Indium partitioning between silicate melts and magmatic fluids: implications for indium ore genesis and the tracing of magma degassing

Abstract

Most of the global indium resources are from deposits associated with felsic rocks. Considering that the indium concentration in the average upper crust is extremely low, magmatic-hydrothermal processes play a critical role in the enrichment and mineralization of indium. However, the extraction efficiency of indium by magmatic fluids remains unclear. We performed experiments at 800 °C, 150 MPa and oxygen fugacity of Ni-NiO buffer to determine the partition coefficient of indium between aqueous fluids and granitic melts (DInf/m). To counteract the rapid loss of indium into the noble metal container wall, the DInf/m were obtained by the method of local equilibrium between microscopic-sized fluid bubbles and surrounding silicate melt. The results show that indium has the highest fluid-melt partition coefficients of all metals investigated so far. At a constant aluminum saturation index (ASI = 1.07–1.12), DInf/m correlates linearly with the total Cl concentration in the coexisting fluid (mCltotal), increasing from 61 ± 11 (1σ) at mCltotal = 1.0 mol/kg H2O to 895 ± 105 (1σ) at mCltotal = 16 mol/kg H2O. When the HCl concentration in the solution increases from 0.13 to 0.49 mol/kg H2O at a fixed mCltotal = 2 mol/kg H2O, DInf/m increases parabolically from 129 ± 21 to 320 ± 24. The observed partitioning data suggest that indium was dominantly present as In(OH)2Cl in the low-HCl and In(OH)Cl2 in high-HCl aqueous fluid at the experimental conditions. Numerical modeling indicates that the extraction efficiency of indium from S-type felsic magmas is very high (>85 %) and magmatic fluids may be the major source of indium for most indium deposits. Due to the high DInf/m values, fluid saturation and exsolution result in a sharp drop of indium concentration in both the residual melt and crystallized minerals, which makes the indium concentration in whole rock and minerals an indicator of fluid exsolution.