Wade, Claire1, 2, Payne, Justin3, Barovich, Karin2, Gilbert, Sarah4, Wade, Benjamin4 , Crowley, James5, Reid, Anthony1, 2 and Jagodzinski, Elizabeth1
1Geological Survey Of South Australia, , Australia, 2Department of Earth Sciences, University of Adelaide, Adelaide, Australia, 3UNISA STEM, University of South Australia, Adelaide, Australia, 4Adelaide Microscopy, University of Adelaide, Adelaide, Australia, 5Department of Geosciences, Boise State University, Boise, United States of America
The trace element signatures of zircon from Phanerozoic porphyry-related magmatic rocks and their mineralising systems have recently been applied as a means to assess potential magmatic suite fertility. The iron oxide-copper-gold (IOCG) and iron oxide-apatite (IOA) deposit family share some genetic attributes with porphyry Cu deposits, including subduction-modified magmatic sources, association with calc-alkaline to mildly alkaline magmas, and highly oxidised magmas. The observed relationship between magma fertility and zircon chemistry in porphyry Cu deposits raises the possibility that the trace element signature of zircon could also be used to assess the fertility of magmatic systems associated with IOCG and IOA systems.
Significant IOCG deposits in southern Australia (Gawler Craton) and IOA deposits in the south-central USA are associated with extrusive and intrusive felsic rocks formed as part of silicic large igneous province magmatism. Zircons from early rhyolitic and granitoid rocks coeval with IOCG mineralisation in the Gawler Craton are distinguished from younger rhyolite and granitoid zircons by their higher Eu/Eu*, Ce/Ce* and Ti values and separate magma evolution paths with respect to Hf. Higher zircon Ce/Ce* and Eu/Eu* correspond to more oxidising magmatic conditions and lower degrees of fractionation and/or crustal assimilation, respectively. Higher zircon Ti contents correspond to higher magmatic temperatures in the magmas coeval with mineralisation. In this respect, we consider higher oxidation state, lower degrees of fractionation and higher magmatic temperatures to be features of fertile magmas in southern Australian IOCG terrains.
Similar zircon REE characteristics are shared between Australian IOCG magmatic rocks and IOA rhyolites from the St Francois Mountains, Missouri. IOCG and IOA magmatic rocks have high Ce/Ce* and high Dy/Yb ratios in zircon, which are indicative of oxidised and dry magmas, respectively. Syn-mineralisation IOCG and IOA magmatic rocks are distinguished from unmineralised ones by their higher Eu/Eu* zircon signature, and higher magmatic temperatures. Zircon Dy/Yb are generally higher and Eu/Eu* are generally lower in IOCG and IOA magmatic rocks when compared with fertile porphyry Cu deposit magmatic rocks. The dry and more fractionated nature of the IOCG and IOA associated magmas contrast with the hydrous and unfractionated nature of fertile porphyry Cu deposit magmas, highlighting differences in setting and magma formation of porphyry Cu deposits and the IOCG-IOA deposit family. As indicated by high zircon Ce/Ce* ratios, the oxidised nature of mineralised IOA magmatic rocks coupled with lower degrees of fractionation and higher magmatic temperatures, are akin to fertile IOCG magmatic rocks and considered to be key elements in magma fertility in IOCG-IOA terrains.
Claire is an employee of the Geological Survey of South Australia undertaking a PhD at the University of Adelaide. Her background is in igneous geochemistry and isotope geochemistry. Her PhD is investigating the link between magmatism and mineralisation in Mesoproterozoic mineral systems in South Australia.