison are aligned along a northeast-trending isotopic boundary. Similar boundaries occur between the eastern and western parts of the Pilbara Craton and between the Yilgarn Craton and the Albany–Fraser Orogen. These isotopically defined discontinuities may be important clues to the earliest architectural elements in Western Australia.
Dr. Yongjun Lu is Senior Geochronologist Isotope Geologist at GSWA. Yongjun has over 15 years’ geological experience, extensive collaboration with industry, government and academia. He has made important contributions to mineral systems science, highlighted by being the 52nd recipient of the Waldemar Lindgren Award of Society of Economic Geologists (SEG).