Multiple stages of skarnization in the world-class Zhuxi scheelite skarn deposit, South China

Abstract

Although previous studies consistently revealed that the world’s largest W deposit, Zhuxi (3.44 Mt. WO3 @0.54 %) was formed by a single metallogenic stage in the Late Jurassic (∼150 Ma), this contribution identifies an unrecognized, postdate metallogenic event. After detailed field investigation, we noted that the early disseminated scheelite-bearing garnet–pyroxene massive skarn ores are crosscut by vein-type skarn ores, mainly composed of garnet, coarse-grained scheelite, and apatite. The mineral assemblage of the vein-type skarn ore is different from the scheelite-bearing veins that formed during the retrograde alteration stage. Moreover, some massive skarn ores crosscut by vein-type skarn ores have undergone obvious retrograde alteration. Given that a complete metallogenic event in skarn deposits is characterized by retrograde alteration following the prograde skarn stage, the component of the vein-type skarn ores shows affinity to that of the prograde massive skarn ores and the vein-type skarn is superimposed on both previous skarns and their retrograde altered ores, indicating another skarnization event occurred in conjunction with a new metallogenic event in the Zhuxi deposit.The studied massive and vein-type skarn scheelites are characterized by low Sr contents (usually less than 40 ppm), positive Eu anomalies (δEu = 2–105), and low Mo contents (usually less than 300 ppm); thus, the massive and vein-type skarns were both formed by reduced magmatic hydrothermal fluids, with no oxidized meteoric water that is commonly prevalent in the retrograde alteration stage. Compared to the massive skarn garnets, the vein-type skarn garnets are richer in Al and poorer in Fe, indicating lower oxygen fugacity conditions during their crystallization and arguing against the involvement of oxidized meteoric water in their formation. The vein-type skarn garnets have higher Sn contents, lower U contents, and similar W contents compared to the massive skarn garnets; thus, the massive and vein-type skarns did not form from the same hydrothermal fluid system at different evolution stages. This is because garnets crystallized from an evolved hydrothermal fluid system under lower oxygen fugacity conditions are expected to have lower Sn and W contents with higher U contents than their predecessors. The in situ U–Pb age (142.8 ± 2.4 Ma) of apatite from the vein-type skarn reveals that apart from the widely known Late Jurassic metallogenic event (∼150 Ma), within the Zhuxi deposit, an Early Cretaceous metallogenic event also occurred, and the latter has been widely identified in the other districts of the Jiangnan tungsten belt. From this multistage metallogenic event superposition, early disseminated scheelite could leach and recrystallize as larger scheelite grains, forming high-grade, world-class scheelite skarn deposits, such as the Zhuxi (3.44MtWO3) deposit and similar deposits worldwide.