MEGWA Talk: Fiji Epithermal Gold – Ken Collerson

MEGWA Talk: Fiji Epithermal Gold – Ken Collerson


 

Ken Collerson – Fiji Epithermal Gold

Wednesday, 15 July 2015

Large epithermal Au-Ag and porphyry Cu-Au-PGM deposits occur in K-rich alkaline igneous rocks (shoshonites) in many parts of the circum-Pacific and circum- Mediterranean. To understand the origin of these shoshonite-hosted deposits, and shoshonite magmatism in general, it is important to understand the source of metals, the role of element transfer into the mantle wedge during subduction, the cause of melting of the sub-arc mantle wedge and the timing of magmatism.

During work for a client in Fiji, a number of previously unrecognised, highly prospective shoshonite suite (absarokites to shoshonite) calderas have been discovered. They occur in the southern Yasawa Islands, and in central-northern and central-western Viti Levu. By integrating field observations, petrology, geochemistry, geochronology, gravity, reprocessed magnetic data and geodynamic models we have modelled the tectonic control on shoshonite distribution and improved understanding of their metallogenic evolution. Trace element/isotopic chemistries indicate that shoshonite magmagenesis involved melting of hydrated supra-subduction zone mantle wedge and also the impact of a lower mantle-derived plume (OIB) component.

The upwelling plume is interpreted to have provided heat for melting, and contributed Au and PGE’s. Accessing the melt generation zone by the upwelling plume required the existence of a slab conduit or window. The shoshonite suites in the Yasawa Islands and on Viti Levu occur in zones of back-arc extension above slab windows. These tears migrated southward during roll-back of the New Hebrides-Vanuatu subduction zone. By contrast, the shoshonites in the Lomaiviti Islands east of Viti Levu, occur along the edge of the Vanuatu-New Hebrides subduction zone, which has been seismically imaged to >400 km. This new model “cracks the metallogenetic code” for the occurrence of Au-bearing shoshonites. It has regional and global implications for alkaline-hosted epithermal and porphyry style mineralisation.