SEEKING THE MISSING ANIONIC SPECIES IN LOW SALINITY CU-RICH FLUIDS FROM MAGMATIC-HYDROTHERMAL SYSTEMS

The recognition of Cu-rich, low salinity, commonly CO₂-bearing magmatic ore fluids in porphyry deposits points to the possible role of non-Cl complexing of certain metal species in these systems. 213nm UV LA-ICP-MS analyses of metal contents in low-salinity (3-5 wt% NaCl equivalent) aqueo-carbonic (3-8 mol% CO₂) fluid inclusions from deep, early quartz-dominated porphyry veins at Butte, Montana, reveal high copper contents of up to 9000 ppm and Cu is typically one of the dominant cations along with K, Na and Fe. The common failure to detect Cl – despite large ablation signals for other elements and with Cl concentrations inferred from PIXE analysis well below those derived from microthermometry assuming NaCl dominance – suggests that an additional anion may be important in these fluids. Further, the extreme variability in measured Cu/Cl ratios suggests that Cu is not principally complexed by chloride. If additional anions are present, the quantification of LA-ICP-MS data which depends on modeling of microthermometric data in simple salt-water systems will be compromised. The presence of daughter minerals in fluid inclusions can provide qualitative information on additional anionic species. Tetrahedral or scalenohedral chalcopyrite daughters are common. Transparent solids that are commonly interpreted as daughters and identified by laser raman microprobe include feldspar, anhydrite (diagnostic peak at ˜1017 cm^-1), and possibly other metal sulfates (Pb, Ba). Additional, unidentified Fe±Mn(±Cu) translucent phases were also imaged by PIXE. CO₂ was confirmed as the dominant gaseous phase, with trace N₂ in one inclusion. In the aqueous phase, CO₂ was ubiquitous and sulfate (~980 cm^-1 peak) was sometimes identified. The recognition of hexavalent sulfur as sulfate in solution appears to contradict thermodynamic data that predict H₂S and SO₂ as major species at magmatic temperatures and pressures. Although hydrogen diffusion could have affected sulfur speciation post-trapping, limited experimental data from the literature support the possibility of the occurrence of sulfate under moderately oxidizing conditions at elevated P-T. This could have a profound effect on metal transport and redox-related depositional mechanisms and could help to explain the unusual copper concentrations observed in these fluids.