Reaction between Cu-bearing minerals and hydrothermal fluids at 800 °C and 200 MPa: Constraints from synthetic fluid inclusions

Transport and deposition of copper in the Earth's crust are mainly controlled by the solubility of Cu-bearing phases and the speciation of Cu in magmatic-hydrothermal fluids. To improve our understanding of copper mobilization by hydrothermal fluids, we conducted an experimental study on the interaction between Cu-bearing phases (metallic copper, Cu2O, CuCl) and aqueous chloride solutions (H2O ± NaCl ± HCl; with Cl concentrations of 0 to 4.3 mol kg-1). The experiments were run in rapid heat/rapid quench cold-seal pressure vessels at 800 °C, 200 MPa, and logfO2 ~ NNO+2.3. Either Cu or Au capsules were used as containers. The reaction products were sampled in situ by the entrapment of synthetic fluid inclusions in quartz. Fluid composition was subsequently determined by analyzing individual fluid inclusions using a freezing cell and laser ablation inductively coupled plasma-mass spectrometry. Our results show that large isolated and isometric inclusions, free of late-stage modifications, can be preserved after the experiment even when using a high cooling rate of 25 K s-1. The obtained results demonstrate that: (1) reaction between native Cu, NaCl solution, and quartz (± silica gel) leads to the coexistence of fluid inclusions and Na-bearing silicate melt inclusions. Micrometer-to submicrometer-sized cuprite (Cu2O) crystals have been observed in both types of the inclusions, and they are formed most probably due to the dissociation of CuOH. (2) When Cu0 reacts with HCl and CuCl solutions, or Cu+ reacts with NaCl solution, nantokite (CuCl) formed due to oversaturation has been found in fluid inclusion. Copper concentration in the fluid shows a strong positive dependence on the initial chlorine content, with Cu/Cl molal ratios varying from 1:9 to 1:1 in case 1 and case 2, respectively. When Cl is fixed to 1.5 m, initial fluid acidity has a major control on the Cu content, i.e., 0.17 ± 0.09 and 1.29 ± 0.57 m Cu were measured in fluids of case 1 and case 2, respectively. Cu solubility in pure water and in 1.5 m NaCl solutions are 0.004 ± 0.002 and 0.16 ± 0.07 m, respectively. The main responsible Cu-bearing complexes are CuOH(H2O)x in water, NaCuCl2 in NaCl solutions and HCuCl2 in alkali-free solutions. These results provide quantitative constraints on the mobility of Cu in hydrothermal solutions and confirm that Cl is a very important ligand responsible for Cu transport. The first observation that silicate melt can be generated in the fluid-dominated and native-copper-bearing system implies that transitional thermosilicate liquids can coexist with metal-rich fluids and may enhance Cu mobility in magmatic-hydrothermal systems. This may have important implications for the formation of Cu deposits in the systems with low S activities.

Two methods for temperature measurement in super-hot geothermal systems based on synthetic fluid inclusions

<p>Super-hot geothermal systems are promising targets for near future geothermal exploration either for direct fluid exploitation or as potential reservoirs of Enhanced Geothermal Systems. Although reservoir conditions assessment is crucial for the evaluation of the geothermal resources, temperature measurement is still a major challenge in super-hot systems since their extreme conditions (i.e. very-high temperature, possible presence of aggressive fluids) preclude the use of conventional logging methods. During two EU projects (i.e. IMAGE (FP7) and the DESCRAMBLE (H2020)) two methods based on fluid inclusions synthesis were developed for in-situ measurements of very high-temperature (i.e. ≥400°C). Synthetic fluid inclusions are produced by trapping fluid within pre-fractured minerals, free of natural fluid inclusions, placed in a gold capsule together with an aqueous solution. Laboratory tests showed that fluid inclusions in quartz form in a relatively short time (down to 48 hours) if an alkaline-saline solution (0.4 M of NaOH + 10 to 20 wt.% NaCl) is used. In the first method synthetic fluid inclusions in quartz chips are produced within gold capsules placed inside a micro-reactor containing a volume of de-ionised water in such amount that the density of water in the micro-reactor has the critical value. Under these conditions, the trapping temperature of synthetic inclusions can be computed by the intersections between inclusion isochores, determined through microthermometry, and the critical isochore of water. Thus, if the micro-reactor is kept for at least 48 hours at the depth of measurement in a geothermal well, the trapping temperature of fluid inclusions formed in capsules would correspond to the well temperature at that depth. The second method consists in the production of fluid inclusions in gold capsules in direct contact with the environment of the geothermal well. Under the conditions of the super-hot systems characterized by relatively low pressure (such as the deepest part of the Larderello-Travale geothermal system in Italy), pressure-temperature conditions would cause fluid immiscibility in the gold capsule (i.e. the saline-alkaline fluid splits in a high-salinity liquid and a low-salinity vapor). In this case, the trapping temperature of both high-salinity and low-salinity inclusions is equal to their homogenization temperature. Laboratory tests demonstrated that the trapping temperatures of fluid inclusions produced by both methods can provide a good estimate of the experimental temperatures. Two field tests following the first method were performed in geothermal wells of Krafla (Iceland) and Larderello-Travale (Italy) characterized by measured temperature at the test depth of 336°C and 249°C, respectively. These tests showed that synthetic fluid inclusions trapping temperatures closely approach the temperature measured using conventional methods. Finally, a field test was also attempted in the Venelle 2 (Larderello-Travale) geothermal well characterized by super-hot conditions. Trapping temperatures of fluid inclusions formed at 2900 below ground level (b.g.l.) by both methods resulted compatible with independent measurement by an electronic device which gave 444°C at 2810 m b.g.l..</p><p>The research leading to these results has received funding from the EC Seventh Framework Programme under grant agreement No. 608553 (Project IMAGE) and from the Horizon 2020 Programme under grant agreement 640573 (Project DESCRAMBLE).</p>

Interaction of B-rich supercritical magmatic fluids with granite: first report of dumortierite in Larderello, Italy

In a peraluminous two-mica monzogranite cored at 4.5 km depth in well Radicondoli 29, dumortierite occurs together with andalusite, Li-rich tourmaline, fluorite and fluorapatite. The average composition is SiO2: 30.9, Al2O3: 58.6, FeO: 0.44, MgO: 0.47, CaO: 0.02, F: 0.15. Dumortierite crystallized from supercritical magmatic fluids at T =520-620°C, P = 100 ± 30 MPa. Synthetic fluid inclusions yielded T = 510 ± 10°C, P = 42 ±3 MPa at 2.9 km depth in well Venelle 2. All data suggest present-day T = 450-550°C, P = 40-80 MPa. The supercritical magmatic fluids were hypersaline brines with ~ 30 wt % LiCl and up to 2.4 wt % F, extreme contents that can be found only in pegmatites, aplites and leucogranites. Larderello granites derived from partial melting of a lower crust made up of interlayered metasediments and amphibolites. Extensive melting took place in the lower crust during the last 10 Ma owing to extensional tectonics, lithosphere thinning, roll back or break down of the west merging Adria plate and swelling of the asthenosphere below the western side of the Apennines.