Tracking the Evolution of Mineral Systems Using Apatite Petrochronology and Geochemistry: A Case Study from the Ernest Henry Iron Oxide Copper-Gold Deposit, Australia

Abstract Understanding multistage alteration and mineralization in iron oxide copper-gold (IOCG) systems remains a challenge in economic geology, requiring robust chronological constraints combined with microstructural and geochemical data. This study uses apatite petrochronology and geochemistry to identify major hydrothermal alteration stages and mineralization at the Ernest Henry deposit, one of Australia’s premier copper-gold deposits and a global-type locality for magnetite-dominant IOCG systems. Petrographic observations, in situ U-Pb geochronology, and compositional analyses of apatite reveal a sequence of distinct events reflecting multiple post-peak metamorphism deformation events of the Isan orogeny and magmatism: regional Na alteration, regional Na-Ca alteration, pre-ore high-temperature K-Fe(-Mn) alteration, syn-ore K-Fe alteration and ore deposition, and post-ore alteration. These apatite groups from within and around the deposit exhibit systematic geochemical variations, notably an increase in F, S, As, and Ba over time, and a decrease in Na and Cl, reflecting physicochemical evolution of hydrothermal fluids (which could serve as constraints on metal distribution models) and demonstrating that economic mineralization is a product of a dynamic, multistage process occurring over extended periods. The integration of apatite petrochronology and geochemistry not only unravels the temporal and chemical complexity of the Ernest Henry IOCG system but is applicable to investigating IOCG deposits globally.