Garnet major and trace element evidence of the alteration and mineralizing processes associated with genesis of the Qiaomaishan skarn deposit, Xuancheng ore district, eastern China

Abstract

The Qiaomaishan deposit is a skarn-type Cu–W deposit located within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt of eastern China. The mineralization within the deposit is predominantly located along the contact between a porphyritic granodiorite intrusion and surrounding Carboniferous limestone units. The deposit is associated with a well-developed and zoned skarn system that includes both endoskarn and exoskarn zones. This study presents new garnet major, trace, and rare earth element (REE) geochemical data for these skarn zones, discusses the factors (e.g., crystal chemistry, oxygen fugacity and water/rock interaction) that control the incorporation of trace elements into garnet, and constrains the formation of skarn within the Qiaomaishan deposit. Grossular (Adr21–70Grs29–78; type 1) garnet is generally located within the endoskarn and is light REE (LREE) enriched but heavy REE (HREE) depleted with weakly negative Eu anomalies and higher total REE contents than the exoskarn garnet in the study area. Type 1a grossular garnet is further split from type 1b grossular garnet by the presence of positive Pr and Nd anomalies and high LREE concentrations. Exoskarn garnet has pure andradite compositions and is split into type 2 and 3 garnets, where type 2 andradite garnet has variably positive Eu anomalies and variable total REE contents that are indicative of formation under transitional or oscillating conditions (e.g., pH, oxygen fugacity). In comparison, the type 3 andradite garnet has low total REE contents and positive Eu anomalies. Endoskarn garnet Al2O3 and total REE contents positively correlate, indicating that REE is incorporated into this garnet through coupled YAG-type substitution. The differences between the total REE contents of the type 2 exoskarn and endoskarn garnets may reflect host rock buffering as well as variations in the crystal chemistry of the two garnets present within the different parts of the skarn system (grossular and andradite). In comparison, the differences in the total REE contents of the two types of exoskarn garnet may reflect variations in water–rock ratios and crystal growth rates. The different Eu anomalies and U contents present within the different types of garnets record variations in the pH and oxygen fugacity of the hydrothermal fluid over time. These changes may have promoted the crystallization of magnetite and scheelite and the formation of the tungsten mineralization within the deposit. This study demonstrates the usefulness of garnet in tracking variations in mineralizing conditions and processes and confirms the validity of these minerals as vectors or indicator minerals that can be used in combination with traditional exploration approaches to determine whether a system is mineralized or not during the early stages of exploration.