Program > Browse abstracts by speaker > Hu Bin

Hydrothermal iron speciation governs metal transport efficiency and redox conditions in ore-forming systems. Yet, conventional experimental approaches often obscure authentic high-temperature behaviour through carbon/metal-mediated or quench-induced artefacts. We present an innovative high-pressure/high-temperature (HP/HT) reactor optimised for synchrotron X-ray absorption spectroscopy (XAS), resolving these limitations through key advances: (1) carbon-free sample container using bilateral sealed ultra-pure quartz capillaries; (2) annular heating enabling stable operation ≤700°C while preserving 90˚ fluorescence and 180° transmission detection geometries; (3) can accommodate several independent samples with the same temperature profile; (4) concurrent XAS measurement and optical monitoring. Pressures are regulated via temperature-dependent fluid expansion and are limited by capillary choices. This configuration eliminates redox interference from autoclave components, such as O-ring or metal gaskets, prevalent in traditional hydrothermal reactors. Our results demonstrated sustained Fe(III) stability in acidified FeCl3 + HCl solutions over 250 ˚C, contradicting prior in-situ XAS studies.

System validation through salinity-modulated experiments reveals two temperature-dependent chloride coordination regimes in Cl-bearing solutions. Below 300 ˚C, dominant [FeCl4-] complexes progressively transition to distorted [FeCl3(aq)] configurations at higher temperatures, with XANES spectra confirming tetrahedral/trigonal geometry shifts. These coordination changes, quantified through pre-edge intensity and EXAFS bond length distributions, reduce the number of chloride ligands.

In the presence of magnetite, however, Fe(III) was not retained in solution. This suggests that in natural systems, Fe(III) may be stable only in the absence of catalytic surfaces, e.g. in hydrothermal channels. Our results better constrain the relationships among speciation, temperature, and Fe concentrations in crustal fluids, imposing mechanistic limitations on iron oxide-copper-gold (IOCG) mineralisation, where chloride complexations are important for iron mobility. 

This newly developed setup for in-situ XAS measurements provides a favourable environment for examining the geochemical behaviours of redox-sensitive elements in magmatic-hydrothermal conditions, with practical uses in advancing the modelling of ore-forming processes and analysing mineral exploration geochemical datasets.