Geochemistry and origin of a new-type of Zr-Nb-Y-REE deposit in highly-evolved alkali granite, Chungju area, South Korea

In the Chungju area, Korea, highly enriched Zr-Nb-Y-REE deposits occur in subhorizontal layered Paleozoic granitic rocks (331±1.5 Ma), which can be divided into layered alkali granite, alkali aplite, and pegmatite. The rocks are mainly composed of alkali feldspar, quartz, and microcrystalline zircon. The ubiquitous zircon is the distinctive feature of the alkaline rocks, which plot with within plate granite, anorogenic granite, and ultrapotassic rocks, and show very similar REE patterns. Alkali aplite has especially high total rare earth elements and negative europium anomalies compared to the layered alkali granite. The Zr-Nb-Y-REE mineralization occurs as zircon-magnetite bands that are associated with several REE minerals. Repeated graded textures of layered alkali granites with interlayered Zr-Nb-Y-REE mineralization can be explained by gravity accumulation in the late magmatic stage. The compositions of zircons plot between the late magmatic and hydrothermal fields. The REE patterns of zircon-rich mineralization shows slightly negative slopes, whereas zircons show positive slopes. This can be explained by the HREE being strongly partitioned into zircon grains from the melt. Zircons with low total REE contents show high positive Ce anomalies. Although zircon analyses were conducted on one sample from a small area, it shows variable Ce anomalies and TREE, which indicates the zircons crystallized under conditions of rapidly changing oxygen fugacity, as the REE contents of zircon are related to the oxygen fugacity of the melt. The limited Th/U ratios of zircons indicate that they crystallized during a simple magmatic event, and were not affected by hydrothermal alteration and metamorphism. Here we suggest a flotation, aggregation, and gravity accumulation model can explain settle down of microcrystalline zircon and magnetite grains in fluid rich alkaline melt. This is the first report on highly evolved alkali granite that associated with Zr-Nb-Y-REE mineralization. The features displayed in these deposits have important implications for the evolution of alkali magmas.