Characteristics of REEs and trace elements in scheelite from the Zhuxi W deposit, South China: Implications for the ore-forming conditions and processes

摘要

The Zhuxi W deposit in Jiangxi, South China, with a resource of 2.86 Mt W and an average grade of 0.54% WO3, is the largest W deposit in the world. The deposit occurs at the contact between a late Mesozoic granitic intrusion and Carboniferous - Permian carbonate rocks and Neoproterozoic metamorphic rocks. The ores are divided into three types: skarn ore (dominant), sulfide ore and greisen ore. Based on occurrences and petrographic relationships, scheelite (the dominant ore mineral) is classified into three types, i.e. type I (greisen), type II (skarn, specifically in the retrograde stage), and type III (hydrothermal sulfides). Type I and type II are time equivalent but spatially separated, whereas type III is later. The different types of scheelite have distinctly different REE and trace element compositions. Type I scheelite is characterized by an average Sigma REE of 67.9 ppm, LREE/HREE ratio of 24.4, delta Eu of 8.5, Mo of 0.1 ppm and Sr of 1058 ppm, whereas type II scheelite features an average Sigma REE of 48.1 ppm, LREE/HREE ratio of 10.6, delta Eu of 0.67, Mo of 1892 ppm and Sr of 90.1 ppm. Type III scheelite has an average Sigma REE of 53.9 ppm, LREE/HREE ratio of 4.8, delta Eu of 17.5, Mo of 92.9 ppm and Sr of 37.1 ppm. The extremely high concentrations of Sr and highly positive Eu anomalies in type I scheelite are likely related to release of these elements from plagioclase in the granite during greisenization, whereas the extremely low concentrations of Mo may be related to the reducing and high-temperature nature of the magmatic fluid - Mo probably was mainly in the Mo4+ state and could not replace W6(+) in the scheelite lattice. In contrast, the negative Eu anomalies and high Mo concentrations in type II scheelite suggest that the mineralizing environment in the retrograde alteration stage was relatively oxidizing, causing Eu to exist mainly as Eu3+, which is difficult to replace Ca2+ in scheelite, and Mo mainly as Mo6+, which is easy to replace W6+ in scheelite. In the sulfide mineralization stage, the fluid became relatively reducing again, and Eu2+ could easily replace Ca2+ in scheelite causing positive Eu anomalies, whereas Mo mainly exists as Mo4+ and precipitated as molybdenite, and so has limited substitution for W6+ in scheelite. The REE and trace element compositions of the dominant W mineral (type II scheelite), characterized by high concentrations of Mo, high LREE/HREE ratios and negative Eu anomalies, indicate that the ore-forming materials were mainly derived from the granitic intrusions. The variation of REE and trace elements in scheelite in time and space reflects a complex magmatic-hydrothermal mineralization process involving various fluid-rock reactions within the intrusion and in the country rocks. The formation of the Zhuxi W deposit is linked with a large fertile granitic intrusion, which delivered a large amount of W to the site of mineralization through convection within the magma chamber, and W precipitation in the skarn was largely caused by an abrupt drop in temperature and increase in pH of the ore-forming fluid.