CONSTRAINTS ON THE FORMATION OF GRANITE-RELATED INDIUM DEPOSITS

The use of indium in modern technologies has grown in recent decades, creating a growth in indium demand; thus, there is a need to constrain the spatial and temporal distribution of indium-bearing, granite-related deposits. Toward this end, a conceptual model and exploration vectors for the formation of granite-related indium deposits have been developed. The magmatic-hydrothermal system is modeled by consideration of crystal-melt and vapor-melt equilibria. The model calculates the efficiency of removal of indium from a melt into a volatile phase, which may serve as a component of an ore-forming fluid. The results of the model suggest that as the proportion of ferromagnesian minerals increases in the associated granites, the probability of indium ore formation decreases. Further, for a given modal proportion of ferromagnesian minerals, as the modal proportion of amphibole increases, the probability of indium ore formation decreases. Lastly, for a given modal proportion of biotite, as the magnesium content of the biotite increases (as would result from increasing oxidation of the magmatic system), the probability of indium ore formation decreases. Granites with the highest probability of being associated with indium ore formation will typically be part of A-or S-type igneous systems and will likely be highly fractionated (e.g., A-type topaz granites). I-type granites will generally have a lower potential of being associated with indium-bearing deposits. However, some I-type granites may be associated with indium-bearing deposits if the deposits contain granites (sensu stricto) or other related rocks (e.g., alaskites) that lack amphibole or other ferromagnesian phases.