Zircon Trace Element Geochemistry as an Indicator of Magma Fertility in Iron Oxide Copper-Gold Provinces

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Extrusive and intrusive felsic magmas occur throughout the evolution of silicic-dominated large igneous province magmatism that is temporally related to numerous economically significant iron oxide copper-gold (IOCG) deposits in southern Australia. We investigate zircon trace element signatures of the felsic magmas to assess whether zircon composition can be related to fertility of the volcanic and intrusive suites within IOCG-hosted mineral provinces. Consistent with zircon forming in oxidizing magmatic conditions, the rare earth element (REE) patterns of zircon sourced from both extrusive and intrusive magmatic rocks are characterized by light REE depletions and a range of positive Ce and negative Eu anomalies. The timing of the major phase of IOCG mineralization overlaps with the early part of the first phase of Lower Gawler Range Volcanics magmatism (1593.6–1590.4 Ma) and older intrusive magmatism of the Hiltaba Suite (1593.06–1590.50 Ma). Zircon in these mineralization-related intrusives and extrusives is distinguished from zircon in younger, mineralization-absent rocks by higher Eu/Eu*, Ce/Ce*, and Ti values and separate magma evolution paths with respect to Hf. These zircon characteristics correspond to lower degrees of fractionation and/or crustal assimilation, more oxidizing magmatic conditions, and higher magmatic temperatures, respectively, in magmas coeval with mineralization.

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 magmas associated with southern Australian IOCG and iron oxide-apatite (IOA) rhyolites from the St. Francois Mountains, Missouri, namely high Ce/Ce* and high Dy/Yb, indicative of oxidized and dry magmas, respectively. The dry and more fractionated nature of the IOCG- and IOA-associated magmas contrasts with the hydrous and unfractionated nature of fertile porphyry Cu deposit magmas. As indicated by high Ce/Ce* ratios, the oxidized nature is considered a key element in magma fertility in IOCG-IOA terrains. In both IOCG and IOA terrains, the trace element compositions of zircon are able to broadly differentiate fertile from nonfertile magmatic rocks.