Distinguishing magmatic zircon from hydrothermal zircon: A case study from the Gidginbung high-sulphidation Au–Ag–(Cu) deposit, SE Australia

Zircons within the mineralized, silica-pyrite zone of the Gidginbung (Temora) high-sulphidation Au–Ag–(Cu) deposit in the Lachlan Orogen, Australia have been analyzed by both ion microprobe and laser ablation inductively coupled plasma mass spectrometry for titanium and rare earth element (REE) concentrations, as well as their oxygen and hafnium isotopic compositions. These zircons were previously interpreted to be hydrothermal in origin, but the newdata are indicative of a magmatic origin. The zircons exhibit chondrite-normalized REE patterns that are characterized by a steep positive slope from La to Lu with a large positive Ce-anomaly and a relatively small negative Eu-anomaly, similar to igneous zircons from the nearby Middledale Gabbroic Diorite and the Boggy Plain Zoned Pluton in the Lachlan Orogen and some hydrothermal zircons from the Mole Granite in the New England Orogen. Apparent temperatures calculated using the Ti-in-zircon thermometer for the Gidginbung zircons are 692° to 782 °C, significantly higher than the inferred ore-forming temperature (≤350 °C) but consistent with magmatic crystallization. The Gidginbung zircons have mantle-like δ18O values averaging 5.4±0.9‰ VSMOW (2 standard deviations, n=17), lower than those from the Barmedman granite (7.6±2.0‰, n=17) in the same area. The Gidginbung zircons also have εHf(t=435 Ma) values between 6.4±0.9 and 7.8±0.9 (2 standard errors), while the εHf(t=370 Ma) for zircons from the Barmedman granite is more variable, ranging from 5.1±1.1 to 8.4±0.8. Consideration of the data from this and previous studies indicates that the zircons in the Gidginbung deposit are indistinguishable from magmatic zircons with regards to their morphology, cathodoluminescence patterns, and chemical and isotopic signatures. The large measured oxygen isotopic fractionation between quartz and zircon (average: ∼10‰) indicates that the minerals did not form by the same process and/or from the same reservoir at T≥350 °C. While most inclusions in zircon are interpreted to have been trapped after zircon formation or resorption, it is possible that, like quartz, rutile, and anatase, a few pyrite inclusions occurring wholly within zircons have formed at relatively high temperatures during zircon crystallization in magma. Thus, the U–Pb age of these zircons (436 Ma) represents the igneous crystallization age and the Gidginbung zircons investigated by us are best interpreted as magmatic relicts of the earliest Silurian igneous rocks, which survived the processes of hydrothermal alteration and mineralization