Post-subduction magmatism and mineralisation: the Tuvatu gold-telluride deposit, Fiji.

Post-subduction, alkaline magmas host some of the world’s largest gold deposits, which show enrichments in other elements, including the critical element tellurium. Fiji is home to several of these enigmatic deposits, including Tuvatu. Mineralised magmatic centres across Fiji formed following subduction cessation and plate-boundary reconfiguration. Through detailed geochemical analysis of 93 new samples, I show that compared to syn-subduction magmas, post-subduction suites have elevated concentrations of Ba, Rb, Sr, and K2O, and Nb-Ta depletions. Geochemical modelling suggests syn-subduction fractionation formed cumulates containing biotite, which were re-activated during post-subduction, liberating trace elements and metals.

Through detailed, quantitative mineralogy of Tuvatu I show that the alkaline monzonite there hosts two styles of mineralisation: the Main Zone (laterally extensive veinlets of quartz, carbonates, minor sulfides, tellurides, and native-Au), and the HT zone (vuggy rock containing quartz, biotite, apatite, magnetite, sulfides, native-Au, and lesser carbonate and tellurides). Precious-metal tellurides dominate Au deportment, contributing 5x more Au by weight than native-Au. There is evidence of multiple fluid phases and zoning with depth. Mineral assemblages suggest high temperatures (>250°C) which may allow Au and other metals to remain as semi-metal melts. These aid the transfer of precious-metals into the ore-forming environment, alongside other processes recognised in the crustal continuum of post-subduction ore deposits, including CO2-facilitated transport of metals, and high pH hydrothermal fluids.

Through an assessment of critical metals as by-products, I suggest that recovery of Te alongside Au could impact apportioning of carbon between metals and influence the environmental credentials of a deposit. Post-subduction, alkaline magmas can enable processes which support the development of precious- and critical-metal rich ore deposits and offer a potential source from which those metals might be more sustainably produced together, rather than in separate, single-metal mines.