Tin deposits are formed from source rocks, not cooling

New research from the University of St Andrews has uncovered the primary drivers of tin mineralization, which will have significant implications for the discovery of tin deposits critical to the energy transition.

In a paper published in Geology, researchers from the School of Earth and Environmental Sciences, alongside colleagues at the University of Oxford, have disentangled the primary drivers of tin mineralization, revealing that the defining characteristics of "tin granites" are established at their geological source—i.e., nature—rather than through later magmatic processing—nurture.

Electrification, which is at the heart of the transition to net zero, will only be possible with sufficient supplies of critical metals. Tin is one such metal. Though rarely in the spotlight, it acts as the essential glue, in the form of solder, powering all modern electronics, from mobile phones and batteries to AI data centers. Consequently, there is an urgent need to understand the geological processes governing tin mineralization to unlock new supplies with a smaller environmental footprint.

Researchers investigated the formation of "tin granites," magmatic rocks responsible for the majority of the world's tin supply, such as those found in southwest England. These granites have traditionally been considered the result of extreme fractional crystallization, where tin is concentrated through geological processing as the granite magma cools and crystallizes. However, this new study challenges that view, demonstrating that the key features of tin granites are driven by their source, i.e., what is being melted to form the magma in the first place.

As well as being tin-rich, these granites have long been recognized as highly reduced, compared to copper-rich granites, which tend to be oxidized. Until now, quantitative models have focused on understanding the tin enrichment itself, but not this reduced signature. By employing the latest advances in geochemical modeling, the researchers sought to answer how a magma becomes both tin-rich and reduced.

The authors show that the reduced character of tin granites is inherited directly from the melting of highly reduced source rocks. Furthermore, by coupling the models with recent experimental studies, they demonstrate that the melting conditions which yield a reduced melt occur exactly within a "redox transition zone" where tin is preferentially enriched in the magma.

The modeling highlights a critical flaw in traditional theories: fractional crystallization actually oxidizes the magma; therefore, the process traditionally invoked as being responsible for tin mineralization cannot also produce reduced granites. Instead, the results suggest that tin mineralization necessitates a source rock that is already tin-rich prior to melting.

Lead author Joseph Benson from the School of Earth and Environmental Sciences at the University of St Andrews said, "We've traditionally assumed tin granites become enriched during cooling and crystallization. In fact, our study reveals these magmas are essentially born this way. Unless you start with a tin-rich, highly reduced source rock, you will never form a tin deposit."

The study provides new, quantitative constraints on tin granite petrogenesis, highlighting the critical role of source conditions in governing whether a particular magma can form a tin deposit. It reveals that the scarcity of tin granites is ultimately controlled by the rare convergence of tin-rich, reduced source rocks rather than magmatic processes. Crucially, these constraints can help narrow down the geological terranes which may be prospective for new deposits, hence aiding mineral exploration.