Tin and tungsten have good potentials for increased demand applications particularly in the electrical and energy storage areas. Similar to other critical metals like Li and Co, Sn and W are essential ingredients for many applications and technologies that are important for a sustainable future.
Granite related hydrothermal mineral systems are the predominant source for Sn and W deposits.Cassiterite, wolframite and scheelite are primary Sn and W ore minerals in nature. The distribution of Sn rich areas around the world is uneven, which may reflects that geochemical heritage is fundamental to form Sn and W deposits. Besides, magmatic differentiation has been considered as another efficient way to enrich Sn in various geological reservoirs. The tectonic setting of Sn and W mineralisation is well understood, with most Sn and W deposits having formed at active margin settings. A comparison between the Tethyan and Andean Sn-W mineral systems confirmed that Sn and W mineral systems can form under thickened continental crust associated with an oceanic crust subduction. The importance of granitoids for the formation of Sn and W mineral systems is well understood. The genetic affinity of causative intrusions can be either S-type, I-type or A-type, but a common feature is that they are reduced (or ilmenite series) and highly evolved (high SiO2 content and high Rb/Sr ratio). Another prominent feature for Sn and W mineral systems is their high concentration of critical metals, including Li, Ce, Ta and In etc. Therefore, Sn and W mineralisation has a close association with other critical metal mineralisation. Overall, the precipitation mechanisms of W (wolframite and scheelite) and Sn (cassiterite) ore minerals from the hydrothermal fluid include (1) fluids mixing, (2) boiling and, (3) water-rock interaction.
Recent studies have highlighted discrepancies in Sn mineralisation and W mineralisation conditions. Although Sn- and W-associated granites have substantial overlapping characteristics, many of their physico-chemical natures (e.g., aluminum-saturation index (ASI) values, zirconium saturation temperatures and crystal fractionation degrees) are distinctive, suggesting Sn- and W-granites may form under different geological conditions. The difference between Sn mineralisation and W mineralisation is also evident by their contrasting fluid-melt partitioning coefficients. Tungsten strongly partitions into the aqueous fluid and can be transported farther away from the intrusion, but Sn slightly partitions into the silicate melt and can precipitate as magmatic cassiterite or be incorporated into crystallizing micas (which can have >100 ppm Sn). Another area warranting more study is understanding the elemental associations observed in Sn and W mineral systems. It is common to have many other metals in Sn-dominant mineral systems, for example W, Li, Nb, Ta. For W-dominant mineral systems, apart from with Sn, other common associated metals include Mo, Au-Bi and Cu. Nevertheless, the relationship between Sn-W and Cu-Au mineral systems at both the regional/provincial-scale and deposit-scale is an intriguing puzzle, because Sn-W and Cu-Au deposits are generally formed under different geological conditions, though their tectonic setting are similar, i.e., arc-related subduction and continental collision. An emerging field for understanding Sn and W mineral systems is made possible with the development of micro-analytical techniques, e.g., in-situ U-Pb geochronology and O-isotopic analyses on cassiterite and wolframite enable a greater understanding of Sn and W mineralising systems. Since both are the primary ore minerals, U-Pb dating on them can deliver direct age information - an advantage compared with many other commodities types like Cu, Au and Ag. However, unlike those commodities, impactful advances on Sn and W exploration models, techniques, and tools have been deficient in recent years; therefore, more attention and effort is needed to boost Sn and W mineral exploration in the future.
This paper was presented to the 2022 Asian Current Research on Fluid Inclusions IX (ACROFI IX) Conference 12-13 December (http://www.csmpg.org.cn/tzgg2017/202210/t20221011_6522628.html)
