Iron Oxide(-Cu-Au-REE-P-Ag-U-Co) Systems

The Fe oxide(-Cu-Au-REE-P-U) family of Cu, Fe, and/or Au deposits (or IOCG) represents a geochemically coherent but geologically diverse group that formed globally from the Archean to the Holocene. IOCG systems exhibit intense, voluminous Na-Ca-K-Fe(-H) hydrothermal alteration related to flow of moderately to highly saline metal-rich, sulfur-poor brines. These fluids account for the characteristic sulfide-poor, oxide-rich mineralogy and the alkali-rich character of the alteration and for the varied contents of Cu, Au, and other metals. Associated igneous rocks range from mafic to felsic, subalkaline to alkaline. Metal enrichments vary with host-rock type and sulfur availability. Geologic settings are tectonically diverse but commonly have evidence for contemporaneous or older evaporitic environments. Magmatism drives most systems, yet clearly amagmatic examples occur. Geochemical and petrologic studies demonstrate igneous-dominated sources for some solutes and permissive evidence for a connection to magmatic fluids. In many cases, a central role for nonmagmatic saline fluids is evident. The geochemistry of the latter fluids rationalizes the key distinguishing features of the IOCG family. The diversity of the IOCG family parallels that seen in other major families of deposits; their distinctive attributes indicate that they comprise a separate class of (mainly) terrestrial hydrothermal systems.