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

<p> There is a well-established link between world-class gold <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/telluride" target="_blank">telluride</a> <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/mineral-deposit" target="_blank">mineral deposits</a> and alkalic magmatic host rocks, often emplaced post-subduction in arc <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/terrane" target="_blank">terranes</a>. Fiji is home to two such deposits, and provides an ideal location to study the relationship between the alkalic (shoshonitic) host rocks, their <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/geodynamics" target="_blank">geodynamic</a> context, and potential roles in metal enrichment. A total of 93 new geochemical analyses of magmatic rocks from across Fiji are presented, alongside detailed <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/petrography" target="_blank">petrography</a> and <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/mineralogy" target="_blank">mineralogy</a> and a new compilation of literature data. Despite relatively similar mineralogy across <em>syn</em>- 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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/magma" target="_blank">magmas</a>, they are likely insufficient to drive such strong trends. Instead, <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/peridotite" target="_blank">biotite</a> or possibly <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/phlogopite" target="_blank">phlogopite</a> in the lithosphere are suggested as being a key ‘ingredient’. Earlier <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/subduction" target="_blank">subduction</a> can modify the lithosphere through the addition of hydrous phases, both by <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/metasomatism" target="_blank">metasomatism</a> of the lithospheric mantle, or by the formation of residual minerals during <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/fractional-crystallization" target="_blank">fractional crystallisation</a>. Remelting of these phases could liberate trace elements and metals. Biotite in particular could facilitate some of the observed large ion <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lithophile-element" target="_blank">lithophile elements</a> (LILE) enrichments and the alkali-rich (potassic) nature of the magmas. </p>