INDIUM: AN ELUSIVE ELEMENT OF CRITICAL IMPORTANCE

Indium has been characterized by DOE in their Critical Materials Strategy (2010), as most critical: criticality is a measure that combines risk of short-term supply disruption, and importance to clean energy production. Indium is a vital component in solar cells, and in indium-tin oxide, an electrically conductive, optically transparent material that forms the basis for touch screens and high-end LCDs. Despite its importance, we know little about its behavior in magmatic-hydrothermal (MH) systems. Indium is so dispersed in nature that deposits exploited exclusively for indium are very rare. Where indium reaches high concentrations, for example, at the Mount Pleasant Deposit in New Brunswick, enrichment is related to granite magmatism. There, it has been postulated that indium was concentrated in high-T MH fluids exsolved from granite magmas, and subsequently deposited as indium-rich minerals at lower T (Sinclair et al., 2006).

As a first step toward better understanding the behavior of indium, we have evaluated the distribution of indium among rock-forming minerals in the Tuolumne Batholith, CA, which comprises nested plutons with more primitive rocks at the margin, and more evolved rocks at the core. Quartz and feldspar contained indium at levels consistently below detection (10s of ppb). Conversely, indium was concentrated in the major ferromagnesian phases, biotite and hornblende, reaching average concentrations of 0.22 and 0.72 ppm, respectively, in more evolved members of the suite. Given the modes of these phases, calculated whole rock indium concentrations range from 20-90 ppb, which is consistent with the crustal abundance (50 ppb). These results are consistent with Fe-, Mg-bearing silicates playing a key role in the mass transfer of indium in MH systems.

It is imperative to gain a better understanding of the behavior of indium in MH systems, so that better predictive models can be developed. An experimental program is underway to supplement our measurements of indium in natural systems. Experiments are being performed to evaluate the distribution of indium among sulfides, silicates and associated fluids. Results will allow us to relate the indium distribution in MH systems associated with granitic, including pegmatitic systems, so that better predictive models for indium exploration can be developed.