MAGNETITE TRACE ELEMENT CHEMISTRY IN ARC ROCKS: A PERSPECTIVE FROM ALASKAN-TYPE ULTRAMAFIC-MAFIC INTRUSIONS WITHIN THE CANADIAN CORDILLERA

The chemistry of magnetite (Fe₃O₄) is commonly used as a mineralization indicator in ore-forming systems, and trace element variations have the potential to provide new insights into the evolution of mantle-derived magmas that reach magnetite saturation. To assess the significance of magnetite crystallization in arc magmas, Late Triassic to Early Jurassic Alaskan-type ultramafic-mafic intrusions in British Columbia (Hickman, Polaris, Tulameen, Turnagain) within the accreted arc terranes of the North American Cordillera are being investigated. Magmatic magnetite is a common accessory phase in several lithologies (clinopyroxenite, hornblendite, gabbro-diorite), but is most abundant (~10 vol%) in hornblende-rich rocks and locally forms bands and pods of magnetitite (~70 vol%) within clinopyroxenite and hornblendite. Common textures indicate both magmatic (e.g., net-textured interstitial grains with abundant ulvöspinel laths) and subsolidus (e.g., oxyexsolved ilmenite) magnetite crystallization. Major and trace element compositions, acquired by EPMA and LA-ICP-MS, are heavily influenced by the presence of exsolution and mineral inclusions, and exsolved grains were reintegrated to original magmatic compositions by EPMA X-ray mapping. Concentrations of the most compatible elements (Cr, Ni) in magnetite decrease by orders of magnitude from early- (olivine-bearing clinopyroxenite) to late-formed (hornblendite) rocks, effectively tracking fractional crystallization of olivine and clinopyroxene in these intrusions. Magnetite from Turnagain, host to a significant nickel sulfide deposit, is characterized by low concentrations of chalcophile elements (e.g., Co, Ni, Cu) compared to other intrusions and reflects depletion of these elements caused by early sulfide saturation. Higher concentrations of incompatible elements (e.g., Al, Ge, Cu, Mn) in magnetite from Polaris and Hickman compared to Tulameen and Turnagain indicate the relatively early timing of magnetite saturation in these intrusions, highlighting the variable evolutionary pathways of these magmatic arc systems. Our findings demonstrate that magnetite geochemistry is an effective petrogenetic tool for understanding the evolution of arc magmas within the North American Cordillera, with applications to igneous-derived magnetite in convergent margin settings globally.