Determination of absolute trace-element concentrations in fluid inclusions using laserablation (LA) ICPMS requires an internal standard, i.e., the concentration of one element must be independently known from independent observation. Microthermometric determination of the last melting temperature of ice, hydrohalite or halite is routinely used to calculate apparent salinities in wt% NaCl equivalent, using phase relations in the binary H2O-NaCl system to estimate Na concentration. Calculating the concentrations of all other elements requires an empirical correction, if additional salt concentrations are of similar magnitude as that of NaCl. If CaCl2 is the main additional salt component, as in many low-temperature basin and basement brines, absolute Na concentrations (wt% NaCl abs.) can be obtained by observing two melting temperatures (hydrohalite and either ice or halite), uniquely defining the major element composition of the fluid in the ternary model system H2O-NaCl-CaCl2 and allowing Na to be used as internal standard for quantifying all minor and trace elements. Test results for a range of compositions show that calcium concentration can be determined more precisely by microthermometry than by LA-ICPMS analysis, but that both methods agree within error. The combined approach of microthermometry and LA-ICPMS analysis described here permits reliable quantification of major (Ca, Na) as well as trace element concentrations in sodic-calcic brine inclusions, even in Ca-rich host minerals such as fluorite or Ca-bearing carbonates.

The Prominent Hill iron oxide-copper-gold (IOCG) deposit, located in the Gawler craton of South Australia, contains ca. 278 Mt of ore at 0.98 % Cu, 0.75 g/t Au, 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 intermediatecomposition lavas, inferred to be members of the lower Gawler Range Volcanics, host the economically mineralized hematite breccias. The volcanic-sedimentary package was downfaulted and tilted along a major E-W 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 2 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 lithological 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 lithological contacts. The degree...

The Prominent Hill deposit is a large iron oxide Cu-Au (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 old 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 co-magmatic with the Gawler Range Volcanics and show widespread magnetite-rich alteration. Economic mineralization was formed through 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 pre-ore stage was overprinted by the economically important stage II sulfides, deposited from hypogene, oxidized fluids ultimately sourced from the paleo-surface. The high-grade Cu ores contain dominantly chalcocite, bornite, chalcopyrite and gangue minerals including fluorite, barite and minor quartz, hosting mineralization-related fluid inclusion assemblages. Petrography, microthermometry and LA-ICP-MS microanalysis were used to characterize pre-, syn-and post-mineralization 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.% eqv. NaCl) and contains low concentrations of K, Pb, Cs, and Fe (600 ppm), but is rich in Cu (1000 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 the most saline fluid with a total NaCl + CaCl2 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 bornite + chalcocite bearing ores. It is a calcic-sodic brine with 16-28 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 i...

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