REE geochemistry of gangue minerals and their geological significance in the Muli antimony ore deposit in Yunnan, China

Abstract

The Muli antimony deposit is located in the Au–Sb polymetallic metallogenic belt in south-eastern Yunnan, China. In this paper, we investigated the concentrations of trace elements in gangue minerals, mainly calcite, quartz, and pyrite, which were formed at different metallogenic stages. Meanwhile, the host rocks, predominantly composed of limestone, are also analysed for comparison. The calcite from the Nadan ore section is enriched with medium-heavy rare earth elements (M-HREEs), likely due to the presence of a high concentration of Fe and Mn impurities, which results in the preferential enrichment of M-HREEs in the calcite. Alternatively, the calcite may be precipitated from the M-HREE-rich granitic leaching fluid. In the Muli ore section, both quartz and pyrite in the metallogenic period show enrichment with light rare earth elements (LREEs), and the wall rock is also enriched with LREEs, which indicates that the wall rock material was involved in the metallogenic process. The W-shaped tetrad effect of quartz in the late metallogenic stage was interpreted to determine extensive fluid–rock interactions in highly fractionated Si-rich systems. Fe and Mn impurities cause M-HREE to be preferentially enriched with calcite to some extent. Whether mineralization is related to granite deserves further study. Eu and Ce anomalies of different types of gangue minerals indicate that the temperature and the fO2 were constantly changing during mineralization, and the temperature of the main ore-stage was higher than 200 °C in an oxidized state. The various REE patterns, LREE/HREE and (La/Yb)N values, reveal that there may be multi-sources and multi-stage hydrothermal activities in the Muli antimony deposit. The REE distribution patterns of minerals are likely interfered with by many internal and external factors. Studies on REE characteristics of calcite, quartz, pyrite and limestone in the Muli antimony deposit have greatly improved the understanding of ore-forming fluids. When we traced the origin and evolution of ore-forming fluids by means of mineral REE distribution patterns, in addition to the determination of inclusions of ore minerals related to mineralization and the in situ analysis methods performed by LA-ICP-MS, we should also combine the REE characteristics of various minerals or trace the ore-forming fluids with multiple methods.