Source controls on mineralisation: Regional geology and magmatic evolution of Fiji

There is a well-established link between world-class gold telluride mineral deposits and alkalic magmatic host rocks, often emplaced post-subduction in arc terranes. Fiji is home to two such deposits, and provides an ideal location to study the relationship between the alkalic (shoshonitic) host rocks, their geodynamic context, and potential roles in metal enrichment. A total of 93 new geochemical analyses of magmatic rocks from across Fiji are presented, alongside detailed petrography and mineralogy and a new compilation of literature data. Despite relatively similar mineralogy across syn- and post-subduction samples, there are geochemical differences between them, with the latter displaying elevated concentrations of Ba (<2137 ppm), Rb (<218 ppm), Sr (<2648 ppm) and K2O (<10.18 wt%), and depletions in Nb and Ta. Multi-element trends suggest exaggerations of ‘typical’ arc patterns in post-subduction shoshonites. Geochemical data suggest that whilst the fluxing of fluids and sediment melts from a subducted slab may play a role in producing the distinctive post-subduction magmas, they are likely insufficient to drive such strong trends. Instead, biotite or possibly phlogopite in the lithosphere are suggested as being a key ‘ingredient’. Earlier subduction can modify the lithosphere through the addition of hydrous phases, both by metasomatism of the lithospheric mantle, or by the formation of residual minerals during fractional crystallisation. Remelting of these phases could liberate trace elements and metals. Biotite in particular could facilitate some of the observed large ion lithophile elements (LILE) enrichments and the alkali-rich (potassic) nature of the magmas.