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 ( D In f / m ). To counteract the rapid loss of indium into the noble metal container wall, the D In f / 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), D In f / m correlates linearly with the total Cl concentration in the coexisting fluid ( m Cl total ), increasing from 61 ± 11 (1σ) at m Cl total = 1.0 mol/kg H 2 O to 895 ± 105 (1σ) at m Cl total = 16 mol/kg H 2 O. When the HCl concentration in the solution increases from 0.13 to 0.49 mol/kg H 2 O at a fixed m Cl total = 2 mol/kg H 2 O, D In f / m increases parabolically from 129 ± 21 to 320 ± 24. The observed partitioning data suggest that indium was dominantly present as In(OH) 2 Cl in the low-HCl and In(OH)Cl 2 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 D In f / 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.