Iron Oxide Copper-Gold Deposits<subtitle>Geology, Space-Time Distribution, and Possible Modes of Origin</subtitle>

Abstract Many diverse ore systems are classified together as iron oxide copper-gold (IOCG) deposits based on an empirical definition arising primarily from geochemical features that do not specify tectonic setting, geologic environment, or sources of ore-forming fluid, metals, or other ore components. Such deposits have (1) Cu, with or without Au, as economic metals; (2) hydrothermal ore styles and strong structural controls; (3) abundant magnetite and/or hematite; (4) Fe oxides with Fe/Ti greater those in most igneous rocks and bulk crust; and (5) no clear spatial associations with igneous intrusions as, for example, displayed by porphyry and skarn ore deposits. IOCG deposits commonly have a space-time association with Kiruna-type apatite-bearing oxide Fe ores and many examples of the latter contain sulfide minerals, Cu, and Au. Most IOCG deposits display a broad space-time association with batholithic granitoids, occur in crustal settings with very extensive and commonly pervasive alkali metasomatism, and many are enriched in a distinctive, geochemically diverse suite of minor elements including various combinations of F, P, Co, Ni, As, Mo, Ag, Ba, LREE, and U. Iron oxide Cu-Au systems are numerous and widely distributed in space and time; they occur on all continents and range in age from the present at least back into the Late Archean. In economic terms, the most important IOCG deposits are those in the Carajás district, Brazil (Archean, Amazon craton); in the Gawler craton and Cloncurry districts, Australia (late Paleoproterozoic to Mesoproterozoic debated intracratonic or distal subduction-related settings), and in the Jurassic-Cretaceous extended continental margin arc of the coastal batholithic belt in Chile and Peru. IOCG deposits and associated features define distinct metallogenic belts in which other types of Cu and Au deposits are rare or absent. The largest deposits include Salobo, Cristallino, Sossego, and Alemão (Carajás), Olympic Dam (Gawler craton), Ernest Henry (Cloncurry district), and Candelaria-Punta del Cobre and Manto Verde (Chile), and have resources greater than 100 million metric tons (Mt), ranging up to more than 1,000 Mt with metal grades that exceed those in most porphyry-style Cu ± Au deposits. A comparison of larger and well-described IOCG deposits illustrates the geologic diversity of the class as a whole. They occur in a wide range of different host rocks, among which plutonic granitoids, andesitic (meta)volcanic rocks, and (meta)siliclastic-metabasic rock associations are particularly prominent. Host rocks may be broadly similar in age to the ore (e.g., Olympic Dam, Candelaria-Punta del Cobre, Raul-Condestable) but in other cases significantly predate mineralization such that ore formation relates to a quite separate geologic event (e.g., Salobo, Ernest Henry). Mineralization is interpreted to have occurred over a wide depth range, from around 10 km (e.g., several deposits in the Cloncurry district) to close to the surface (e.g., Olympic Dam); where systems have been tilted and exposed in cross section (such as at Raúl-Condestable in Peru), they can display strongly zoned mineral parageneses. Structural and/or stratigraphic controls are pronounced, with deposits characteristically localized on fault bends and intersections, shear zones, rock contacts, or breccia bodies, or as lithology-controlled replacements. Host rocks in the vicinity of orebodies display intense hydrothermal alteration. In the immediate vicinity of the ore, the variable pressure-temperature conditions of alteration and mineralization are reflected in a spectrum of deposits ranging from those in which the dominant Fe oxide is magnetite and alteration is characterized by minerals such as biotite, K-feldspar, and amphibole through to hematite-dominated systems in which the main silicate alteration phases are sericite and chlorite. Where present, Na and Na-Ca alteration tends to be developed deeper or more distal from ore, is more extensive, and commonly predates K-Fe alteration and mineralization. Carbonates are commonly abundant, particularly in association with, or postdating, Cu-bearing sulfides that tend to be paragenetically late and postdate high-temperature silicate alteration in the deeper seated deposits. Independent variation in fO2- fS2-(T) conditions during mineralization produced deposits ranging from pyrite-poor examples, with complex Cu mineral associations, including chalcopyrite, bornite, and chalcocite (e.g., Salobo, Olympic Dam), to others in which pyrite and chalcopyrite are the main sulfides (e.g., Ernest Henry, Candelaria). Fluid inclusion evidence suggests that geochemically complex brines, commonly with a carbonic component, were involved in IOCG genesis. However, the ultimate sources of water, CO2, metals, sulfur, and salinity have yet to be well constrained, and it is possible that these components may have different origins from deposit to deposit. Brines and metals may be sourced directly from underlying magmas, indirectly by interaction of mag-matic fluids with country rocks or other fluids, or independently through modification of basinal or metamor-phic fluids. Ore deposition may primarily involve interaction of voluminous fluid with wall rocks and cooling. However, several studies have emphasized the role of mixing sulfur-poor, metal-rich brines with sulfur-bearing fluids at the site of ore deposition, although characterization of the causative fluids has proven problematic. Uncertainty also exists about the original tectonic settings of several major IOCG districts, and considerably more research is needed before it will be clear whether these deposits are linked by a single family of related genetic mechanisms or whether they can form in a range of fundamentally different geologic environments from fluids of different sources.

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