Cu and Ni solubility in high-temperature aqueous fluids

Abstract

Copper and nickel are generally associated in magmatic sulfide ores formed byimmiscibility in mafic and ultramafic magmas. In contrast, hydrothermal Cu-Ni deposits are uncommon andthese elements usually occur in separate Cu-Fe-sulfide and Ni-Co-Ag-Bi-As-S mineralizations. Among theporphyry-type deposits formed at high temperatures to about 700 °C, there are many copper but no nickeldeposits [1], pointing to a higher solubility of Cu relative to Ni in aqueous fluids at such conditions. The aim ofthis study is to measure the solubilities of Cu and Ni sulfides in high-temperature hydrothermal fluids in-situusing synchrotron-radiation micro-X-ray fluorescence spectrometry.Synthetic CuS or NiS crystals were partly dissolved in aqueous NaCl, NaCl+HCl, or CaCl2 solutions attemperatures of 400 to 600 °C and pressures between 70 and 900 MPa using a modified hydrothermaldiamond-anvil cell with a recess in one diamond [2]. Consecutive XRF spectra of the fluid in the recess werecollected in a confocal mode to exclude signal contributions from the crystals in the sample chamber [3].Equilibrium was assumed if the determined concentrations of the dissolved metals indicated that a steadystate was attained.The measured dissolved Cu concentrations ranged between 22 ppm at 70 MPa, 500 °C and 235 ppm at 306MPa, 600 °C in 0.5 to 1.6 m NaCl solutions. We observed a decrease in Cu concentration with increasingpressure at constant temperature, and for 1.6 m NaCl an increase by a factor of two along an isochore from120 MPa, 500 °C to 306 MPa, 600 °C. Higher Cu solubilities were determined in more concentrated solutions.A preliminary run with a more acidic NaCl+HCl solution (pH ~1) revealed a dramatic increase in the dissolvedCu concentration to 7898 ppm at 170 MPa, 500 °C.The measured dissolved Ni concentrations ranged between 3 ppm at 200 MPa, 500 °C in a 1 m NaCl solutionand 33 ppm at 411 MPa, 500 °C in a 0.75 m CaCl2 solution. A solubility maximum at 500 °C along anisochore was observed for both solutions. The Ni solubility increased with pressure at constant temperature.Experiments with aqueous CaCl2 solutions resulted in higher dissolved Ni concentrations compared to NaClsolutions at similar pressure-temperature conditions.Our experiments suggest that the solubility of Cu and Ni in aqueous fluids is mainly governed by fluidcomposition. For both elements, solubility increased in more chlorine-rich fluids, which could reflect metalchlorinecomplexation. Preliminary results for Cu indicate a strong dependence of the solubility on the pH ofthe fluid. A contrasting solubility behavior of Cu and Ni was observed with increasing pressure, which might beone reason for the difference in hydrothermal ore deposit formation.[1] Barnes (1979) Geochemistry of hydrothermal ore deposits, Wiley. [2] Schmidt and Rickers (2003) Am.Mineral. 88, 288-292. [3] Wilke el al. (2010) J. Synchrotron Rad. 17, 669-675