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Immiscible phases derived from degassing silicate magmas are considered to be precursors of metal-bearing hydrothermal fluids in porphyry deposits. The development of melt-inclusion techniques provides a window into this critical period of porphyry formation, when the cooling, decompression, and crystallization of silicate melts result in the formation of immiscible phases. The record of magmatic to hydrothermal evolution is presented using inclusions in clinopyroxene phenocrysts from the syenitic Balut dike, one of the host-rock lithologies for mineralization at the Dinkidi Cu-Au porphyry deposit, Philippines. Primary inclusions include silicate glass, multiphase aggregates comprising salts, silicates, sulfates, carbonates, sulfides and oxides, and highly saline aqueous fluids. Various analyses, including in situ laser ablation inductively coupled plasma-mass spectrometry of the multiphase inclusions, determined elevated concentrations of Cl, S, As, Tl, K, Na, and a number of metals, including those that form ore-grade deposits (e.g., Cu) and those that do not (e.g., Mo, Pb, Zn, and W) at the Dinkidi porphyry deposit. Silicate melt and multiphase salt-rich inclusions in clinopyroxene are interpreted as having originally formed as immiscible phases at magmatic temperatures.

The eastern Gawler craton hosts Australia's premier uranium-bearing iron oxide copper-gold (IOCG) belt, the >500-km-long Olympic Cu-Au-(U) province. In addition to the Olympic Dam Cu -U-Au and Prominent Hill Cu-Au deposits, numerous barren and weakly mineralized IOCG prospects are present in the province. New geochronological data for hydrothermal minerals combined with constraints from host-rock ages demonstrate that alteration and associated IOCG mineralization formed between ~1570 and ~1600 Ma in three districts of the province. This IOCG hydrothermal activity temporally overlaps with magmatism of the Hiltaba Suite and Gawler Range Volcanics. Titanites in paragenetically early magnetite -bearing alteration in the Olympic Dam mad Prominent Hill districts yield U-Pb ion probe ages of 1576 ± 5 and 1.567 ± 10 Ma, respectively. Molybdenite in veins crosscutting magnetite-biotite and albitic alteration in the Moonta-Wallaroo district have Re-Os ages of 1574 ± 6 and 1599 ± 6 Ma, respectively. These represent minimum ages of the IOCG alteration assemblages in this district. A muscovite 40Ar/39Ar age of 1575 ± 11 Ma provides a minimum age of paragenetically later hematitic alteration in the Olympic Dam district. Neodymium isotope compositions were determined for 44 whole-rock samples from barren and weakly mineralized Cu-Au prospects and host rocks in the Olympic Dam and Prominent Hill districts. The new geochronological framework enables comparison of the Nd isotope data across two IOCG districts at the time of formation of the Olympic Dam deposit (ca. 1590 Ma). Magnetite-rich weakly Cu -mineralized alteration from five prospects yields a relatively narrow range of εNd(1590) values of-5.8 to -4.1. Both hematite- and magnetite-rich alteration yield generally similar εNd(1590) values that match values from fresh and weakly altered Paleoproterozoic metasedimentary and metagranitic rocks (-6.6 to -3.5) as well as from most felsic Hiltaba Suite intrusions and Gawler Range Volcanics in the eastern Gawler craton (ca. -6 to -4). These data are consistent with crustal sources for REE and, by implication, for associated copper in the barren and weakly mineralized prospects. Mineralization and alteration in these minor IOCG systems can be geochemically discriminated from the giant Olympic Dana deposit, where greater inputs of mantle-derived REE and other ore components are evident. [ABSTRACT FROM AUTHOR]

Geologic observations suggest two stages of hydrothermal activity at a number of presently subeconomic iron oxide copper-gold systems in the Olympic Dana district, eastern Gawler craton. They contain high-, and moderate -to low-temperature Fe oxide-rich hydrothermal alteration. The mineral assemblages include magnetite-calc -silicate-alkali feldspar ± Fe-Cu sulfides and hematite-sericite -chlorite-carbonate ± Fe-Cu sulfides ± U, REE minerals. In all documented prospects, the minerals of the hematitic assemblages replace the minerals of the magnetite-rich assemblages. The bulk of the subeconomic Cu-Au mineralization is associated with the hematitic alteration assemblages. Microanalysis by proton ion probe (PIXE) of hypersaline fluid inclusions in magnetite-rich assemblages, however, demonstrates that significant amounts of copper (>500 ppm) were transported by the early-stage high-temperature (>400°C) fluids responsible for the magnetite-rich alteration. These brine inclusions contain multiple solid phases (liquid + vapor + multiple solids) including chalcopyrite in some cases. In comparison, inclusions of the hematitic stage are relatively simple liquid + vapor types, with homogenization temperatures of 200° to 300°C and containing 1 to 8 wt percent NaCl equiv. The Br/Cl ratios of the magnetite-forming fluids measured by PIXE lie beyond the range of typical magmatic and/or mantle values, allowing for the possibility that the fluids originated as brines from a sedimentary basin or the crystalline basement. Sulfur isotope compositions of chalcopyrite and pyrite demonstrate that sulfur in both alteration assemblages was derived either from cooling magmas and/or crystalline igneous rocks carried by relatively oxidized fluids (ΣSO42- ≈ ΣH2S, δ34Ssulfides from -5 to +2‰) or from crustal sedimentary rocks (δ34Ssulfides from +5 to +10‰). Oxygen and hydrogen isotope compositions of waters calculated for minerals of the magnetite-rich assemblage have also δ18O values of +7.7 to +12.8 per nail and δD values of-15 to -21 per mil. The only available δ18O and δDfluid values for the hematitic assemblage are +4.7 mad -9 per mil, respectively. The isotopic compositions of both fluids, coupled with the available literature data, can be explained in terms of fluid reequilibration with felsic Gawler Range Volcanics or other felsic igneous rocks in the region and with metasedimentary rocks of the Wallaroo Group at low water-to-rock ratios prior to their arrival at the mineralization sites. The lack of significant copper mineralization associated with magnetite-forming fluids that carried copper suggests that there was no effective mechanism of saturation of copper minerals or the quantity of these fluids was not sufficient to produce appreciable copper mineralization. Association of the copper-gold mineralization with the hematitic alteration in the subeconomic prospects can be explained by a two-stage model in which preexisting hydrothermal magnetite with minor associated copper-gold mineralization was flushed by late-stage oxidized brines that had extensively reacted with sedimentary or metamorphic rocks. The reduction of these brines, driven by conversion of magnetite to hematite, resulted in precipitation of copper and gold. The oxidized brines may have contributed additional copper and gold to the system in addition to upgrading preexisting subeconomic Cu-Au mineralization. When compared to published models for the Olympic Dam deposit, the new data for fluids in subeconomic Fe-oxide Cu-Au prospects of the Olympic Dam district indicate the diversity of origins of iron oxide-copper-gold systems, even within the same geologic region. [ABSTRACT FROM AUTHOR]

Abstract: The Geochemists Workbench® (GWB) is a versatile geochemical modelling software package capable of reaction path modelling, equilibrium fluid speciation and activity diagramming, although it is restricted by the availability of thermodynamic (log K) data files with a temperature range of 0–300°C and pressures on the critical curve for water. The K2GWB (log K to GWB) utility is used to produce a usable Geochemists Workbench data file from a generic log K and system file generated by the UT2K utility complementing the UNITHERM thermodynamic database system. The GWB data files can be generated for pressure–temperature conditions within the bounds of the modified Helgeson–Kirkham–Flowers model (0–1000°C, 1–5000bar). Data can be generated with any combination of pressure–temperature log K grids required, allowing the Geochemists Workbench to be used to model a wide range of geological scenarios under polythermal and polybaric conditions, as well as the generation of activity diagrams over a wide range of pressures and temperatures. A comparative HCh and Geochemists Workbench model using the same thermodynamic dataset was run to benchmark test the validity of the new data files. Modelling the cooling of 2molal NaCl–H2O fluid in equilibrium with a ganodiorite-type rock from 600 to 300°C gave close to identical results between the two codes, validating the thermodynamic data conversion process described here. An investigation of the speciation of Au across a temperature–pressure gradient from 550°C and 1500bar to 150°C and saturated vapor curve pressures has been used to illustrate the usefulness of coupling thermodynamic data file generation at wide temperatures and pressures from a central database and the functionality of The Geochemists Workbench. [Copyright &y& Elsevier]