Effect of fluid salinity and melt composition on the fluid-melt partitioning of Cu-Zn evidenced by ex situ and in situ measurements.

The growing worldwide demand for metals for industrial purposes has led to the development of new experimental approaches to quantify the extraction of metals from magmatic sources. This implies calculating metal partition coefficients between fluid and melt phases at high pressure and high temperature conditions. Such constraints are necessary to understand metal mobility and deposition in magmatic-hydrothermal environments. Cu and Zn have recently received particular attention, being among the 5 more demanded metals for energy transition due to their wide use in industrial domains.

Previous Cu-Zn fluid-melt partitioning experiments involved fluid inclusion synthesis in cold seal pressure vessels (CSPV) and internally heated pressure vessels (IHPV) with post-mortem analysis of quenched fluids in equilibrium with felsic melts. These studies showed a high scattering of partition coefficient values (from 0.5 to 433 for Cu and from 0.01 to 136.1 for Zn), which may arise from the different P-T conditions investigated and the compositions of melts and fluids [1,2,3,4]. Yet this scatter also points to potential issues and limitations with the employed methods such as uncertainties while measuring.

To overcome those limitations, an approach combining in situ and ex situ techniques has been employed. The ex situ technique involves an IHPV with quench melt analysis by LA-ICP-MS and quench fluid analysis performed by ion chromatography for cations/anions and solution ICP-MS for trace elements; it was employed for dacite and rhyolite melts [5, 6]. However, this method relies on the analysis of post-mortem samples, which do not preserve HP-HT information. A newly developed experimental method for in situ measurements also is thus presented involving an IHPV provided with transparent windows allowing a laser or Synchrotron X-ray beam to be transmitted through. Both methods have been comparatively applied to calculate Cu-Zn fluid-melt partition coefficients in pure water and a 0.2m NaCl solution with haplogranite-rhyolite-andesite melts (1000 bar – 800-1000 °C). This data allows to discuss the effect of fluid salinity and melt composition. Preliminary analyses show higher contents of Zn in the melts compared to Cu (> 500 ppm against ~45 ppm) suggesting that Zn has a stronger affinity for the melt relative to Cu.

[1] Bai and Koster van Groos, 1999. GCA 63, 1117-1131

[2] Williams et al., 1995, Contrib. Mineral. Petrol. 121, 388-399

[3] Urabe, 1987, Eco. Geol. 82, 1049-1052

[4] Zajacz et al., 2008, GCA 72, 2169-2197

[5] Iveson et al., 2019, 516, 18-41

[6] Gion et al., 2022, Chem. Geol. 121061