Iron oxide-copper-gold (IOCG) deposits formed by magmatic-hydrothermal fluids (MH-IOCG) share many similarities with, but have important differences from, porphyry Cu ± Mo ± Au (porphyry) deposits: MH-IOCG deposits predominantly occur in Precambrian rocks, are Fe oxide rich, and have volumetrically extensive high-temperature alteration zones, whereas porphyry deposits occur almost exclusively in Phanerozoic rocks, are Fe sulfide rich, and have narrower high-temperature alteration zones. We propose that these deposit types are linked by common subduction-modified magmatic sources, but that secular changes in oceanic sulfate content and geothermal gradients at the end of the Precambrian caused a transition from the predominance of S-poor arc magmas and associated S-poor MH-IOCG systems, to S-rich arc magmas and associated S-rich porphyry deposits in the Phanerozoic. Phanerozoic MH-IOCG and rare Precambrian porphyry deposits are explained by local or periodic fluctuations in oceanic oxidation state and sulfate content, or remobilization of previously subduction-modified lithosphere in post-subduction tectonic settings.

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