Detrital minerals indicate hidden reservoirs to a REE-mineral placer in Westland, New Zealand

Tracing detrital mineral sources is important for understanding economically prospective rare earth element (REE)-rich placers, but identifying their origins in complex geological settings remains challenging. Prospective amounts of detrital REE minerals occur in some of New Zealand's beach sands. However, other than coming from the actively exhuming Southern Alps, their sou-rces are poorly characterised. Here, we attempt to constrain the sources of a fluvial REE (xenotime and monazite) placer within this active orogen using textural, elemental and isotopic compositions of these and associated heavy minerals. Prominent Eu anomalies and U-Pb dating of xenotime (~101 Ma) and monazite (~112 Ma), along with their euhedral textures, are consistent with two distinct but proximal igneous sources. Garnet major element compositions also point to a magmatic source, and although Cretaceous plutons are outcrop extensively in the catchment, the detrital zircon population has only a very weak Cretaceous signal ( n = 6/121). Furthermore, although xenotime bulk rock εNd (i) compositions (average = −5.0) do align with local plutons, the ~101 Ma age is approximately 10 Myr younger than that of the known plutons. The xenotime source may therefore be a suite of granitic pegmatitic dikes that cross-cut these the plutons and gneisses. In contrast, the monazite ages overlap those of the locally dominant plutonic rocks but their in-situ εNd (i) values (−11 to +0.9) extend to much more radiogenic values. These data point to an as yet undiscovered proximal magmatic source rock with a strong depleted mantle component, in part mixed with crustal material to yield the spread in εNd. Although the specific sources of the REE minerals remain unknown, the mineral textures and isotopes strongly indicate that these sources are proximal, largely magmatic zircon-free, and not related to the most abundant mapped rocks in the region. Single grain textural, elemental and isotopic data therefore provide constraints on the origins of different mineral sources, and the implications of these data would be useful in future, advanced exploration. • Detrital REE minerals (xenotime and monazite) have distinct sources. • Geochemical characteristics indicate igneous origins. • Isotope compositions and age of minerals suggest hidden magmatic source. • Sedimentary inputs have varied in space and time in tectonically complex region.