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Current Understanding of Iron Oxide Associated-Alkali Altered Mineralised Systems;
Part 1 - An Overview; Part 2 - A Review.
T.M. (Mike) Porter, Porter GeoConsultancy Pty Ltd, Adelaide, South Australia.

in - Porter, T.M. (ed.), 2010 - Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, v. 3, Advances in the Understanding of IOCG Deposits; PGC Publishing, Adelaide.   pp. 5-106.


   This two part paper discusses the classification, definition and characteristics of what may be termed "iron oxide-alkali altered" mineralised systems - a grouping that collectively incorporates both iron oxide copper-gold (IOCG) sensu stricto ores, and otherwise similar deposits that also have abundant related hydrothermal iron oxides and associated alkali alteration, but are copper-gold deficient. It both summarises and reviews the lithospheric- to deposit-scale setting, tectonic and structural controls, associated magmatism, temporal distribution, implied crustal-scale sources and circulation dynamics of ore-related fluids, and the resultant alteration and mineralisation patterns, for most of the world's provinces hosting significant examples of these mineralised systems.
   These iron oxide-alkali altered deposits, and IOCG sensu stricto ores in particular, are characterised by: (1) 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; (2) the vertical depth of formation window within which they may occur (from >12 to <2 km); (3) the regional (>10 to >1000 km2) and vertical (surface to at least mid-crustal) scale of surrounding alteration systems; and (4) 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 (1) extensive outpourings of comagmatic bimodal basaltic-andesitic and felsic lavas and pyroclastics, in varying relative proportions; and/or (2) 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 reflect 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: (1) the period of major crustal generation during the Neoarchaean from 2.8 to 2.4 Ga, and (2) 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 km2.
   Iron oxide-alkali altered mineralised systems are interpreted to have been the result of one or more of: (1) CO2- 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; (2) 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; (3) fluids produced by high temperature metamorphism induced by an intraplate, and/or anatectic magmatism; (4) sedimentary formation/basinal waters; (5) 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.
   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 (IOA) 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.

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