Apatite as a pathfinder to tin mineralisation: prospects and caveats

Granite-related mineral deposits are major primary sources of the critical metals tin (Sn) and lithium (Li). The utility of accessory minerals such as zircon and apatite as pathfinders to these ore deposits has been a subject of great interest in recent years, with a number of geochemical discriminants having been developed to distinguish barren from metal-fertile and mineralised intrusions. Here, we study the potential of apatite as an indicator mineral for tin and lithium mineralisation using a compilation of published apatite trace element data as well as new data for the mineralised Cornubian batholith and barren Bhutanese leucogranites. Critical examination of common geochemical discriminants tracing magma fractionation and redox conditions (Mn, Eu/Eu*, La/Yb_N and Sr/Y) reveals large and overlapping data scatter for both barren and Sn-fertile intrusions. This calls into question the utility of these petrogenetic indicators to pinpoint tin metallogeny. Instead, prima facie metal concentrations directly related to tin mineralisation (i.e., Sn and Li) are consistently elevated in apatite from fertile and mineralised intrusions. Based on our data compilation, Li and Sn concentrations in apatite are the most robust indicators for Sn (and Li) mineralisation, and we encourage the community to include Li and Sn in their analytical routines to further test these observations and explore their implications for tin metallogeny.