Oral Presentation
IOCG Sensu Stricto Deposits as a Subset of Broader Iron Oxide Associated-Alkali Altered Mineralised Systems.
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T. M. (Mike) PORTER
Porter GeoConsultancy Pty Ltd, Adelaide, S.A., Australia
Abstract: "Iron oxide associated-alkali altered mineralised systems" define metallogenic provinces on all continents, incorporating a diverse group of deposits, including IOCG sensu stricto ores, otherwise similar copper-gold deficient deposits and intrusion related mineralisation. The large vertical range these mineralised systems form within, and the regional-scale and near-surface to mid-crustal extent of related sodic/calcic/potassic+magnetite/hematite alteration, illustrate the lithospheric scale of the responsible fluid-circulation regimes. Virtually all are temporally associated with extensive batholithic(-volcanic) complexes comprising varying proportions of anatectic granitoids and fractionated mantle-related, mafic to intermediate phases. Isotope studies indicate these systems result from one or more fluids of: mantle-derived, CO2-rich, magmatic-hydrothermal; hypersaline, iron-alkali-rich, magmatic-hydrothermal; metamorphic; formation/basinal; or bittern brine, origin. Earliest fluid circulation, usually magmatic-hydrothermal, produces regional-scale sodic-calcic±iron alteration (albite/scapolite±magnetite; >500°C), evolving, temporally and vertically, and becoming progressively more localised, to potassic (biotite/K feldspar±magnetite; ~450°C), to iron-sodic-calcic (magnetite-scapolite-apatite-actinolite) or iron-potassic-calcic (magnetite-K feldspar-actinolite±carbonate) at deeper or shallower levels respectively, culminating in iron oxide-apatite deposition. IOCG sensu stricto deposits post-date this oxidised, sulphur deficient stage, and reflect the complementary influence of another fluid, usually of basinal/bittern or further magmatic-hydrothermal origin. IOCG sensu stricto ores occur at all alteration stability levels, from deep sodic-potassic, to shallow-level chlorite-muscovite-sericite-hematite dominant assemblages (<250°C). The scale of fluid circulation, range of structures, lithologies and metal sources traversed, and variety and combination of fluids that may be involved, produce the diversity of ore deposits. An understanding of the broader encompassing system is important to the appreciation of the genesis and characteristics of individual IOCG sensu stricto deposits.
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Poster
Tectono-Magmatic and Hydrothermal Setting of Iron Oxide Associated-Alkali Altered Mineralised Systems and IOCG Sensu Stricto Deposits.
T. M. (Mike) PORTER
Porter GeoConsultancy Pty Ltd, Adelaide, S.A., Australia
Abstract: "Iron oxide associated-alkali altered mineralised systems", which may incorporate IOCG sensu stricto ores, define large metallogenic provinces on all continents. Related regional-scale (>10 to >1000 km2) alteration systems extend from near-surface to at least mid-crustal levels, and associated deposits may form within a >12 to <2 km depth window. Isotope studies indicate these systems result from one or more circulating fluids of either: mantle-derived, CO2-volatile-rich, magmatic-hydrothermal; hypersaline, iron-alkali-rich, magmatic-hydrothermal; high-temperature metamorphic; formation/basinal; or bittern brine, origin. Virtually all are coeval with batholithic complexes, comprising varying proportions of mingled anatectic granitoids and fractionated mantle-related, mafic to intermediate phases, commonly accompanied by extensive outpourings of comagmatic bimodal basaltic-andesitic and felsic lavas and pyroclastics, and/or coeval sparse, small, juvenile mafic dykes/plugs/sills to large layered complexes, distributed over areas of up to >100 000 km2. These observations illustrate the lithospheric scale of magmatic and related fluid-circulation regimes. The extensive igneous provinces are interpreted to result from underplating at the base of the sub-crustal lithospheric mantle, and/or associated intraplates immediately below the Moho density filter, emplaced at specific periods in the Earth's evolution. Both comprise large fractionating mantle-derived magma chambers, the result of either crustal delamination and detachment, or mantle plume events, that in either case, triggered decompression melting in the upper mantle, generally at depths of <100 km. These under- and intraplates, resultant anatectic magmatism and high temperature metamorphism, acted both as fluid sources and heat engines, driving fluid circulation cells, metal scavenging and alteration over large volumes of the crust, and concomitant ore deposition.
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