Petrogenetic Processes Leading to Rare Earth Element Mineralization in the A-type French Creek Granite, West Coast, New Zealand

Anorogenic alkaline igneous complexes are commonly enriched in rare earth elements (REEs) and other high-field-strength elements: unique metals increasingly required in the renewable energy, military and advanced technology sectors. The 84-82 Ma silica-oversaturated French Creek Granite (FCG) and cogenetic mafic Hohonu Dyke Swarm (HDS) located on the West Coast of New Zealand represent this type of alkalic complex potentially enriched in REEs. Geochemical stream assays from previous mineral exploration in the area consistently indicated anomalous Ce, La, Nb, Y and Zr; however, no detailed petrogenetic study on the host rock or nature of the REE mineralization has been conducted. Field mapping was used to establish relationships between the FCG, HDS and the type of hydrothermal alteration present. A comprehensive analytical program has been undertaken including mineral identification via microscopy and SEM-EDS; whole rock geochemistry by XRF and ICP-MS/AES; and zircon trace element chemistry via LA-ICP-MS. Preliminary results indicate a composite and highly evolved granitoid, which has undergone variable degrees of magma mixing, undercooling and solid-liquid phase separation. The FCG has a complex history including early subsolvus crystallization; a later, high-level hypersolvus stage; a dynamic syn-tectonic environment that spans its cooling history; and late-stage hydrothermal circulation common in such highly evolved intrusions. Evidence for multiple generations of hydrothermal fluids exists, resulting in sericite, chlorite, carbonates, kaolinite, pyrite and quartz. The REEs are concentrated in the FCG relative to the parental magma (up to 850 ppm ƩREE+Y), and initial results indicate REE-Y-Nb-Zr partitioning into carbonates (bastnäsite and parisite), phosphates (xenotime, monazite and apatite), silicates (chevkinite, allanite and zircon) and titanate (brookite and possibly perovskite) minerals. This enrichment is likely a result of solid-liquid segregation and carbonate-rich fluids, possibly sourced from the mantle, exploiting localized zones of extension during the initial opening of the Tasman Sea.