The Great Bear magmatic zone (GBMZ) in the Northwest Territories, Canada, contains the NICO (Au-Co-Bi±Cu-W) and Sue Dianne (Cu-Ag-Au-±U-Co) deposits, and the DAMP and FAB prospects, all of which represent ironoxide dominated polymetallic systems with comparable alteration and mineralization styles similar to iron-oxide copper gold (IOCG) deposits in Chile and Australia. Also in the study area, is the vein hosted Nori/RA (Mo-U±Cu-REE), which has less affinity to the IOCG class of deposits. In

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... s work, trace and major element analyses, along with stable (C, H, S, O and Cu), radiogenic isotopes (Re-Os), and fluid inclusion studies were used to constrain the origin of the IOCG systems, and the Nori/RA showing, and to suggest the potential of magnetite as a mineral indicator in till sampling exploration surveys. Regionally, the Cr/Co ratio is higher in barren and pre-ore magnetite compared to that of magnetite co-precipitated with ore minerals and/or present in ore-rich veins and breccias. At DAMP and Sue Dianne the Co/Ni ratio is extremely high and clearly different from those of other GBMZ magnetite samples. Collectively, the analytical data supports a magmatic-hydrothermal origin for all these systems that are temporally associated with the emplacement of the Great Bear volcanic arc between 1875 and 1865 Ma. However, some sulfur, copper and arsenic could have been recycled from carbonate-rich iii metasedimentary and rocks and incorporated in the ore-bearing fluids as is suggested for the NICO deposit. Some copper might have also been leached from felsic volcanic rocks and metasedimentary rocks and incorporated into the GBMZ mineralized systems. In NICO deposit, Ca-Na-Bi(Au?)-bearing, and saline (~20 wt.% NaCl equiv + CaCl 2 equiv) to hyper-saline aqueous fluids (>42 wt. % NaCl equiv.) formed at 4 to 8 km depth. The Au mineralization in this system occurred via: (i) remobilization of refractory gold and Bi(?) from Fe-S-As-S mineral phases during re-crystallization of the latter, and (ii) scavenging of Au by Bi melts from co-existing hydrothermal fluids. Despite the stable isotopes signature, and the geochronological constraints, mass balance calculations shows that felsic volcanic and metasedimentary rocks hosting these deposits are feasible sources of metals (e.g., Au, Co and Cu) for the GBMZ IOCG systems. However, for this to be true, it would require high permeabilities and efficient fluids capable of leaching, concentrating and reprecipitating metals at a single site, and a suitable hydraulic regime. iv