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The Prominent Hill deposit is a large iron oxide copper-gold (IOCG) resource located in the Olympic IOCG province of South Australia. The deposit is hosted by brecciated sedimentary rocks and structurally underlying lavas of the ca. 1.6 Ga Gawler Range Volcanics. Both rock units are altered and mineralized, forming characteristic hematite breccias. They are located in the footwall of the Southern overthrust separating the host-rock package in the footwall from Paleoproterozoic metasedimentary rocks in the hanging wall. The metasedimentary rocks were intruded by the Hiltaba Suite granites, which are comagmatie with the Gawler Range Volcanics and show widespread magnetite-rich alteration. Economic mineralization was formed dirough a two-stage process. Early pyrite and minor chalcopyrite were deposited from moderately reduced fluids during sulfide stage I and are hosted in subeconomic magnetite skarns and in the brecciated sedimentary host rocks. This preore stage was overprinted by the economically important stage II sulfides, deposited from hypogene, oxidized fluids ultimately sourced from the paleosurface. The high-grade Cu ores contain dominantly chalcocite, bomite, chalcopyrite, and gangue minerals including fluorite, barite, and minor quartz hosting mineralization-related fluid inclusion assemblages. Petrography, microthermometry, and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) microanalysis were used to characterize pre-, syn- and postmineralization fluid inclusion assemblages. The results permit discrimination of four fluid end members (A, B, C, and D). Fluid A is the main ore fluid and hosted in fluorite and barite intergrown with Cu sulfides in the breccia matrix. It is weakly saline (< 10 wt % NaCl equiv) and contains low concentrations of K, Pb, Cs, and Fe (600 ppm) but is rich in Cu (1,000 ppm) and U (0.5-40 ppm). A magmatic origin of the salinity is supported by the low molar Br/Cl ratio of 0.003. We suggest that the solute inventory was derived from shallow fluid exsolution and degassing of late Gawler Range Volcanics and subsequent complete oxidation of the fluid via contact with atmospheric oxygen. Fluid A migrated through oxidized aquifers to the site of the Prominent Hill deposit, where it became the main driver of stage II copper mineralization. Fluid B occurs in fluid inclusions in siderite + quartz-bearing veins crosscutting the hematite breccia. It is die most saline fluid, with a total NaCl + CaCb concentration of 36 to 45 wt % and a low Ca/Na mass ratio of 0.3. Fluid B is rich in K, Fe, Pb, and Cs and contains modest Cu (-70 ppm). Its composition is typical of a moderately reduced magmatic-hydrothermal brine modified by fluid-rock interaction. Fluid C is hosted by fluid inclusions in fluorite and barite within bomite + chalcocite-bearing ores. It is a calcic-sodic brine witii 16 to 28 wt % NaCl + CaCl2 and has an elevated Ca/Na (0.6) and high Br/Cl ratios characteristic of basin brines of residual bittern origin. It is quite rich in Cu (-200 ppm) and likely contributed metals to economic mineralization. Fluid D is hosted by inclusions in fluorite in late veins crosscutting the hematite breccia. The total NaCl + CaCl2 salinity ranges between 19 and 30 wt %, and the concentrations of K, Mn, Cs, and Pb are similar to those of fluid C. Fluid D is a basement brine with characteristically high Ca/Na ratios of about 2 and contains modest Cu (-100 ppm). The high-grade copper mineralization in the Prominent Hill deposit originated dominantly from magmatic components, including both die metal inventory (Cu, minor Au, and U) and sulfur. The essential factor controlling the high-grade IOCG mineralization was wholesale oxidation of magmatic Cu and S from Gawler Range volcanic eruptions; volcanic gases reacted with atmospheric oxygen at or near the Earth's surface-for example, in an acidic volcanic lake environment. Oxidized surface fluids charged with Cu, U, and sulfate and high acidity then infiltrated the host rocks at Prominent Hill and became reduced by ferrous iron in residual magmatic brine, in nonmagmatic basin brine, and by ferrous iron-bearing minerals. Copper sulfides and hematite precipitated in a zone of fluid mixing and by interaction of the fluids with acid-neutralizing host-rock strata. Limited sulfide for the low-sulfidation ore mineral assemblage was supplied, to presently unknown proportions, from magmatic-hydrothermal pyrite derived in a preore stage of mineralization and from magmatic sulfate reduced by aqueous Fe2+ to precipitate hematite. Exploration-relevant factors for Prominent Hill-style high-grade copper ores are the combination of (1) active volcanism during the late stages of the Gawler Range Volcanic Province, (2) a fluid-mixing interface between deep brines and an acidic, surface-oxidized but magmatically charged water as the main ore fluid, and (3) a mappable gradient of acid neutralization in brecciated and chemically reactive host rocks, ideally containing carbonates and some earlier pyrite. [ABSTRACT FROM AUTHOR]

The Prominent Hill iron oxide-copper-gold (IOCG) deposit, located in the Gawler craton of South Australia, contains ca. 278 million metric tons (Mt) of ore at 0.98% Cu, 0.75 g/t An, and 2.5 g/t Ag. In contrast to the predominantly granite-hosted Olympic Dam IOCG deposit, Prominent Hill is mainly within unmetamorphosed sedimentary rocks comprising coarse clastic to laminated argillaceous lithologies with some volcaniclastic components and variable carbonate, including local massive dolomite. Essentially unmetamorphosed sedimentary rocks and structurally underlying mafic to intermediate-composition lavas, inferred to be members of the lower Gawler Range Voleanics, host the economically mineralized hematite breccias. The volcanic-sedimentary package was downfaulted and tilted along a major east-west fault, north of which similar but regionally low-grade metamorphosed rocks were affected by subeconomic skarn mineralization, and (on a more regional scale of the Mount Woods domain) intruded by granitic and gabbroic bodies. Hydrothermal alteration and mineralization at Prominent Hill involved pervasive and texturally destructive replacement of formerly calcareous, dolomitic, and siliciclastic breccia components. Hydrothermal alteration minerals comprise hematite, magnetite, siderite, ankerite, quartz, sericite, chlorite, kaolinite, fluorapatite, fluorite, barite, REE-U minerals (including monazite), uraninite, and coffinite, together with Cu sulfides including chalcopyrite, bornite, and chalcocite in the highest grade ore. Brecciation and replacement caused mechanical mixing as well as chemical alteration of primary lithologies, such that sedimentary contacts became obscured. Mass-balance calculations identify Al, Ti, Si, and Zr as least mobile components during hematite-chlorite-sericite to weak hematite-quartz alteration. Because Zr was not regularly assayed in drill cores, we use concentration ratios of Ti, Al, and Si from the deposit-scale assay database to delineate the distribution of lithochemical units prior to hydrothermal alteration and Cu mineralization. The resulting lithochemical model, based on one horizontal and five vertical cross sections, is used as a basis for mapping alteration patterns calculated from molar (Fe + Si)/(Fe + Si + Al), K/Na, and K/Al ratios. These chemical patterns, in conjunction with mineral stoichiometry, indicate that the spatial distribution of hematite, chlorite, variably phengitic sericite (and/or illite) ± kaolinite ± quartz-bearing alteration is superimposed on the pattern of interpreted lithologic contacts. The alteration patterns confirm visual logging results, showing that hematite enrichment correlates only partially with the distribution of Cu grades of >0.25 wt %. A subvertical body of complete replacement by hematite and quartz with consistent but subeconomic gold enrichment forms a Cu-barren core in the central and eastern parts of the deposit. Zones of increasing K/Al and K/Na ratios extend upward and westward from this Cu-barren core, transgressively overprinting lithologic contacts. The degree of hematite-quartz replacement can be measured by a hematite-quartz alteration index, here termed the HMSI value [(Fe + Si)/(Fe +Si + Al)], which inversely correlates with the normal probability for Cu grade. Areas of highest Cu grade (>1 wt %) spatially correlate with irregular zones having intermediate molar alteration indices: 0.34 < K/Al < 0.40, 20 < K/Na < 36, and HMSI < 0.98. Hematite breccias and Cu ore deposition developed after tilting of the host sequence into its present steep orientation, as indicated by geopetal structures within the breccia matrix. Thus, the economic mineralization occurred late in the deformational history of the region and after extrusion of the lower Gawler Range Voleanics. The formation of the Prominent Hill orebodies occurred during or after upthrusting of deeper seated rocks containing subeconomic Cu in skarns north of the fault. Faulting as well as ore formation may be related to orogenic processes in the central and northern part of the Mount Woods domain. Iron oxide introduction was decoupled from, and at least partly preceded, hydrothermal deposition of high-grade Cu. Geochemical and petrographic data indicate that economic Cu mineralization occurred together with mildly acidic hematitechlorite-sericite ± siderite alteration of originally carbonate-, illite-, and feldspar-bearing sedimentary rocks. The presence of copper enrichment wath an intermediate degree of cation leaching from the host rocks indicates that pH neutralization of initially highly acidic metal-transporting fluids was an essential factor causing Cu sulfide deposition. Distinct ranges in Na-K-Al ratios and low HMSI values offer potential as exploration indicators pointing toward higher ore grades. These results from Prominent Hill are consistent with recently published mineralogical studies at the giant Olympic Dam deposit, indicating similar ore depositional controls despite lithologically different host rocks. [ABSTRACT FROM AUTHOR]

Presents research which quantified the physical and chemical mechanism of ore precipitation in the Yankee Lode tin deposit, Mole Granite, Australia. Trace-element microanalysis of fluid inclusions; Use of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS); Tin precipitation; Magmatic vapor phase transporting copper and boron.