Integrated O, Fe, and Ti Isotopic Analysis Elucidates Multiple Metal and Fluid Sources for Magnetite from the Ernest Henry Iron Oxide Copper Gold (IOCG) Deposit, Queensland, Australia

Iron oxide copper gold (IOCG) deposits are globally important sources of Cu; however, their origins remain poorly understood with respect to the metal and fluid sources involved in their formation. In this work, we utilize integrated Fe, Ti, and O isotopic data for magnetite to trace major metal and fluid inputs for the Proterozoic-aged Ernest Henry IOCG deposit-the largest known IOCG deposit in the Cloncurry District, Queensland, Australia. Magnetite separates from ore stage and pre-ore biotite-magnetite (bt-mgt) and magnetite-actinolite (mgt-act) alteration assemblages from Ernest Henry were analyzed for their O, Fe, and Ti isotope abundances, reported as 818O (VSMOW), 856Fe (IRMM-14), and 849Ti (OL-Ti). Select magnetite samples were also analyzed for 17O (reported as Delta iota 17O0.5305) and range from-0.118 to-0.056 %o -suggesting evaporitic input. The 818O values from ore stage (+1.57 to +7.36 %o), bt-mgt (+0.34 to +5.68 %o), and mgt-act (+1.68 to +2.10 %o) samples are consistent with a magmatic-hydrothermal origin for ore and pre-ore mineralizing fluids at Ernest Henry; non -magmatic 818O values > c. 5 %o may be explained through localized carbonate wall rock assimilation. Despite this, 856Fe values for ore stage (-0.50 to +0.33 %o), bt-mgt (-0.65 to +0.38 %o), and mgt-act (-0.26 to +0.30 %o) magnetite are generally isotopically lighter than the accepted range (c. +0.06 to +0.49 %o) for igneous and magmatic-hydrothermal magnetite and exhibit a relatively large range of c. 1 %o, suggesting Fe source mixing within the deposit. Ore stage magnetite 849Ti (-1.64 to + 3.79 %o; avg: +1.49 %o; 2c5 = 2.63 %o) compositions are generally higher and more variable than either the bt-mgt (-0.44 to + 1.49 %o; avg: +0.62 %o; 2c5 = 1.13 %o) or mgt-act (+0.27 to + 1.89 %o; avg: +1.05 %o; 2c5 = 1.56 %o) and suggest that Ti isotope fractionation occurred due to differential mobility in the fluid. The data are best explained by models invoking both magmatic and non -magmatic metal and fluid input. Fluid flow channeled between the footwall and hanging wall shear zones introduced non-magmatic Fe that may have been leached from local mafic units by magmatic-hydrothermal fluids, resulting in neoformed and regeneratively replaced magnetite with magmatic 818O, but non-magmatic 856Fe values during pre-ore alteration. These fluids may have mixed with Fe-poor evaporitic fluids prior to magnetite formation. Later magmatic Fe and O contributions during ore stage mineralization resulted in magnetite with variable 856Fe and irresolvable 818O overprinting. Ernest Henry is the first known example of an IOCG deposit with a major leached metal component identified through metal stable isotope geochemistry.