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

Sang-Gun No,Maeng-Eon Park

doi:10.3390/min9090566

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.

Highlights

The Chungju deposit is located in the central part of the southern Korean Peninsula (Figure 1)
The deposit contains especially high grades of zirconium (ZrO2 = 9.6–25.3%), niobium (Nb2 O5 = 1.2–2.3%), yttrium (Y2 O3 = 0.5–1.5%), and total rare earth elements (REE; REE2 O3 = 1.07–2.7%) [1,2]. This deposit is hosted in the Gyemyeongsan Formation [1,2,3], which belongs to the Chungju unit and is located in the northwestern region of the Okcheon Metamorphic Belt (Figure 1)
We suggested that the Zr–Nb–Y–REE mineralization occurred within the alkali granite during the fractional crystallization of alkali magma in the Paleozoic period

Summary

Introduction

The Chungju deposit is located in the central part of the southern Korean Peninsula (Figure 1). The deposit contains especially high grades of zirconium (ZrO2 = 9.6–25.3%), niobium (Nb2 O5 = 1.2–2.3%), yttrium (Y2 O3 = 0.5–1.5%), and total rare earth elements (REE; REE2 O3 = 1.07–2.7%) [1,2]. This deposit is hosted in the Gyemyeongsan Formation [1,2,3], which belongs to the Chungju unit and is located in the northwestern region of the Okcheon Metamorphic Belt (Figure 1). Previous studies [1,2,4,5,6,7]

Methods

Results

Conclusion