Zealandia's deep earth noble and crucial metal endowment; Au, PGE, Ni, Co, Zn, and Cu in exposed mantle rock

Abstract

Noble metals – Au, Pd, Pt, Rh, Ru, Ir and Os – and crucial metals – here constrained to Ni, Co, Zn and Cu – are critical metal resources utilised in the innovation of green energy production, a paramount technological advancement for today’s warming world. While primary extractable concentrations of noble and crucial metals unequivocally reside in ore deposits, Earth’s mantle is an important reservoir for metal retention and replenishment. Processes modifying the deep Earth should, therefore, lead to certain geological settings becoming more favourably endowed than others. As such, the residence and geochemical behaviours of noble and crucial metals in mantle rock needs to be defined. Zealandia and its surrounding ocean basins uniquely host an array of exposed deep Earth ultramafic bodies, denoting it as an ideal location for this study. Utilising a diverse suite of local continental (SCLM xenoliths) and oceanic (OLM ophiolite and ocean exposure) ultramafic rocks, this thesis analyses peridotites, orthopyroxenites, chromitites and serpentinites at varying degrees of melt depletion, chemical enrichment and alteration. Synthesising whole rock chemistry with scanning electron microscopy (SEM) mineral chemistry and imaging, laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) spot analyses, and X-ray fluorescence microscopy (XFM) in- situ element mapping, informs the investigation into the abundance, residence and mobilising mechanisms of noble and crucial metals in Zealandia’s ultramafic rocks from the whole rock to elemental scale. Crucial metals are found to occur in higher concentrations in OLM rocks than in SCLM xenoliths, despite recording histories of more melt depletion and less metasomatic re-enrichment. Maximum whole rock crucial metal values are: 2900 ppm Ni (dunite), 213 ppm Co (chromitite), 280 ppm Zn (chromitite), and 230 ppm Cu (serpentinite). Observed sulphides and alloys are primarily crucial metal-rich, predominantly occurring as species within the Ni-Fe-Cu system, and commonly include ancillary Co. There are no observed occurrences of Co-dominant phases, and Zn-rich sulphides are only found in S-elevated serpentinites. In addition to presenting as the dominant constituent of mantle metallic phases, the Ni budget is intimately tied to olivine abundance, with an average SEM analysis of 0.3 wt% Ni residing in the olivine crystal lattice. Cobalt and zinc trend well with whole rock iron abundance and are found to occur in Cr-rich spinel phases, and in conjunction with iron exsolution in highly serpentinised samples. Copper does not significantly occur within silicates or spinel, but Cu-metallic phases tend to be spatially associated with OPX grains. Like crucial metals, noble metals measure higher whole rock abundances in OLM rocks compared to SCLM xenoliths. Primary ophiolitic peridotites are shown to be noble metal enriched, and chromitite cumulates, despite reporting (Pd/Ir)N enrichment indices <1 (indicating PGE depletion), host the highest concentrations of Au and PGE (except for Pd), and are particularly IPGE-rich. Maximum whole rock noble metal values are: 4.92 ppb Au (chromitite), 29.01 ppb Pd (serpentinite), 29.73 ppb Pt (chromitite), 40.25 ppb Rh (chromitite), 750.58 ppb Ru (chromitite), 268.37 ppb Ir (chromitite), and 297.29 ppb Os (chromitite). Observable noble metal-rich sulphides and alloys, predominantly occurring as laurite (RuS2) or IPGE-rich alloys, are only observed in OLM rocks, with chromitites hosting the highest abundance. XFM element mapping suggests that noble metals additionally reside within Cr-rich spinel phases. No Au-rich metallic phases occur coarse enough for measurement; however, LA-ICP-MS spot analyses suggest that Au is occurring as discreet mineral inclusions. Furthermore, data may suggest that the Au budget correlates with PGE enrichment, but whole rock Au was not found to display significant quantifiable behavioural relationships with any other element or geochemical variable investigated in this study. Data collected here suggest that carbonatite metasomatism may have a significant influence on the PGE budget of SCLM rocks, but there is no evidence to suggest that this affects Au or the crucial metals. Serpentinisation is shown to highly mobilise both noble and crucial metals, often concentrating metal budgets and altering element residence preferences from host silicate and spinel phases to coarse sulphides and/or alloys. Extensive serpentinisation is associated with an influx of sulphur and a relative elevation in incompatible metals: Au, Pd, and Cu. Melt removal is an effective mechanism for mobilising noble and crucial metals, particularly the incompatible elements. Additionally, melt interaction, like that which produces chromitite formations, is remarkably effective at promoting the retention of noble metals, especially IPGE.