Copper solubility in mineral-buffered supercritical fluids — some preliminary results from fluid inclusion synthesis experiments

A. Hack and J. Mavrogenes

Hydrothermal fluxes of metals in the crust and mantle are an important geological phenomena but remain poorly understood because of the scarcity of experimental high-temperature and -pressure mineral solubility data. Simply, this paucity of relevant information is due to the limitations of traditional experimental methods. However, previously uninvestigated pressure and temperature regions are accessible by utilising a synthetic fluid inclusion technique coupled with analysis of individual fluid inclusions by excimer laser-ablation (LA—ICPMS) and/or proton induced x-ray emission (PIXE). 

In the present study copper solubility has been measured in supercritical fluids as a function of salinity at 700^oC and 300MPa under mineral-buffered conditions in the system H₂O-Cu-Fe-K-H-Cl (Figure 1). 

Figure 1: Solubility data for Cu, K, Fe in moles per kilogram H₂O at 700^oC and 300MPa as a function of total chlorine measured in individual quartz-hosted fluid inclusions by LA—ICPMS and PIXE. Fluid was buffered by the mineral assemblage quartz-sillimanite-K—feldpar-magnetite-hematite. Errors = 1s

The experimental design allows the solubility data to be interpreted thermodynamically thereby allowing information on copper complexation and its speciation to be derived. For instance the data can be treated by way of a general dissolution reaction (1).    

       (1) 

  The solubility data are plotted for this reaction in Figure 2. K-H-Cl speciation was solved to give a_HCl°_(aq) and f_H2(g).  

This involved simultaneously solving a system of equations relating mass action, charge balance, and potassium mass balance. Fe data could not be fit to the speciation model but since it is a minor component in these fluids can probably be neglected without significant effect. For simplicity activity coefficients for ionised aqueous species were calculated using the Davies revision of the extended Debye-Hückel expression and neutral species were assigned a value of unity. The standard states were defined as the pure mineral and liquid at the temperature and pressure of interest for solid phases and H₂O. For aqueous species a hypothetical ideal 1 molal solution referenced at infinite dilution at the temperature and pressure of interest, and the pure gas taken at 1 bar and the temperature of interest for gases.

  Figure 2: Thermodynamic representation of the copper solubility data with respect to reaction 1. Curve fit by eye. 

The preliminary solubility data are not detailed enough to allow derivation of the copper chloride speciation, but a simple thermodynamic treatment indicates a significant ‘salting out' (or saturation) effect for copper in the most saline experiments (Figure 2). Further experiments to constrain copper speciation and investigate solubility as a function of salinity, temperature and pressure are continuing.