Characteristics of REEs and Trace Elements in Scheelite and Muscovite Ar–Ar Isotopic Dating of the Daping Tungsten Deposit

Abstract

The recently discovered Daping tungsten deposit is located about 25 km north of Tongcheng County, Hubei Province, in the northern margin of the Sijiapu granite deposit of the Mufushan composite batholith. The ore body is produced in the northeast-oriented greisenization granite and quartz vein, and belongs to the greisen-vein-type scheelite deposit. The resources of the Daping W deposit have a value of 7935 t W and the average grade is 0.201% WO3. Based on mineralogical and petrographic studies, scheelite is classified into two types. A study of the geochemical characteristics of rare earth elements (REEs) and trace elements indicated that some scheelite specimens showed LREE depletion. Meanwhile, the total amount of scheelite rare earth elements (REEs) is low and the ratio of LREE/HREE ranges from 0.40~2.72 in the Daping W deposit. The contents of REEs and trace elements in the two types of scheelite differ significantly. Type I scheelite has an average ∑REE content of 195.65 ppm, an LREE/HREE value of 0.5, an Eu anomaly (δEu) of 0.78, Mo of 366.20 ppm, Sn of 22.62 ppm, and Sr of 264.80 ppm. However, type II scheelite features an average ∑REE of 111.28 ppm, an LREE/HREE ratio of 1.43, a δEu of 1.56, Mo of 188.48 ppm, Sn of 0.15 ppm, and Sr of 829.93 ppm. The content level of Mo in the two types of scheelite is not high, indicating that this whole metallogenic environment is a reduction environment. However, this is a complex process. The presence of type I scheelite with negative Eu anomalies and relatively high Mo content indicates that the ore-forming environment in the early period of the greisen stage was relatively oxidizing. In contrast, type II scheelite contains large amounts of Sr and large positive Eu anomalies, which are likely to be released from plagioclase in the granite during greisenization. The extremely low composition of Mo in type II scheelite is closely related to the reducing environment in the later period of the greisen stage. Because Mo probably exists in its Mo4+ state, it may be difficult for it to replace W6+ in the scheelite lattice. Additionally, comparing the contents of Sn and Sr in different types of scheelite shows that the metallogenic environment changes from relative oxidation to the reduction of scheelite. The variation in trace elements and REEs in scheelite over time reflects a complex magmatic–hydrothermal mineralization environment. Additionally, the Ar–Ar system dating results for muscovite that is closely associated with scheelite in the greisenization granite vein indicate that a muscovite 40Ar/39Ar plateau age of about 133 Ma represents the time of tungsten mineralization. This muscovite 40Ar/39Ar dating result is close to the previous zircon U-Pb age data of the biotite monzogranite (ca. 140–145 Ma), which is the largest intrusion in the orefield. Meanwhile, the new age data reported here confirm that the Daping tungsten deposit represents a Mesozoic magmatic–hydrothermal mineralization event with a setting of lithospheric extension in the Mufushan composite batholith.