Abstract Iron oxide copper-gold (IOCG) and Kiruna-type iron oxide-apatite (IOA) deposits are commonly spatially and temporally associated with one another, and with coeval magmatism. Here, we use trace element concentrations in magnetite and pyrite, Fe and O stable isotope abundances of magnetite and hematite, H isotopes of magnetite and actinolite, and Re-Os systematics of magnetite from the Los Colorados Kiruna-type IOA deposit in the Chilean iron belt to develop a new genetic model that explains IOCG and IOA deposits as a continuum produced by a combination of igneous and magmatic-hydrothermal processes. The concentrations of [Al + Mn] and [Ti + V] are highest in magnetite cores and decrease systematically from core to rim, consistent with growth of magnetite cores from a silicate melt, and rims from a cooling magmatic-hydrothermal fluid. Almost all bulk δ 1 8 O values in magnetite are within the range of 0 to 5‰, and bulk δ 56 Fe for magnetite are within the range 0 to 0.8‰ of Fe isotopes, both of which indicate a magmatic source for O and Fe. The values of δ 1 8 O and δ D for actinolite, which is paragenetically equivalent to magnetite, are, respectively, 6.46 ± 0.56 and −59.3 ± 1.7‰, indicative of a mantle source. Pyrite grains consistently yield Co/Ni ratios that exceed unity, and imply precipitation of pyrite from an ore fluid evolved from an intermediate to mafic magma. The calculated initial 187 Os/ 188 Os ratio (Os i ) for magnetite from Los Colorados is 1.2, overlapping Os i values for Chilean porphyry-Cu deposits, and consistent with an origin from juvenile magma. Together, the data are consistent with a geologic model wherein (1) magnetite microlites crystallize as a near-liquidus phase from an intermediate to mafic silicate melt; (2) magnetite microlites serve as nucleation sites for fluid bubbles and promote volatile saturation of the melt; (3) the volatile phase coalesces and encapsulates magnetite microlites to form a magnetite-fluid suspension; (4) the suspension scavenges Fe, Cu, Au, S, Cl, P, and rare earth elements (REE) from the melt; (5) the suspension ascends from the host magma during regional extension; (6) as the suspension ascends, originally igneous magnetite microlites grow larger by sourcing Fe from the cooling magmatic-hydrothermal fluid; (7) in deep-seated crustal faults, magnetite crystals are deposited to form a Kiruna-type IOA deposit due to decompression of the magnetite-fluid suspension; and (8) the further ascending fluid transports Fe, Cu, Au, and S to shallower levels or lateral distal zones of the system where hematite, magnetite, and sulfides precipitate to form IOCG deposits. The model explains the globally observed temporal and spatial relationship between magmatism and IOA and IOCG deposits, and provides a valuable conceptual framework to define exploration strategies.

Abstract This paper deals with the earliest stages of formation of porphyry Cu deposits, starting with the production of partial melts in the mantle and ending where upper crustal magmas reach their solidus and potentially have exsolved a metal-bearing hydrothermal fluid. During all these stages magmatic sulfides exert a major control on the budget of ore-forming metals in the magma. High metal concentrations in mafic arc magmas are favored by low degrees of partial melting in the mantle source region, and by limited removal (or effective redissolution) of magmatic sulfides in the lower crust. Ascending magmas accumulate in large, compositionally stratified magma chambers in the upper crust (5- to 15-km depth), which represent the exsolution source of the mineralizing fluids for the shallower porphyry Cu deposits. Interaction between mafic and felsic magmas in these magma chambers leads to partial mingling/mixing, volatile release, and the formation of magmatic sulfides that incorporate large amounts of Cu and Au, but only little Mo. For porphyry Cu mineralization, it is essential that these magmatic sulfides are subsequently destroyed and thereby release their contained metals to the mineralizing fluids. Evidence from experimental phase equilibria studies and melt inclusions hosted in phenocrysts from plutonic and volcanic rocks in arc environments, combined with fluid inclusion evidence from porphyry Cu deposits, suggest that silicate melts that ultimately give rise to porphyry Cu deposits are likely saturated first with a CO 2 -rich fluid and later give way to single-phase, low-salinity (typically 5–10 wt % NaCl equiv) aqueous fluids. At the typical f O2 conditions of porphyry Cu-forming magmas (ΔFMQ + 1 to ΔFMQ + 3), sulfur occurs mostly as SO 2 in the fluid. Efficient Cu removal from the magma into the overlying porphyry environment is favored by the exsolution of an S-bearing volatile phase that has a low HCl/alkali chloride ratio. The ability of the ore fluid to scavenge and transport Cu increases with increasing f O2 and the concentration of K in the aqueous fluid, and may be maximized at high ratios of SO 2 /H 2 S of the fluid. Once formed, efficient focusing of the ore fluid into the upper portions of the magma chamber may be favored by the development of permeable melt channels that act as conduits for the ascent of ore fluid in a pressure gradient through the crystallizing magma. These conduits likely facilitate the contribution of S, Cu, and other metals from mafic silicate melt that ponds at deeper levels of the magma system.