Textures and Chemical Compositions of Nb-Bearing Minerals and Nb Mineralization in the Shuangshan Nepheline Syenite Pluton, East Qinling, China

Hong Wang,Ying-Wei Song,Yong Tang,Hui Zhang,Zheng-Hang Lv,Yu-Sheng Xu,Shan-Xian Qin

doi:10.3390/min11111163

Abstract

The Shuangshan alkaline complex located in the Henan province of China is a newly discovered, potentially giant niobium (Nb) deposit. A variety of Nb-bearing minerals including pyrochlore, zircon, and titanite have been identified in this deposit. Distinct textural and chemical differences of pyrochlore and zircon indicate that both have different origins. The magmatic pyrochlore and zircon both have euhedral grains with small sizes. On the other hand, hydrothermal pyrochlore is mainly intergrown on the edge or inside of hydrothermal zircon in the form of an aggregate. Compared with magmatic pyrochlore, the contents of F, Ca, and Na in hydrothermal pyrochlore are obviously high. The texture and composition of hydrothermal pyrochlore and zircon indicate that Ca-bearing hydrothermal alteration resulted in the migration of Nb from Nb-bearing zircon and the reprecipitation of Nb to form aggregate pyrochlore. However, the quantitative calculation shows that the amount of Nb migrated from zircon is very small. Therefore, this study suggests that hydrothermal alteration plays a certain role in the redistribution of Nb, but the enrichment of Nb is limited.

Highlights

Alkaline igneous rocks constitute only a small part of the Earth’s crust in volume, they have gained considerable attention recently
Pyrochlore is mainly found in nepheline syenite and is the main ore mineral found in the study area
We provided the evidence that the hydrothermal pyrochlore in the Shuangshan nepheline syenite was formed by Nb recrystallization that migrated from Nb-rich zircon under the action of late Na- and Ca-bearing hydrothermal fluid

Summary

Introduction

Alkaline igneous rocks constitute only a small part of the Earth’s crust in volume, they have gained considerable attention recently. One important reason for this is that they are extremely rich in high-field strength elements (HFSE), Zr, Nb, Y, and Rare. Earth Elements (REEs) [1]. These elements, known as critical metals, are widely used in “high-tech” applications, such as defense systems, high-strength and corrosionresistant alloys, and energy generation and storage [2]. The enrichment of HFSE is associated with a multi-stage process. These elements behave incompatibly during most magmatic processes, and the first step toward ore formation usually involves magma formed as the product of extensive fractional crystallization [3]. The subsequent upgrading of metal concentrations has been attributed to a series of processes, including liquid immiscibility [4,5], magmatic crystal accumulation [6,7], and in particular, hydrothermal alteration [8,9,10,11,12]

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