Cu isotope fractionation between Cu-bearing phases and hydrothermal fluids: Insights from ex situ and in situ experiments

Cu isotope ratios have been widely applied to fingerprinting metal sources, tracking fluid pathways, and tracing mineralization processes, for which knowledge of isotope fractionation is required. This paper presents new experimental calibration data of Cu isotope fractionation between Cu-bearing minerals (native Cu and cuprite) and Cl-bearing hydrothermal fluids at 25-800 degrees C and 0.1-200 MPa. The experiments were performed either using a polytetrafluoroethylene (Teflon) beaker on a hotplate, a rapid heat/rapid quench argon cold seal pressure vessel (RH/RQ Ar-CSPV), or a large volume fluid reactor, which enabled fluid sampling ex situ (the former two) and in situ (the latter). Three setups were designed to investigate Cu isotope fractionation (Delta65Cu<INF>fluid-mineral</INF>): (1) associated with Cu(I) dissolution, Cu(I) disproportionation as well as oxidation of Cu(0) (native copper) and Cu(I) in Cu +/- Cu<INF>2</INF>O + HCl systems at temperatures of up to 800 degrees C and pressures of up to 200 MPa; (2) related to Cu-Au alloying processes in the Cu + NaCl/HCl system at 600 degrees C and 150 MPa; (3) to evaluate the influences of different sampling techniques (ex situ vs. in situ) and superimposed cooling processes. The selected system is Cu<INF>2</INF>O + NaCl + 0.2 m HAc/NaAc (pH-buffer), and runs were conducted at 200-250 degrees C and 20 MPa. Delta 65Cu<INF>fluid-mineral</INF> shows the least variation during solid separation from source fluids, generally <0.16 +/- 0.03 parts per thousand. Positive Delta65Cu<INF>fluid-mineral</INF> values are found in most runs, excluding cuprite dissolution in NaCl/HCl solutions at 200-250 degrees C, 20 MPa, and 800 degrees C, 200 MPa. Cu oxidative leaching into Cu(I) results in decreasing Delta65Cu<INF>Cu(I)-Cu(0)</INF> values from 0.12 +/- 0.05 parts per thousand to 0.07 +/- 0.05 parts per thousand, and -0.30 +/- 0.03 parts per thousand at 25, 50, and 200 degrees C, respectively. In contrast, Cu dissolution at high temperatures (600-800 degrees C) leads to Cu(I)-containing quench fluids and subsequent native Cu precipitates. Both products are enriched in heavy isotopes by up to 5 parts per thousand, caused by preferential enrichment of 65Cu at the surface layer during the alloying-induced diffusion transport process. Cuprite oxidative leaching in HCl leads to fluids enriched in Cu(II), and the corresponding Delta65Cu<INF>Cu(II)-Cu(I)</INF> increases from 0.52 +/- 0.04 parts per thousand at 25 degrees C to 0.89 +/- 0.02 parts per thousand at 50 degrees C. Fluids are enriched with light isotopes (63Cu) during cuprite dissolution at 200, 250, and 800 degrees C, i.e., 0-0.55 +/- 0.04 parts per thousand lower than the precursor. At 250-300 degrees C, Cu(I) disproportionation into Cu(II) and Cu(0) dominates the observed isotope fractionation, yielding Delta65Cu<INF>fluid-Cu<INF>2</INF>O</INF> up to 0.59 +/- 0.03 parts per thousand and Delta65Cu<INF>Cu(0)-Cu<INF>2</INF>O</INF> up to -0.28 +/- 0.02 parts per thousand. Rapid cooling (3-25 K s-1) relative to slow cooling (0.014 K s-1) can cause phase separation as well as significant isotope fractionation, particularly if fluids cool from an intermediate high temperature to ambient temperature (e.g., from 200-300 to 25 degrees C), which highlights the importance of kinetic processes that may potentially alter the isotope composition of natural ore-forming fluids.