IOCG deposits are a distinct class of mineral deposit characterized by an abundance of magnetite, hematite and other Fe-bearing minerals. Economic mineralization consists of one or more of the following; Fe, Cu, Au, U, rare earth elements (REE), Ag, Co, Bi, Mo, and generally contains enrichments in S, As, Ni, Sn, F, P, W, and Hg. Sites of mineralization are intensely Fe- and K-metasomatized and are commonly characterized by distinct magnetic and gravity geophysical signatures. Regionally, these deposits share a complex configuration and setting. They occur generally within silicic-alkalic volcanic and plutonic terrains that were intracratonic or occurred along cratonic margins during the time of mineralization. Structural control over the mineralization is evident with mineralization commonly hosted in inter-related breccias, shear zones and vein-filled fractures that occur along major structural or deformational lineaments.
These deposits also occur as complex breccia systems, as dike-like or sill-like lenses of numerous steeply-dipping and coalescing breccia bodies. Early alteration of host rocks is typically pervasive sodic metasomatism at depth, with sodic-calcic metasomatism grading into potassic alteration towards the surface. Alteration zones are commonly structurally controlled along major faults or splays off regional structural trends. The sodic (Na) alteration is occasionally accompanied by the formation of magnetite-rich zones. Minor to extreme potassic and/or hydrolytic alteration with associated Fe-oxides overprint the earlier Na-alteration, but do not necessarily display any clear spatial association with the earlier alteration. Deposits with well-developed K-alteration often have intense and pervasive Fe-oxide mineralization comprised of magnetite. Deposits with hydrolytic alteration contain both magnetite and hematite. Sericite-altered, near-surface rocks are frequently silicified, and often have narrow to wide zones of quartz-vein stockworks.
In general, alteration within these deposits indicates a continuously evolving system that is depth and temperature dependent. Permeability within structurally prepared zones and the permeability of the surrounding host rocks largely control patterns of mineralization. Mineralogy is variable and attributed to host rock and ore fluid compositions, physico-chemical conditions at the time of deposition, and the degree of interaction of magmatic with connate or meteoric fluids. Economic mineralization is variable and dependent largely on the volatile composition of the parent magma and chemical composition of the host rock. Ore can occur from several kilometres depth (4-6km) to the paleosurface and can often be divided into magnetite-dominated and hematite-rich zones. The magnetite-dominated parts can be ambiguous as to depth of formation. Although hematite-rich zones generally have textures and mineral assemblages of near-surface environments, such as goethite around clasts. As a result of dynamic mixing of clasts within the diatreme and mineralogical and textural overprinting, magnetite- and hematite-rich fragments can be mixed at various levels within the breccia complexes (Camier, 2002).
Hitzman (2000) provides a comprehensive outline for the characteristics of an IOCG deposit and states the following:
- Age. The deposits are aged between the Early Proterozoic and Pliocene.
- Tectonic Setting. These class of deposits are generally characterized by a) intercontinental orogenic collapse; b) intra-continental anorogenic magmatism; c) extension along a subduction-related continental.
- Association with igneous activity. IOCG deposits do not seem to have a direct spatial association with specific intrusions or magmatic compositions.
- Association with evaporates. Many IOCG deposits occur within districts that have marine or lacustrine halite facies evaporates.
- Structural control. IOCG deposits occur along high to low angle faults that are splays off major crustal scale faults.
- Morphology. IOCG deposits form by the metasomatic replacement of original host rock generally as stockwork breccia zones, but may also occur as stratabound sheets.
- Mineralogy. IOCG deposits are characterized by an abundance of Fe-oxide (magnetite – hematite) minerals. They generally contain significant carbonate, Ba, F, or P minerals or combinations of these minerals, and anomalous concentrations of rare earth elements.
- Alteration. IOCG deposits are intensely Fe-O altered either hematite and/or magnetite depending on oxidation state and temperature of emplacement. Also, they are generally associated with peripheral Na-alteration, proximal strong K-alteration, or both, and may exhibit hydrolytic alteration (sericite and clays) depending on degree of mixing with meteoric or connate fluids. Therefore a source of Fe-O
- Ore fluid composition. IOCG deposits are associated with saline, oxidized, sulphate-bearing, fluids that are generally lower in temperature, but may exhibit multiple influxes of higher temperature fluids and/or mixing of higher and lower temperature fluids, or retrograde reaction of higher temperature fluids (i.e. magnetite overprinting hematite – hematite overprinting magnetite).
- However, the most significant characteristics for developing an IOCG deposit, is the influx of non-magmatic, oxidized, saline and relatively Cu-enriched fluids from either meteoric or metamorphic sources, or both, and the mixing of these fluids with relatively Fe-rich hydrothermal fluids derived from intrusive magmatic bodies.
Hitzman, M.W., 2000 – Iron Oxide-Cu-Au Deposits: What, Where, When and Why; in Porter, T.M. (ED.), Hydrothermal Iron Oxide-Copper-Gold & Related Deposits: A Global Perspective, Volume 1; PGC Publishing, Adelaide, pp 9-25.