U-Pb Geochronology and Geochemistry of Stratiform Garnet from the Aqishan Pb-Zn Deposit, Eastern Tianshan, Xinjiang, NW China: Constraints on Genesis of the Deposit

The Aqishan lead-zinc deposit, located in the Jueluotag metallogenic belt of eastern Tianshan, Xinjiang, Northwest China, has a stratiform occurrence in the marine volcanic tuff of the Yamansu Formation. The ore body has a typical double-layer structure, having a stratified, stratoid, lenticular upper part and a veined, stockwork-like lower part. The occurrence of the upper orebody is consistent with that of the volcanic tuff wall rock. The ore minerals are mainly chalcopyrite, pyrite, sphalerite, galena and magnetite, the altered minerals mainly being silicified, such as sericite, chlorite, epidote, garnet. The garnetized skarn, being stratiform and stratoid, is closely related to the upper part of the orebody. Geological observations show that the limestone in the ore-bearing Yamansu Formation is not marbleized and skarnized. Spatially, it is associated with the ferromanganese deposits in the marine volcanic rocks of the Yamansu Formation. These geological features reflect the likelihood that the Aqishan lead-zinc deposit is a hydrothermal exhalation sedimentary deposit. The results from the EPMA show that the garnet is mainly composed of grossular-andradite series, contents being in a range of 34.791-37.8% SiO2, 32.493-34.274% CaO, 8.454-27.275% FeO, 0.012-15.293% Al2O3, 0.351-1.413% MnO, and lower values of 0.013-1.057% TiO2. The content of SiO2 vs. CaO and FeO vs. Al2O3 has a significant positive correlation. The results of ICP-MS analysis for the garnet show that the REE pattern is oblique to right in general. The total amount of rare earth elements is relatively low, sigma REE = 71.045-826.52 ppm, which is relatively enriched for LREE and depleted for HREE. LREE/HREE = 8.66-4157.75, La-N/Yb-N = 23.51-984.34, with obvious positive Eu and Ce anomalies (delta Eu = 2.27-76.15, delta Ce = 0.94-1.85). This result is similar to the REE characteristics of ore-bearing rhyolite volcanic rocks, showing that the garnet was formed in an oxidizing environment and affected by clear hydrothermal activity. The U-Pb isotopic dating of garnet by fs-LA-HR-ICP-MS gives an age of 316.3 +/- 4.4 Ma (MSWD = 1.4), which is consistent with the formation time of the Yamansu Formation. According to the study of deposit characteristics and geochemical characteristics, this study concludes that the Aqishan lead-zinc deposit is a hydrothermal exhalation sedimentary deposit, the garnet being caused by hydrothermal exhalative sedimentation.