Hydrothermal alteration and rare earth element mineralisation in the French Creek Granite, Westland, New Zealand (2016)
AuthorsMorgenstern, Regineshow all
Alkaline igneous complexes are one of two primary sources of rare earth elements (REEs), which are unique metals crucial for the economic growth of a country. Understanding REE metallogenesis in these systems is often complicated, with evidence of both magmatic and hydrothermal processes present. The A-type French Creek Granite (FCG), located on the West Coast of New Zealand, is a poorly-studied example of such a complex system in which anomalous REEs have previously been reported. The purpose of this thesis was to undertake a comprehensive field, petrological and geochemical study of the FCG, its hydrothermal alteration and, to a lesser extent, the cogenetic Hohonu Dyke Swarm (HDS), in order to better understand the type, style and location of REE mineralisation. Whole rock geochemical analyses of 54 samples using XRF and ICP-MS/AES established that the ca. 82 Ma FCG is a composite granitoid dominated by a ferroan, peraluminous biotite granite that was emplaced into a high-level (ca. 3 km) syn-tectonic setting. A syenite shell and genetically related basaltic–rhyolitic dykes are present, and trace element content, and disequilibrium textures in phenocrysts in dykes, are evidence of magma mixing. Maximum ƩREE+Y content are higher in the felsic FCG (847 ppm) relative to the mafic HDS (431 ppm). Primary REE-Zr-Y enrichment in the FCG is a function of partial melting of an enriched mantle source and subsequent extensive differentiation. Primary REE mineralisation was identified via SEM-EDS and is defined by modal allanite, zircon, apatite, fergusonite, monazite, perrierite and loparite, which typically occur with interstitial biotite. This association, and LA-ICP-MS analyses of REE-bearing giant (500 μm) zircon, indicate REE enrichment in the residual melt was likely due to high magmatic fluorine and late-stage water saturation, in addition to differentiation. Extensive sericitisation, chloritisation, hematisation, carbonate alteration and kaolinisation were identified in the altered FCG using field observations, microscopy and XRD. A zone of propylitic alteration in the Little Hohonu River and a smaller, phyllic alteration assemblage in the Eastern Hohonu River are defined, both of which generally correlate with higher REE anomalies than fresh FCG. Quartz protuberances, microscopic fractures and dyke emplacement indicate the phyllic alteration is structurally controlled, and REEs are hosted in bastnäsite group minerals, zircon, monazite and xenotime. This zone is consistently enriched (607 ppm average ƩREE+Y), indicating remobilisation and secondary REE-Zr-Y enrichment by hydrothermal fluids. Stable 13C and 18O isotopes from secondary carbonates indicate low temperature (~250°C) magmatic-hydrothermal fluids sourced from the cooling FCG, which were likely part of a late-stage porphyry-type system operating during the same mantle degassing and extension episode that was associated with initial Tasman Sea spreading.