Isotopic and elemental distribution of copper between Cu-bearing minerals and aqueous fluids : implications of an experimental study

Abstract

Transport and deposition of copper in the Earth’s crust are mainly controlled by the speciation of Cu and solubility of Cu-bearing phases in magmatic-hydrothermal fluids. In order to improve our understanding of mobilization of copper by hydrothermal fluids, we conducted experiments with Cu-bearing phases (metallic copper, Cu2O, CuCl) and aqueous solutions (H2O, NaClaq, KClaq, HCl, acetate solutions with/without pH buffer) at 25°C-800°C and 0.1 -200 MPa. The high temperature and high pressure (i.e., 800°C and 200 MPa) experiments were conducted in rapid heat/rapid quench argon cold seal pressure vessel using the synthetic fluid inclusion technique. The experimental charges Cu2Os ± CuCls + H2O ± NaClaq ± HClaq (chloride concentration: 0 to 4.3 mol/kg) were loaded in either Cu or Au capsules. Two types of quartz cylinders were used to trap in-situ hydrothermal fluids: (i) pre-cracked and (ii) intact prior to experiment. Fluid composition was subsequently determined by analyzing individual fluid inclusions using laser ablation inductively coupled plasma mass spectrometry. Two types of inclusions, i.e., fluid inclusion and Na-bearing silicate melt inclusion, have been formed exclusively in metallic Cu-NaCl system. Moreover, micron- to submicron-sized cuprite has been observed in both types of inclusions. In HCl±CuCl-bearing systems, fluid inclusion trapping potential nantokite (CuCl) is observed. The Cu content is strongly enhanced by initial chloride content, and can reach up to 4.3 wt% and 11 wt% in 4.3 m NaCl and 1.9 m HCl solutions, respectively. Fast cooling which is avoided by most researchers shows advantages of preservation of ample inclusions in Cu-NaCl system. In addition, the fluid inclusions after rapid quench (25 K/s) yields much smaller variation of Cu content in comparison to the usually favored slow quench process (0.5 K/s). The H-D exchange experiment demonstrates that only H2O is present in isolated, isometric inclusions whereas D2O has been measured in necking-down inclusions, implying isolated (and isometric) inclusions are well sealed and are representative of fluid present at run conditions. This study confirms that synthetic fluid inclusion is an effective method to preserve in situ hydrothermal fluid at high P-T conditions. Two coexisting phases, i.e. hydrothermal brine and silica-rich melt phases, may be responsible for Cu transport and enrichment. The moderate temperature and pressure (100-250°C, 5-30 MPa) experiments were conducted in a Parr autoclave allowing for in-situ sampling of liquid phase. The partitioning of Cu between cuprite and hydrothermal fluids (KClaq, H2O, pH buffered KClaq and H2O, where pH buffer refers to 0.2 m HAc/KAc) has been investigated from two aspects: Cu concentration and isotope fractionation. Experimental products are native copper and tenorite. Native copper is formed at 250°C and occurs in H2O and KCl-bearing runs and short-termed (≤24 hours) acetate-bearing runs. Tenorite formed in 150°C and 250°C long-termed (72 hours) acetate-bearing runs. Four competing reactions control the Cu partitioning, i.e., cuprite dissolution, Cu(I) disproportionation into Cu(II) and native Cu, decomposition of acetate into methane and carbon dioxide and oxidation of dissolved Cu(I) to Cu(II). It is worth noting that the last reaction exclusively occurs in Cu2O-acetate systems. During the cuprite dissolution stage (<~6 hours), Cu content in pH buffered solution is by an order of magnitude higher than that without. In pH buffered solutions: (i) Cu content in KCl solutions is up to two orders magnitude higher than that of acetate solutions; (ii) temperature and salinity can significantly affect Cu content, whereas the effect of pressure is insignificant. The subsequent coupled Cu(I) disproportionation and acetate decarboxylation processes result in a reduction of dissolved Cu. Conjointly, the resulting isotope fractionation is 0.10±0.10‰ (Δ65Cu [Cu(0)-Cu(I)]). The oxidation substantially increases Δ65Cu Cu(II)O-Cu(I) to 0.35±0.05‰. The low temperature-pressure (5-80°C, 0.1 MPa) experiments were conducted to get insights into the mechanisms of isotope fractionations induced by reduction processes (Cu2+ + 2e- = Cu0). All experiments have been conducted in aqueous CuSO4 solutions using Cu electroplating method. In all cases, the plated Cu metal is enriched in the light isotope (63Cu) with respect to the solution. At room temperature the Cu isotopic fractionation between the electroplated Cu and electrolyte is found to increase with electrolyte concentration and stirring speed, and to decrease with current and run duration. These trends can be explained by three competing processes: copper transport in the solution, the kinetics of electrochemical reduction of copper ions and the surface diffusion at the electrode, i.e., transport becomes important at low copper concentration, low stirring speed, high currents and large amount of copper precipitation. Copper isotope fractionation has a maximum (Δ65Cu = -2.66±0.02‰) near 35°C, decreasing both towards higher and lower temperatures. Our findings in comparison to other studies imply that transformation of fivefold to sixfold coordinated aquacomplexes of Cu2+ to linear Cu+ complexes is a key step during reduction of copper in aqueous solutions, inducing large negative copper isotope fractionation. These findings support that copper isotopes can be used as effective tracers of redox processes. This may have implications to various hydrothermal ore deposits, such as supergene processes, black smokers and volcanic-hosted massive sulfide.