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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Fluid inclusions are commonly the best available source of information on the compositions of fluids in past geologic environments. Microanalytical data, predominantly from LA-ICPMS, allow assessment of the relative abundances of chemical elements in fluid inclusions. Such data show that geologic fluids commonly contain appreciable concentrations of multiple salts in addition to NaCl, particularly KCl, CaCl 2 , and FeCl 2 as major components. Quantification of absolute salt concentrations generally requires an internal standard concentration, which is typically derived from microthermometric measurements interpreted according to the vaporsaturated liquidus relations of simpler systems such as H 2 O-NaCl or H 2 O-NaCl-CaCl 2. Here, we review and reassess compositional information obtainable from microthermometric measurements in multicomponent chloride-dominated aqueous systems. To do so, we investigate the systematics of vapor-saturated liquidus phase equilibria in complex multicomponent electrolyte solutions through thermodynamic modeling based on Pitzer's equations. We focus on low-to intermediate-salinity chloride-dominated inclusions, in which ice is the liquidus phase, and on the temperature range from subsolidus conditions to <25 °C. On the basis of measured ice and hydrohalite melting temperatures, fluids with predominantly monovalent (Na ± K) chlorides, or mixtures of monovalent and divalent cation chlorides can be identified and a robust value of m Na as a proportion of total cations be calculated. We show that microthermometric measurements laone do not allow unequivocal determination of the identity of salts that are present in addition to NaCl. In combination with microanalytical determination of cation ratios, however, robust compositional results for multi-salt aqueous fluid inclusions can be obtained using microthermometric measurements interpreted with generic H 2 O-(Na,K)Cl-ΣX n+ Cl n phase stability relations.

The Xiong’ershan district in central China hosts broadly coeval porphyry Au (Qiyugou deposit), porphyry Mo (Leimengou deposit), and barren (Huashan pluton) systems. The key controls on the ore potential and different mineralization styles in these systems are not well understood, with first-order differences in fluid chemistry and melt sources being the main alternatives. The fluid inclusion characteristics of all three porphyry systems have been studied using an integrated approach that combines field geology, petrography, microthermometry, and laser ablation−inductively coupled plasma−mass spectrometry analysis of single fluid inclusions. The results permit a reconstruction of the magmatic-hydrothermal evolution of the ore-forming fluids, and to elucidate whether specialized hydrothermal fluids strongly enriched in ore metals (i.e., Mo, Au, Cu) were essential to form the economically significant deposits. The fluid compositions across the three hydrothermal stages from the Qiyugou Au deposit remain approximately the same over time, suggesting that progressive magma fractionation, fluid-rock reaction along fluid path, and mineral precipitation had a limited effect on fluid composition. The syn-ore stage fluids of the Leimengou Mo deposit are characterized by higher Cs/Na, Sr/Na, and B/Na, but lower K/Na and Cl/Na ratios, and also have salinities and homogenization temperatures distinct from the earlier fluids. This demonstrates that Mo mineralization was caused by a second pulse of fluid input from a highly fractionated felsic magma subsequent to the pre-ore stage. At the Huashan barren pluton, fluids from phase II have higher Cs/Na, B/Na, Li/Na, and Rb/Na ratios with lower homogenization temperatures than fluids occurring in porphyritic rocks of phase III, reflecting a higher degree of magma fractionation of this plutonic complex. The Huashan pluton does not host economic mineralization which is likely caused by the low ore metal tenor, inefficient fluid extraction from the melt, or the flat-roof geometry preventing accumulation of a large volume of fluid in the apical part. The Au tenor of the Qiyugou deposit was most likely contributed by mantle-derived material of higher Mg/Na, Fe/Na, Pb/Na, and Zn/Na ratios. Taken together, the metal charged magmatic-hydrothermal fluids, steeply dipping geometry, and small volume of the porphyry stocks all suggest that a much larger magma chamber feeding the porphyry systems should be present at deeper levels with good potential for Mo mineralization below the current level of exposure at Qiyugou deposit.

Iron oxide copper-gold (IOCG) deposits form in spatial and genetic relation to hydrothermal iron oxide-alkali-calcic-hydrolytic alteration and thus show a mappable zonation of mineral assemblages toward the orebody. The mineral zonation of a breccia matrix-hosted orebody is efficiently mapped by regularly spaced samples analyzed by the scanning electron microscopy-integrated mineral analyzer technique. The method results in quantitative estimates of the mineralogy and allows the reliable recognition of characteristic alteration as well as mineralization-related mineral assemblages from detailed mineral maps. The Ernest Henry deposit is located in the Cloncurry district of Queensland and is one of Australia’s significant IOCG deposits. It is known for its association of K-feldspar altered clasts with iron oxides and chalcopyrite in the breccia matrix. Our mineral mapping approach shows that the hydrothermal alteration resulted in a characteristic zonation of minerals radiating outward from the pipe-shaped orebody. The mineral zonation is the result of a sequence of sodic alteration followed by potassic alteration, brecciation, and, finally, by hydrolytic (acid) alteration. The hydrolytic alteration primarily affected the breccia matrix and was related to economic mineralization. Alteration halos of individual minerals such as pyrite and apatite extend dozens to hundreds of meters beyond the limits of the orebody into the host rocks. Likewise, the Fe-Mg ratio in hydrothermal chlorites changes systematically with respect to their distance from the orebody. Geochemical data obtained from portable X-ray fluorescence (p-XRF) and petrophysical data acquired from a magnetic susceptibility meter and a gamma-ray spectrometer support the mineralogical data and help to accurately identify mineral halos in rocks surrounding the ore zone. Specifically, the combination of mineralogical data with multielement data such as P, Mn, As, P, and U obtained from p-XRF and positive U anomalies from radiometric measurements has potential to direct an exploration program toward higher Cu-Au grades.