Compositional Characteristics And Geological Significance Of Garnet From Skarn W And Sn Deposits In Southern Hunan

The southern Hunan region is an important part of the Nanling metallogenic belt in China,with numerous skarn polymetallic deposits,which provide a huge amount of resources of W,Sn,Bi,and various other metals.The Xianghualing Sn deposit and Xintianling W deposit represent two of the many skarn deposits in this region,and have been selected for investigation attempting to provide insights into the ore-forming processes and guidance for exploration of skarn W/Sn mineralization.Detailed mineralogy and mineral geochemistry of these two deposits have been conducted in this study;the garnets from both deposits have been analyzed by LA-ICP-MS to quantify the concentrations of their major elements,trace elements,as well as rare earth elements.The results reveal that the garnets from the Xianghualing and Xintianling deposits mainly belong to andradite-almandine solid solution;the former tends to be richer in Al and is mainly composed by grossular,while the latter show a transitional trend with garnets dominated by either grossular or andradite,or else both.These two deposits show different REE patterns.The Xianghualing deposit is characterized by HREE enrichment and LREE depletion,with pronounced negative Eu anomaly,indicating relatively reduced fluids during garnet crystallization.By comparison,The Xintianling deposit shows relatively complicated REE patterns,including two patterns:(1)HREE enrichment and LREE depletion with negative Eu anomaly,and(2)LREE enrichment and HREE depletion with positive Eu anomaly.This suggests that the garnets crystallization under variable fluid redox conditions,which have been reflected in the oscillatory zonations.In addition,the geochemical compositions of the Xianghualing and Xintianling deposits reveal that the Sn concentrations are relatively high in both deposits,with the highest values of 5766 ppm(average4295 ppm)and 987 ppm(average 425 ppm),respectively.Moreover,the LA-ICP-MS mapping results of the Xianghualing garnets show obviously positive correlations between Sn and Fe,suggesting that garnet prefers to incorporate Sn in their lattice by replacing Fe³⁺in the octahedral position.Notably,there is also a well positive correlations between Sn and W of the garnets from both deposits,and therefore it can be inferred that the garnet can incorporate W via a similar mechanism like Sn.Combined with previous studies,the available data suggest that garnet in skarn deposits with economic W-Sn mineralization is generally enriched in Sn,while the concentrations of W vary significantly.Nevertheless,the W concentrations in garnet from skarn deposits with economic W-Sn mineralization are still higher than those without W-Sn mineralization.Therefore,it is reasonable to come to the conclusion that the Sn and W concentrations of garnet in a skarn deposit or prospect can provide considerable insights in predicting the potential economic metal(s)in the deposit or prospect,which might be helpful in guiding exploration.It should also be mentioned that due to limited data from the current study,the conclusions drawn here still need further examination with large numbers of data from a great amount of skarn deposits.