The role of fluorine in hydrothermal mobilization and transportation of Fe, U and REE and the formation of IOCG deposits

Abstract

The fluorine(F)-rich nature in iron oxide-copper-gold (IOCG) deposits has received much attention since it was recognized that the giant Olympic Dam (OD) deposit contains >2 wt% fluorite. Yet, the significance of the F-U-REE-Fe association remains poorly understood, with four existing hypotheses regarding the role of F: (i) fluoride increases the solubility of metals by forming stable aqueous complexes; (ii) fluoride acts as a precipitating rather than a transporting agent, due to the low solubility of some fluoride minerals; (iii) high F contents may simply reflect the source of the ore fluids; and (iv) the presence of HF(aq) in acidic aqueous solutions may improve leaching of metals and create fluid pathways. In this study, we investigated these hypotheses using thermodynamic modelling, and performed complementary experiments to evaluate the nature and stability of Fe(II/III)-F complexes at elevated temperature.Our in-situ X-ray absorption spectroscopy (XAS) data show that at room temperature, the Fe(II/III) fluoride complexes are more stable than corresponding Fe(II/III) chloride complexes, and the Fe(III) fluoride complexes are important in F-only solutions at low temperature (≤100 °C). Increasing temperature causes precipitation of Fe from F-only solutions, so that above 200 °C there was little detectable Fe left in the solution. In mixed F-Cl solutions, the experimental data and the thermodynamic calculations show that Fe(III)-F complexes are important at low temperature (~≤150 °C) while Fe(II)-Cl complexes predominate at temperatures higher than 200 °C, causing an increase in Fe solubility.We further investigated the potential of granitic rocks as a source for F and metals (Fe, REE and U) using thermodynamic calculations (hypotheses (i), (ii) and (iii)). Our results show that U and La solubilities are mainly controlled by temperature: U solubility is relatively high at T < 200 °C; La solubility is relatively high at T > 250 °C. Fluoride significantly enhances the solubility of U and La compared to F-free system by affecting fluid chemistry or forming stable complexes (La).Our simulations also show that Si solubility is enhanced in F-bearing solutions (hypothesis (iv)), mainly attributed to the increased solubility of H3SiO4− and NaHSiO3(aq). Moreover, the solubility of Si-F increases dramatically. These indicate that F may help breaking the SiO bond in silicates and releasing Si into fluids. Therefore, fluorine may play a key role in enhancing the porosity of the wall rock and breccia, and in mineral replacement reactions. Collectively, these would enhance fluid-rock reaction, and thus may help to drive metal precipitation.Overall, the thermodynamic considerations suggest that the common F + Fe + REE ± U association in IOCGs may not only reflect the source of the metals, but also that F contributes to the metal endowment of some IOCG deposits via a combination of processes, including increasing the metal (U and REE, but not Fe) carrying capacity of the fluids; and dissolving silicates and enhancing the porosity of the breccia and the wall rocks, thus contributing to increased fluid pathways and ore-forming reactions.