EXPERIMENTAL GEOCHEMISTRY AND MODELING AS AN AIDE TO EXPLORATION: AN INDIUM CASE STUDY

The techniques used to evaluate the potential of mineral prospects vary and are specific to deposit type, target metals, and locality. Constraints on ore formation can be evaluated through observing deposits, conceptual modeling, or experimental geochemistry. Regardless of method, conclusions should provide exploration vectors for the exploration enterprise, which may be concerned with grade and tonnage, or where in space and time deposits were likely to form. In this study we construct exploration vectors for indium-bearing systems based on experimentally-derived data interpreted in light of existing deposits. To construct these vectors, experiments were performed in cold-seal pressure vessels at 800°C, 100 MPa, for durations of 1-8 weeks. Nernst-type partition coefficients between biotite/melt (D^Bt/Melt), amphibole/melt (D^Amph/Melt), and vapor/melt (D^Vapor/Melt) were determined. The run product glass is a high-silica rhyolite with an ASI of ~1.08 and ~0.18 wt% Cl. D^Bt/Melt decreases with increasing annite component of biotite and ranges from 0.6 to 17. The average D^Amph/Melt is 35 ± 3 (1σ) and the average D^Vapor/Melt is 13 ± 2 (1σ). Because Cl likely influences the exchange of indium between vapor and melt, the D^Vapor/Melt is an estimate of indium exchange in absence of a formal equilibrium constant. These partition coefficients are used to model the evolution of magmatic-hydrothermal deposits with respect to indium. This model predicts the efficiency of ore formation based on the melt fraction, proportions of biotite and amphibole, and percent of volatile saturation. Modeling suggests that for every modal percent of magmatic amphibole, the efficiency of ore formation decreases 23%. In contrast, magmas lacking amphibole and containing minimal biotite (<5 modal percent), such as A-type granites, have higher potential to form indium-bearing deposits. Additionally, when indium behaves incompatibly, the efficiency of ore formation is enhanced by late stage volatile saturation (due to the presence of F). Exploration vectors suggest that Climax-type deposits and deposits associated with topaz rhyolites are prime candidates for indium exploration, so long as suitable indium host minerals are precipitated from magmatic ore-fluids.