The tin granite trade-off: Modeling redox evolution during magmatic processing

Most primary tin (Sn) deposits are genetically related to Sn-rich, reduced, ilmenite series “tin granites” derived from partial melting of a metasedimentary source. Petrogenetic models for tin granites invoke varying contributions of fractional crystallization, source Sn enrichment, and specific melting conditions. Tin can speciate as either Sn4+, which is compatible in minerals such as biotite, or Sn2+, which is generally incompatible. A key, but poorly constrained, factor in tin metallogeny is how melting and magmatic evolution modify magma redox state (expressed as molar XFe3+ = Fe3+/ Fetotal), and therefore redox-dependent Sn speciation and compatibility. We use phase equilibria modeling to quantify redox evolution during crustal melting and subsequent fractional crystallization. We show that ilmenite series magmas (XFe3+ < ∼0.31) can only be generated through partial melting of strongly reduced metapelitic protoliths (XFe3+ < ∼0.1). Partial melting conditions that generate an ilmenite series melt fall within a redox transition zone with enhanced Sn incompatibility, which is extremely sensitive to small changes in source oxidation state. By contrast, fractional crystallization of hydrous magmas progressively oxidizes the melt through crystallization of Fe2+-rich minerals; in particular, biotite. Hence, there is a critical trade-off between Sn enrichment and increasing XFe3+. Petrogenetic models invoking extreme degrees of fractional crystallization (>90%) cannot produce reduced ilmenite series melts characteristic of tin granites. Instead, our results necessitate Sn “pre-enrichment” during anatexis of a reduced, Sn-rich protolith prior to magmatic differentiation to generate Sn-fertile magmas.