HR: 0830h
AN: V51H-0382    [PDF]
TI: Copper Solubility and Speciation in Mineral-Buffered Fluids at Crust to Upper Mantle Conditions
AU: * Hack, A C
EM: alistair.hack@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road Acton, Canberra, ACT 0200 Australia
AU: Mavrogenes, J A
EM: john.mavrogenes@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road Acton, Canberra, ACT 0200 Australia
AU: Mavrogenes, J A
EM: john.mavrogenes@anu.edu.au
AF: Department of Geology, Australian National University, Acton, Canberra, ACT 0200 Australia
AU: Berry, A J
EM: andrew.berry@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Mills Road Acton, Canberra, ACT 0200 Australia
AB: Fluid inclusions, synthesised in a piston-cylinder apparatus, were used to trap representative high {\it P-T} fluid samples under mineral-buffered conditions in the systems Cu$_{2}$O-MgO-SiO$_{2}$-HCl-H$_{2}$O and Cu-K$_{2}$O-Al$_{2}$O$_{3}$-SiO$_{2}$-Fe$_{3}$O$_{4}$-Fe$_{2}$O$_{3}$-HCl-H$_{2}$O at up to $850\deg$C and 1.7 GPa, and as a function of salinity to 11 mol/kg Cl. Copper solubility and speciation were obtained by analysing individual fluid inclusions by excimer laser ablation inductively coupled mass spectrometry (LA-ICP-MS), proton induced X-ray emission (PIXE) and Cu K-edge X-ray absorption near edge structure (XANES) spectroscopy. Quenched capsule fluids were also analysed. At $710\deg$C copper-cuprite-talc-quartz solubility in aqueous fluid containing 1 mol/kg Cl increases with {\it P} to at least 1.7 GPa. Conspicuously, with increasing {\it P} ($>$ $\sim$ 0.5 GPa) talc solubility increases and molal Cu concentrations exceed those of Cl. Isothermal Cu solubility appears to mimic the solubility isopleths in the SiO$_{2}$-H$_{2}$O system. Solubility trends suggest that the stability field of copper(I) hydroxide complexes (e.g. Cu(OH)$_{aq}$) expands to higher salinities such that H$_{2}$O may become an effective ligand at high-{\it P}. At constant {\it P} (e.g. 0.35 GPa) solubility decreases with increasing {\it T} (i.e. $>$ $525\deg$C). High-T Cu K-edge XANES spectra of single homogenised synthetic fluid inclusions indicate that highly coordinated chlorocopper(I) complexes (e.g. Cu:Cl, 1:3 to 4) predominate at high salinity, whereas lower-order linear Cu-Cl coordination predominates at lower salinities, in fluids buffered by quartz-talc-copper-cuprite. This is consistent with the interpretation of the solubility data. At equivalent salinity, {\it T} and {\it P} conditions, spectra for fluids buffered by native copper-orthoclase-sillimanite-quartz-magnetite-hematite show no evidence for higher-order chlorocopper(I) complexes. Preliminary extended X-ray absorption fine structure data for these latter inclusions indicate that [CuCl$_{2}$]$^{-}$ predominates. The stability of higher-order complexes is strongly coupled to HCl concentrations, which at constant {\it P} and {\it T} is determined by both the specific mineral assemblage and total salinity. This is the first spectroscopic evidence for highly coordinated chlorocopper(I) complexes in supercritical fluids. Furthermore, the speciation dependence on the buffering mineral assemblage has not been recognized previously. Similarly, this is the first experimental confirmation that copper concentrations in mineral-buffered fluids can be extremely high, e.g. $\sim$ 10 wt%, substantiating inferences based on natural fluid inclusions associated with porphyry copper ore deposits.
DE: 1000 GEOCHEMISTRY (New field, replaces Rock Chemistry)
DE: 8045 Role of fluids
DE: 8424 Hydrothermal systems (8135)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting