Petrological and geochemical characteristics of REE mineralization in the A-type French Creek Granite, New Zealand

Abstract

The French Creek Granite, New Zealand, is an alkaline intrusion with enrichment in rare earth elements (REE). Petrography and whole-rock geochemical signatures demonstrate that the c. 82–84-Ma French Creek Granite is a composite granitoid, dominated by a ferroan, weakly peraluminous biotite alkali feldspar granite to syenogranite, with subordinate quartz-alkali feldspar (QAF) syenite. Maximum total REE + Y contents are higher in the granite and felsic dikes relative to the marginal QAF syenite, cogenetic mafic Hohonu Dike Swarm, and pegmatites. Our findings suggest that REE and high-field-strength element enrichment in the unaltered French Creek Granite is a function of partial melting from an enriched HIMU-like lithospheric mantle source, minor crustal assimilation, and extensive magmatic differentiation. Primary (magmatic) REE enrichment is defined by the occurrence of allanite, zircon, apatite, fergusonite, monazite, perrierite, and loparite, which are commonly associated with interstitial biotite. French Creek Granite samples from a phyllic-sericitic alteration zone in the Eastern Hohonu River are strongly elevated in REE relative to unaltered French Creek Granite, indicating remobilization and secondary REE enrichment by hydrothermal fluids. The REE are hosted in bastnasite group minerals, monazite, xenotime, and zircon. Quartz protuberances, dilatational microfractures, and dike emplacement indicate that this alteration is structurally controlled. The δ13C and δ18O values of secondary carbonate veinlets are consistent with mixed low-temperature (~ 250–260 °C) magmatic hydrothermal fluids containing mantle-derived carbon. These fluids were likely part of a late-stage porphyry-type system operating during the same mantle degassing and extensional episode that was associated with the emplacement of the French Creek Granite and lamprophyric dikes of the Hohonu Dike Swarm, and initial Tasman Sea spreading. Results from this study provide an important insight into the complex processes responsible for REE enrichment in alkaline igneous systems.