INDIUM IN MAGMATIC-HYDROTHERMAL SYSTEMS: UNDERSTANDING ITS BEHAVIOR TODAY TO MEET TOMORROW'S DEMAND

Indium is an essential component in indium-tin oxide (ITO), an electrically conductive, and optically transparent material that forms the basis for touch screens and high-end LCDs. Increases in production can result from better models for exploration, which can be aided by better models for the formation of indium-bearing ores, yet little is known about the behavior of indium in these systems. As a first step toward solving this problem, we have evaluated the distribution of indium among rock-forming minerals in the well-characterized Tuolumne Batholith, CA, which comprises nested plutons with more primitive rocks at the margin, and more evolved rocks at the core. Minerals from all plutons were analyzed for select major and minor elements. Quartz and fsp contained indium at levels consistently below detection (10s of ppb). Conversely, indium was concentrated in the major ferromagnesian phases, bt and hbl, reaching average concentrations of 0.22 and 0.72 ppm, respectively, in the more evolved members of the suite.

We also performed experiments to evaluate the partitioning of indium between a sulfide(pyrrhotite, po) (Fe1-xS) and silicate melt (m). Experiments were performed at 800 °C, 100 MPa, and fO2 ≈ NNO in a vapor-brine-rhyolite melt system for 5 to15 days. Three separate series of experiments were conducted in which each differed by the aqueous solution added: 1) pure water; 2) 1.01 M Cl- solution; and, 3) 0.35 M CuCl2-bearing solution. Changes in starting material produced changes in the composition of the run product po and glass. The partition coefficients D(po/m) from the experiments were on the order of ≈ 10, ≈ 1.5, and ≈ 3, respectively. Although the D varies depending upon the composition of the starting aqueous solution, D is ~ 4 is a reasonable, preliminary, order of magnitude estimate D(po/m). By using reasonable estimates of the mode of po that crystallizes in magmatic systems, the proportion of indium sequestered by po can be evaluated. The results indicate that po sequesters <0.5% indium from a crystallizing silicate melt. Results presented here are consistent with the inability of po alone to limit the capacity of a magmatic-hydrothermal system to yield an indium-rich ore fluid, and we suggest that Fe-, Mg-bearing silicates can play a key role in the mass transfer of indium in magmatic-hydrothermal systems.