LI Su-xin1, 2, ZHONG Hong1, 2 *, BAI Zhong-jie1, ZHU Wei-guang1

(1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China; 

2. University of Chinese Academy of Sciences, Beijing 100049, China)

Abstract: The ore-hosting syenitic dikes of the Luku and Baicao alkaline rocks-related Nb-Zr-REE deposits in the Panzhihua-Xichang linear structural belt of the Emeishan Large Igneous Province intruded into the Hongge layered gabbros. LA-ICPMS U-Pb dating results of titanite grains show that the titanite was formed at 258±2.1 Ma, indicating that the mineralization of this deposit was closely related to the Emeishan mantle plume activity. In this paper, the controversial ore-forming mechanism of the alkaline rock related Nb-Zr-REE deposit has been further discussed based on mineralogical characteristics and in-situ analyzed major and trace elemental compositions of accessory minerals. This study shows there are magmatic-stage and hydrothermal-stage mineralization in the Luku and Baicao deposits. At the magmatic-stage, the crystallization differentiation of rock-forming minerals, such as K-feldspar and albite, had resulted in the enrichment of high field strength elements (HFSE) in the residual melt which then formed abundant ore minerals enriched in Nb, Zr and REE, such as Ce-U pyrochlore, zircon and titanite. This is the main factor controlled the Nb-Zr-REE mineralization. At the hydrothermal-stage, the matasomatism of early ore minerals, such as Ce-U pyrochlore, zircon, and titanite, by hydrothermal fluid resulted in the reactivation of high field strength elements from these minerals to have formed hydrothermal zircon and Ta-pyrochlore. Therefore, it is suggested that the crystallization of pyrochlore with high REE content and low Nb/Ta ratio could result in the low REE concentrations and high Nb/Ta ratios in residual melt and fluid. The crystallization differentiation is the key factor resulting in the Nb-Zr-REE mineralization in this area, while the late stage hydrothermal metasomatism only lead to the increase of the Nb-Zr-REE grades of local ore bodies. 

Keywords: zircon; titanite; pyrochlore; the Nb-Zr-REE mineralization; the Panzhihua-Xichang area

ACTA MINERALOGICA SINICA Vol. 43, No. 5, 2023, Page 627