Iron oxide-alkali altered (IOAA) systems and Iron Oxide Copper Gold (IOCG) deposits


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Iron oxide copper gold (IOCG) sensu stricto ores are members of a larger grouping of what may be termed 'iron oxide-alkali altered' (IOAA) mineralised systems that also includes otherwise similar deposits that have abundant related hydrothermal iron oxides and associated alkali alteration, but are copper-gold deficient.

These iron oxide-alkali altered deposits, and IOCG sensu stricto ores in particular, are characterised by: i). the large to giant size (>100 Mt to >9 Gt @ 0.5 to 1.5% Cu, 0.3 to 0.8 g/t Au + REE, ±U, ±Ag) of the more significant examples; ii). the vertical depth of formation window within which they may occur (from >12 to ≤2 km); iii). the regional (>10 to >1000 km2) and vertical (surface to at least mid-crustal) scale of surrounding alteration systems; and iv). the alkali-iron oxide rich nature (sodic/calcic/potassic+magnetite/hematite) of both regional- and deposit-scale alteration/mineralisation patterns. These characteristics illustrate the lithospheric scale of the regimes responsible for their generation. All significant IOCG and related deposits are characterised by a clear temporal, but (usually) not close spatial association, with batholithic complexes, composed of both anorogenic granitoids and varying proportions of mantle related, fractionated, mafic to intermediate phases. These magmatic events are accompanied by either i). extensive outpourings of comagmatic bimodal basaltic-andesitic and felsic lavas and pyroclastics, in varying relative proportions; and/or ii). by numerous and equally widespread, but generally small (although sometimes large) coeval juvenile mafic dykes, plugs, sills and layered complexes. These magmatic complexes extend over areas of tens of thousands of km2, representing extensive igneous provinces, interpreted to refl ect underplates at the base of the sub-crustal lithospheric mantle (SCLM), and/or intraplates immediately below the Moho density filter. The under- and intraplates comprise large fractionating mantle-derived magma chambers, the result of either crustal delamination and detachment, or mantle plume events that triggered decompression melting in the upper mantle, generally at depths of <100 km.

Igneous events of this type, coincident with iron oxide-alkali altered mineralised systems, are distributed throughout the geological record from the Neoarchaean to Tertiary. However, those related to significant IOCG sensu stricto deposits would appear to be restricted to: i). the period of major crustal generation during the Neoarchaean from 2.8 to 2.4 Ga, and ii). the periods following consolidation of the Nuna/Columbia, Rodinia, (the short-lived) Pannotia and Pangea supercontinents, coinciding with extensional phases accompanying the commencement of break-up from 1.60 to 1.45, 0.85 to 0.75, 0.57 to 0.51 and 0.165 to 0.095 Ga respectively. The under- and intraplates, and associated high temperature metamorphism and anatectic magmatism, acted both as heat engines, driving fluid circulation cells and consequent alteration over large volumes of the crust, and as fluid sources. Structurally controlled fluid cell circulation is reflected by concomitant alteration, occurring as either linear corridors of alteration up to tens x hundreds of kilometres, or by more equidimensional regions associated with orthogonal patterns of faulting or fracturing that may cover tens to >1000 km

Iron oxide-alkali altered mineralised systems are interpreted to have been the result of one or more of: i). CO
2- and volatile-rich, magmatic-hydrothermal fluids/vapours released directly from fractionating mantle-derived magma chambers or related mafic intrusions in the lower- to mid-crust; ii). hypersaline, iron- and alkali-rich, magmatic-hydrothermal fluids, exsolved within fractionated anorogenic and/or mafic to intermediate juvenile batholiths, which have inherited volatiles, water and other components from the related intraplate; iii). fluids produced by high temperature metamorphism induced by an intraplate, and/or anatectic magmatism; iv). sedimentary formation/basinal waters; v). surface derived bittern brines, or re-dissolved buried evaporites. Any of these fluids may carry components related to the processes involved in their formation, or exsolved or scavenged from the rocks through which they are circulated.

Mantle derived mafic magmas release carbonic vapours/brines when mingled with anorogenic granitoid magmas to exsolve voluminous, immiscible, hydrous magnetite dominated melts and quartz-feldspar residues, and expel associated hypersaline, alkalic, metasomatic fluids from the batholith (cf. Lightning Creek, Marcona) that rise to form magnetite-apatite deposits as magnetite intrusions/flows (e.g., Kiirunavaara) and/or overprinting and associated metasomatic magnetite mineralisation and alkalic alteration (e.g., Chilean Iron Belt). The associated fluids are often Cu-rich but sulphur-poor and will only precipitate Cu in the presence of an external sulphur source. Interaction of mantle derived mafic magmas with older crystallised granitoids and sedimentary rocks derived from a granitoid terrane may produce a similar, but less intense response.

In most major IOCG provinces, the earliest fluid circulation and alteration occurs on a regional- or district-scale, progressively reducing in areal extent with time, evolving to deposit-scale zones. Regional scale alteration usually commences at depth with early sodic-calcic±iron (albite/scapolite±magnetite), related to either deeply circulated formation/basinal waters or magmatic-hydrothermal fluids, accompanied by a statistical depletion of ore forming solutes in altered rocks. This alteration usually predates ore, with scavenged solutes potentially sequestered for future reworking. Regional alteration progresses, both temporally and spatially upwards (i.e., with decreasing temperature), to potassic with increasing iron oxides (biotite/K feldspar±magnetite), to iron-sodic-calcic (magnetite-scapolite-apatite-actinolite) or iron-potassic-calcic (magnetite-K feldspar-actinolite±carbonate) at deep or shallower levels respectively, both of which commonly host major iron oxide-apatite accumulations. IOCG sensu stricto deposits, where developed, generally post date this oxidised, sulphur deficient stage. Fluid inclusion and related data are supportive of, but do not in most cases unequivocally prove, the influence of a second fluid in the formation of IOCG sensu stricto deposits, triggering the precipitation of sulphides, most likely of either shallow basinal or of further magmatic-hydrothermal origin.

Alteration patterns associated with these deposits progress both temporally and upwards from the pre-ore regional assemblage at >500°C, to progressively overprinting biotite and then K feldspar (~450°C), to chlorite-muscovite-sericite (hydrolytic) and finally to a muscovite and hematite dominant assemblage high and late in the system, at temperatures of <250°C, often with late carbonate ±quartz veining, and in some instances a late barren siliceous stage. Most iron oxide-alkali altered deposits are related to rock porosity and permeability on a deposit-scale, occurring in shear zones, tectonic and explosive breccias, or volcanic and/or sedimentary breccias that control aggressive to passive ingress and reaction of fluids with wall rocks, clasts and breccia matrix.


The figure above is a schematic diagram illustrating the suggested lithospheric setting of IOCG and other iron oxide-alkali altered deposits (not to scale). The key driver of the system is assumed to be the decompression melted mantle underplate magma chambers indicated or inferred below most iron oxide-alkali altered provinces, and the intraplates emplaced between the base of the sub-crustal lithospheric mantle (SCLM) and the Moho, most likely immediately below the Moho. Such under- and intraplates may be formed by a number of processes, including those related to SCLM delamination and detachment and plume upwelling. The intraplate, which is too dense to buoyantly rise above the Moho, is responsible for anatectic melting and high temperature metamorphism of the lower continental crust, initially forming voluminous anorogenic granites, which rise to form batholiths at their level of buoyancy within the crustbatholiths at their level of buoyancy within the crust (e.g., the Hiltaba Suite - Gawler craton; Williams-Naraku batholiths - Mount Isa Inlier; Eastern Granite-Rhyolite Province - USA; Erinpura Granite/Malani Igneous Suite - Aravalli craton, India; Hooke Granite, Lufilian Arc - Zambia, etc.). Comagmatic volcanic sheets of generally subaerial or shallow water lavas and tuffs are frequently (but not always) emplaced (e.g., Gawler Range Volcanics - Gawler craton; Rooiberg Felsites - Kaapvaal craton, South Africa; Eastern Granite-Rhyolite Province - USA, Malani Igneous Suite - Aravalli craton, India; Sierra Madre Ocidental LIP; etc.). These are frequently 2 to >4 km in thickness and cover areas in excess of 25 000 km2. This magmatism is summarised on the left hand half of the diagram).
   Fractionation within the intraplate magma chamber produces less dense ultramafic to mafic fractions that may initially be injected along dilational fractures within the crust to form widespread, relatively small dykes, sills and stocks, although some large bodies (e.g., the Rustenburg Layered Suite of the Bushveld Complex) may be emplaced. As fractionation (and assimilation) continues, larger volumes of juvenile magma are generated which are sufficiently buoyant to rise through the Moho to form voluminous dioritic to tonalitic and peralkaline granite batholiths and basaltic to basaltic-andesite volcanic sheets which may dwarf some initial anorogenic granites and felsic volcanic developments (e.g., the Coastal batholiths of Chile and Perú and coeval La Negra Formation volcanism in Chile; and possibly the Itacaiúnas Supergroup volcanics and subsequent intrusions of the Carajás Mineral Province in Brazil). This case is represented on the right hand half of the diagram.
   The column below Kiirunavaara represents mingling of mantle derived mafic intrusions which have released carbonic vapours/ brines into anorogenic granitoids to exsolve voluminous immiscible hydrous magnetite dominated melts and fluids from the batholith (cf. Lightning Creek, Marcona) that rise to form magnetite-apatite deposits as magnetite intrusions/flows and/or overprinting and associated metasomatic magnetite mineralisation and alkalic alteration.
   On the far left, is a diagrammatic representation of the release at ~12 km depth of voluminous magmatic K-Na-Ca-Mg-Fe-F and CO
2-rich dominated chloride brines, triggered by carbonate crystallisation from the Palabora fractionated low-degree partial mantle melting derived intrusion. Above it (but not related), is a representation of the Bushveld complex sequence of the Rustenburg Layered Mafic/ultramafic Suite (7 to 8 km thick), Lebowa Granite (3.5 km) and Rooiberg Felsite (3.5 km) and the associated Vergenoeg magnetite-fluorite deposit. Fractionation of the magmatic systems and the high heat flow generated, result in the exsolution and circulation of hydrothermal fluids, the passage of which is strongly influenced by the mechanical porosity and permeability of the rocks transgressed, particularly transcrustal faults/shears (e.g., Cloncurry, Mount Dore and Pilgrim faults - Mount Isa Inlier; Elizabeth Creek Fault Zone - Gawler craton; Cinzento and Carajás Faults - Carajás Mineral Province in Brazil; Atacama Fault System, Chile; Kaliguman Lineament, India; Red River Fault Zone, Vietnam ?). The five varieties of fluids (or deep vapours) indicated on the legend, are circulated by the heat engine represented by this magmatism, exsolving, scavenging and transporting metals. Evaporite bearing units shown at high and mid-crustal levels, and found in many provinces, contribute Na-Ca-K-Cl and sulphate to the basinal fluids.
   Regional scale Na-Ca-Fe alteration is evident within iron oxide-alkali altered provinces, particularly in association with transcrustal faults and their splays, assumed to be channelling either or both magmatic and basinal fluids. At higher levels, this regional alteration is replaced by potassic assemblages and shallow level chlorite-sericite. Note that while density contrasts limit the ascent of mafic to ultramafic rocks above the Moho, there is no such barrier to the transmission of magmatic fluids to the zone of anatexis, or up transcrustal faults. Also, because of the scale of the mineralising systems and the heterogeneity of the crust, there will be considerable variation in the nature of the fluids that dominate in any area, the ligands and metals they scavenge/transport, cause to be exsolved, and eventually precipitate.

For detail and descriptions of the provinces and processes, see Porter (2010) cited below.

The most recent source geological information used to prepare this summary was dated: 2010.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Chiaradia M, Banks D, Cliff R, Marschik R and de Haller A,   2006 - Origin of fluids in iron oxide-copper-gold deposits: constraints from d37Cl, 87Sr/86Sri and Cl/Br : in    Mineralium Deposita   v41 pp 565-573
Groves D I, Bierlein F P, Meinert L D and Hitzman M W,  2010 - Iron Oxide Copper-Gold (IOCG) Deposits through Earth History: Implications for Origin, Lithospheric Setting, and Distinction from Other Epigenetic Iron Oxide Deposits : in    Econ. Geol.   v105 pp 641-654
Hitzman M W, Oreskes N, Einaudi M T  1992 - Geological characteristics and tectonic setting of Proterozoic iron oxide (Cu-U-Au-REE) deposits: in    Precambrian Research   v58 pp 241-287
Huang, X.-W., Boutroy, E., Makvandi, S., Beaudoin, G., Corriveau, L. and De Toni, A.F.,  2019 - Trace element composition of iron oxides from IOCG and IOA deposits: relationship to hydrothermal alteration and deposit subtypes: in    Mineralium Deposita   v.54, pp. 525-552.
Pollard, P.J.,  2006 - An intrusion-related origin for Cu-Au mineralization in iron oxide-copper-gold ( IOCG ) provinces: in    Mineralium Deposita   v.41, pp. 179-187.
Porter T M,  2010 - Current Understanding of Iron Oxide Associated-Alkali Altered Mineralised Systems: Part II, A Review: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.3 pp. 33-106
Porter T M,  2010 - Current Understanding of Iron Oxide Associated-Alkali Altered Mineralised Systems: Part I, An Overview: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.3 pp. 5-32
Richards, J.P., Lopez, G.P., Zhu, J-.J., Creaser, R.A., Locock, A.J. and Mumin, A.H.,  2017 - Contrasting Tectonic Settings and Sulfur Contents of Magmas Associated with Cretaceous Porphyry Cu ± Mo ± Au and Intrusion-Related Iron Oxide Cu-Au Deposits in Northern Chile: in    Econ. Geol.   v.112, pp. 295-318.
Rodriguez-Mustafa, M.A., Simon, A.C., del Real, I., Thompson, J.F.H., Bilenker, L.D., Barra, F,. Bindeman, I. and Cadwell, D.,  2020 - A Continuum from Iron Oxide Copper-Gold to Iron Oxide-Apatite Deposits: Evidence from Fe and O Stable Isotopes and Trace Element Chemistry of Magnetite: in    Econ. Geol.   v.115, pp. 1443-1459.
Skirrow, R.G.,  2010 - Hematite-group IOCG ± U systems: Tectonic settings, hydrothermal characteristics, and Cu-Au and U mineralizing processes: in Corriveau, L. and Mumin, H. (eds), 2010 Exploring for Iron Oxide Copper-Gold Deposits: Canada and Global Analogues, Geological Association of Canada,   Short Course Notes 20, pp. 39-58.
Skirrow, R.G., Murr, J., Schofield, A., Huston, D.L., van der Wielen, S., Czarnota, K., Coghlan, R., Highet, L.M., Connolly, D., Doublier, M. and Duan, J.,  2019 - Mapping iron oxide Cu-Au (IOCG) mineral potential in Australia using a knowledge-driven mineral systems-based approach: in    Ore Geology Reviews   v.113, 31p. doi.org/10.1016/j.oregeorev.2019.103011
Williams P J, Barton M D, Johnson D A, Fontbote L, de Haller A, Mark G, Oliver N H S and Marschik R,  2005 - Iron oxide copper-gold deposits: Geology, space-time distribution and possible modes of origin: in Hedenquist J W, Thompson J F H, Goldfarb R J and Richards J P (Eds.), 2005 Economic Geology 100th Anniversary Volume, Society of Economic Geologists, Denver,    pp 371-405

   References in PGC Publishing Books: Want any of our books ? Pricelist
Barton M D, Johnson D A, 2000 - Alternative Brine Sources for Fe-Oxide (-Cu-Au) Systems: Implications for Hydrothermal Alteration and Metals,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v1  pp 43-60
Buy   Abstract

Pollard P J , 2000 - Evidence of a Magmatic Fluid Source for Iron Oxide-Cu-Au Mineralisation,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v1  pp 27-41
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Haynes D W, 2000 - Iron Oxide Copper (-Gold) Deposits: Their Position in the Ore Deposit Spectrum and Modes of Origin,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v1  pp 71-90
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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,  v1  pp 9-25
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Yardley B W D, Banks D A, Barnicoat A C, 2000 - The Chemistry of Crustal Brines: Tracking their Origins,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v1  pp 61-70
Buy   Abstract

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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