Bands of Zircon, Allanite and Magnetite in Paleozoic Alkali Granite in the Chungju Unit, South Korea, and Origin of REE Mineralizations

Abstract

High-grade Zr–Nb–Y–rare earth element (REE) mineralization occurs as zircon–allanite–magnetite bands in layered Paleozoic alkali rocks which intruded the Gyemyeongsan Formation of the Chungju unit, South Korea. The mineralization period and genesis have been controversial. We investigated the petrological and mineralogical properties of the newly discovered zircon–allanite–magnetite bands and the geochronological properties of zircon within the bands in the alkali granite. We analyzed the zircon with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). The repeated quartz–feldspar-rich layers in the alkali granite show grain-sized grading textures and equilibrium igneous textures. Magnetite and allanite grains in these layers varied in size and exhibited isolated, aggregated, and coalesced textures. In addition, the settling texture of zircon grains onto the other minerals was observed. These observations could reasonably be explained by the process of gravitational accumulation during the solidification of magma. The 206Pb/238U ages obtained from zircon from the zircon–allanite–magnetite-rich layer and the alkali aplite were 331.1 ± 1.5 Ma and 334.5 ± 8.9 Ma, respectively. Therefore, we suggest that the Zr–Y–Nb–REE mineralization developed in the alkali rocks and the Gyemyeongsan Formation in the Chungju unit were formed by fractional crystallization of alkali magma and hydrothermal fluids which evolved from alkali magma fractional crystallization, respectively. The correlation between alkaline granite and REE mineralization found in this study could be used as a tool for REE exploration in other regions where the permeable geological unit is intruded by the alkali granite.

The Geochronology and Geochemistry of Zircon as Evidence for the Reconcentration of REE in the Triassic Period in the Chungju Area, South Korea

“…The granitic rocks that intruded into the Gyemyeongsan Formation occur as plutons, stocks, and dikes ( Figure 1). These granitic rocks include Paleozoic alkali granite [10] and Mesozoic granite [9].…”

“…The quartz-allanite (allanite has not yet been defined according to an official nomenclature) veins were emplaced along the foliation in the metavolcanic rocks (Gyemyeongsan Formation) and crosscut the Paleozoic alkali granite [10] (Figure 2). In some areas, these veins appear similar to coarse-grained pegmatite ( Figure 2).…”

“…The other twenty-five spots yielded discordant ages. The age versus Th/U ratio was presented with Paleozoic magmatic zircon from alkali granite [10] in Figure 4d.…”

“…Since REE and high-field-strength element (HFSE) mineralization was first reported in 1989 in the Chungju area in the central part of the Southern Korean Peninsula [1], a steady stream of studies has been carried out [2][3][4][5][6][7][8][9][10]. The genesis of REE and HFSE mineralization [2,3], the chemical compositions of alkali granite [4], the chemistry of REE-bearing minerals [5,6], and the associated uranium and thorium mineralization [7] have been studied.…”

“…Recently, dating of zircon that is too small (10-20 µm) for sensitive high-resolution ion microprobe (SHRIMP) analysis was conducted with laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) [10]. This analysis revealed that Paleozoic granite (331 ± 1.5 Ma), which intruded into the Gyemyeongsan Formation, is involved in REE and HFSE mineralization [10].…”