Tracing the Origin and Hydrothermal Overprint of Magnetite-Apatite Mineralization in the New Afton Cu-Au ± PGE Alkalic Porphyry Deposit: Insights from Magnetite Textures and Chemistry

Alkalic porphyry deposits are significant sources of Cu-Au ± platinum group elements (PGE), with magnetite being particularly abundant in the Triassic alkalic systems of British Columbia, Canada. The New Afton area hosts both Cu-Au ± PGE mineralization and several massive magnetite lodes, which were historically mined for iron. The origin of these magnetite orebodies and their relationship to alkalic porphyry mineralization remains unclear. This study integrates field observations and magnetite chemistry and textures to elucidate their formation. We identify two stages of magnetite mineralization: an early high-temperature (>500°C) magmatic-hydrothermal stage, characterized by ilmenite exsolution lamellae, high-Ti magnetite, and symplectitic intergrowths with ilmenite, followed by a later stage of hydrothermal reequilibration that precipitated ilmenite-free Ti-depleted magnetite. Rather than preserving its original crystallization signature, magnetite records a complex history of hydrothermal activity, including Cu-Au-PGE-bearing magmatic-hydrothermal fluid circulation and repeated, superimposed coupled dissolution-reprecipitation events. These findings align with those from other magmatic-hydrothermal ore systems such as iron oxide copper-gold and Kiruna-type iron oxide-apatite deposits, emphasizing the widespread nature of magnetite reequilibration and the need for caution when using magnetite as a petrogenetic indicator.