Stephanie Junior

Project description

Gold (Au) is a unique noble metal and a necessary mineral resource, yet it is not fully understood in a geological context. Looking globally at economic deposits, it is evident that Highly Siderophile Elements (HSEs) like Au have an extremely uneven distribution. Concentrated occurrences perpetually deposit throughout Earth's history, pointing to a global replenishing mechanism from a deep earth reservoir with geochemical behaviors yet to be discovered.

The specific deep Earth residence of Au, along with the replenishing mechanism from what is undoubtedly a mantle reservoir, needs to be defined. This project aims to better understand the behaviors of mantle Au by analyzing concentrations in sulfide and alloy mineral sites in peridotites and then investigating the geochemical conditions that promote Au mobility in these contexts. It is possible that crustal Au replenishment is due to percolation by CO2-H2O mantle-derived fluids, a chemical process that has metasomatized much of New Zealand's mantle rock. With known Au deposition and a variety of exposed peridotite localities of varied compositions, including an extensive ophiolite belt and myriad occurrences of mantle-derived xenolith emplacements providing lithospheric mantle rock accessible for sampling, New Zealand provides an ideal locale for exploring Au in the mantle. For this reason, samples used in this project will be predominantly from the Zealandia continent, however, reaction between the mantle and CO2-bearing magmas has been documented elsewhere in the world so results from this study can be further explored in the context of a global mantle process. In this way, it can be determined if Au concentration, distribution and mobility correlates with dramatic chemical changes in the mantle, and we can better understand preferential emplacement of Au in Earth's crust.

Left: Field work at Red Mountain (mantle section of ophiolite complex), Right: Cut lherzolite xenolith from Otago.