Transport processes of Sn and W within the crust: From a peraluminous granitic melt to a mineralising fluid

• Applicant: Julie Michaud, Ph.D.
• Subject Area: Mineralogy, Petrology and Geochemistry
• Term: since 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 521731491

Tin (Sn) and tungsten (W), whose resources are mainly associated with magmatic-hydrothermal systems, have been established as strategic metals and successful exploration of economically valuable deposits depend on a fundamental understanding of ore-forming processes including source and primary concentration, transport, deposition and remobilisation. Together with other high-field-strength (HFSE; i.e., Nb and Ta) and fluxing elements (e.g., Li, P, F), Sn and W commonly occur in close spatial relationship with late to post-orogenic crustal peraluminous granitic systems - including rare-metal granites (RMGs) and pegmatites - and related hydrothermal activity. Tin and W concentration to economic grades results from a combination of melt- (i.e., preconcentration) and fluid-driven (i.e., remobilisation) processes. The transport and redistribution of these elements within the crust depend on different factors such as their solubility in melt and fluid in equilibrium with ore-bearing minerals and their partitioning between melt and fluid at the magmatic-hydrothermal transition. While melt and fluid compositions are important parameters for both elements, W is highly sensitive to temperature and Sn to redox conditions (i.e., storage and transport as Sn2+ or Sn4+) for a given composition. Although main controlling parameters have been identified, there is a need for further investigations to unravel the discrepancies in the literature (e.g., related to the complexity of such systems and to potential experimental problems). To serve that purpose, we plan to focus on several aspects using different approaches: (i) Sn- and W-bearing minerals solubility experiments in melt and fluid considering the geochemical variety of melt and fluid and P-T-fO2 conditions relevant to Sn-W deposits, (ii) experiments designed to determine accurately the fluid/melt and brine/vapour/melt partitioning of Sn and W at the magmatic-hydrothermal transition, (iii) fluid inclusion study on one outstanding example of the Argemela granite system (Portugal) to qualitatively and quantitatively investigate Sn and W concentration in fluids and transport at the magmatic-hydrothermal transition, (iv) investigation of the oxidation state of Sn in different crustal environments to evaluate the redox effect on Sn transport and redistribution within the crust. Altogether, our results will help to reassess the existing models for the formation of Sn-W deposits.

• DFG Programme: Priority Programmes
• Subproject of: SPP 2238: Dynamics of Ore-Metals Enrichment - DOME