Complex Petrogenesis of Porphyry-Related Magmas in the Cowal District, Australia: Insights from LA ICP-MS Zircon Imaging

Zircon-based laser ablation inductively coupled plasma-mass spectrometry (LA ICP-MS) trace element imaging and spot analysis methods are used to investigate the petrogenesis of porphyry Cu-related magmas in the Cowal district, Australia. These data discriminate premineralization magmas from synmineralization fertile magmas and provide insights into a complex petrogenesis that culminated in low-temperature, hydrous, and oxidized magma compositions.

The intragrain trace element distribution in zircons from synmineralization intrusions spatially correlates with abrupt changes in zircon textures demarked by dissolution surfaces. Mapping of key fractionation, temperature, hygrometer, and oxybarometer indices (e.g., Th/U, Gd/Yb, (Ce/Nd)/Yb, and Eu/Eu*) show in some cases that multiple crystal-fractionation events, including the cofractionation of titanite, apatite, and hornblende, are recorded in single zircon grains spatially separated by dissolution surfaces. These resorbed boundaries are interpreted to record periods of discrete magma recharge events that affected the temperature and trace element budget of the magma. In many cases, the highest relative magmatic water content and highest oxidation signature are concentrated near the crystal rims, which suggests magmatic fertility increased late during the evolution of these magmas - evidently triggered as a result of magma recharge. An evolution to cooler, hydrous, oxidized conditions with transient periods of magma recharge and higher temperature conditions are considered prerequisites for fertile magma petrogenesis in the Cowal district. The relationship between zircon complexity and fertility should be a useful tool for zircon-based fertility studies informed by cathodoluminescence imagery.