Sub-solidus hydrology controls the formation of high-grade tin deposits in granites

Tin deposits form primarily in and above the roof of granitic intrusions, commonly with important resources of other strategic and critical raw materials (e.g., W and Li). The metals are extracted by magmatic fluids exsolved from the crystallising intrusion; however, it is unclear how the fluids are collected and focused to form spatially confined high-grade ore bodies. Here, we present numerical simulations showing that: (1) most tin-bearing fluids that exsolve from a granitic intrusion during cooling and crystallisation are not expelled but can remain stored in the crystallising intrusion, and (2) efficient fluid mobilisation, focusing, and ore precipitation occur as naturally emerging sub-solidus processes. Hydrothermal convection in the overlying country rock – driven by the magmatic heat – cools the intrusion, which enhances permeability and induces a lateral flow of mobilised magmatic fluids towards strongly localised zones of intense magmatic-meteoric fluid mixing, an efficient mechanism for tin precipitation. Temperatures, pressures, compositions, and time scales of the simulated fluid evolution closely match field-based observations. We find that different tin precipitation mechanisms proposed in the literature ‒ fluid mixing, cooling, and boiling ‒ may all be reconciled as variations of this hydrological process in response to varying geologic conditions such as intrusion depth and/or minor variations in permeability. Our simulations show that magmatic fluids can remain stored post-crystallisation until near the ductile-to-brittle transition, thereby introducing a novel process for fluid focusing that may facilitate magmatic-hydrothermal ore formation and providing a natural interpretation for sub-solidus granite alteration.