Indium as a critical mineral: A research progress report

Indium (In) is a post-transition metal of Group 13 in the Periodic Table. It is a highly volatile chalcophile element which behaves in a moderately to highly incompatible manner. Abundance of indium in the continental crust is estimated at 0.01 ppm. Indium has been widely used in high-tech products, especially in emerging industry technology and national defense and security sectors. Therefore, it is called "critical mineral" in developed countries. Indium becomes one of the critical metals to support vigorous development of China's emerging strategic industries. More than 90% of indium is recovered as a by-product of zinc mining. The most recent assessment of global indium reserves in 2018 was 79000 tonnes. China accounts for more than 18.2% of global indium reserves, while Peru, Canada, USA and Russia together own more than 62.4%. Indium occurs in different types of ore deposits. Indium-rich deposits include volcanic- and sediment-hosted exhalative massive sulfide deposits, epithennal deposits, polymetallic base metal vein-type deposits, granite-related tin-base-metal deposits, skarn deposits and porphyry copper deposits. These deposits are commonly associated with active oceanic or continental plate margins and orogenic belts with steep geothermal gradients due to enhanced magmatic activities. Most of indium deposits are associated with highly differentiated granites. Biotite is the main carrier mineral of indium. Recent research indicates that the enrichment of indium does not increase with the increase of crystallization differentiation in felsic magmatic systems. The increase of magnesium concentration in biotite reduces the potential of indium mineralization. The joint action of alkaline and sub-alkaline mafic and granitic magmas is an important prerequisite for the formation of high-grade indium deposits. Volcanic systems provide favorable conducive environment for the enrichment of indium. There are few studies on the enrichment of indium in silicate minerals. Garnet is a major tin-bearing skarn mineral at the Dulong Sn-Zn deposit of Yunan Province. Based on LA-ICP-MS analysis, indium concentration is 166.0-629.0 ppm in the garnet that was formed in the early skarn stage and 1.6-10.0 ppm in the garnet that was formed in the late skarn stage. The data indicate that the earlier garnet is enriched in indium than the later one. The data also show that indium has an apparent covariant relationship with tin, but an inverse correlation with aluminum. Multi-stage fluid activities in the evolution of magmatic hydrothermal processes are the key to enrichment of indium. Overprinting of multi-stage hydrothermal processes plays an important role in the formation of high-grade indium deposits. For example, indium-rich sphalerite in the Hiimmerlein skarn polymetallic deposit in Germany occurs around the early indium-free sphalerite grain, demonstrating overprinting of later indium-rich ore fluids. There are three ways for indium to substitute for zinc in sphalerite: (1) Cu++ In3+<-> 2Zn(2+)(2) Cu+/Ag++In2+<-> 2Zn(2+) and (3) In3++Sn3+ +(vacancy)<-> 3Zn(2+). The substitute of indium for zinc in sphalerite is slightly different due to the ore-forming element (Cu, Ag, Sn, Ga, Ge. etc.). Tin can replace zinc with different valence state and other elements depending on the precipitation environment of sphalerite. Recent studies have shown that there are "indium window" and "indium explosion" that exist during the process of indium mineralization. Modern analytical technology and experimental simulation reveal the mechanism for "indium explosion" effect in the Dulong Sn-Zn deposit of Yunan Province. In this mechanism, lattice displacement and vacancies in small number within sphalerite facilitate indium enrichment in {111} direction in sphalerite. In summary, several items should be followed for the future research and exploration of resources of critical mineral indium: (1) Fully understand the occurrence state of indium in major carrier minerals and reveal mechanism for selective super-enrichment of indium; (2) improve our understanding of indium enrichment mechanism in different geological settings, which would effectively guide future exploration of indium deposits; (3) renovate indium recycling technologies and seek its substitutes.