Major and trace elements analyses of garnet from the Haobugao Zn-Fe-Sn polymetallic deposit, northeast China: Implications for skarn formation and hydrothermal fluid evolution

The Haobugao Zn-Fe-Sn skarn deposit in the Southern Great Xing’an Range of NE China contains three generations of garnets that were distinguished using petrography. This paper discusses skarn formation and hydrothermal fluid evolution in the Zn-Fe-Sn skarn system based on the interpretation of diverse textures and compositions of garnets. Crystals of first-generation garnet (Grt 1) from the proximal exoskarn are red-brown, coarse-grained garnets that are subhedral and optically isotropic. They are dominantly grossular (Adr_49-18Grs_51-82) and have the highest REE contents of the garnets at Haobugao. They are LREE-depleted, HREE-enriched, with negative Eu anomalies, and they formed under low water/rock (W/R) ratios and neutral pH conditions. Crystals of second-generation garnet (Grt 2) from the central exoskarn are yellow-green and andradite dominated (Adr_63-97Grs_2-36). They have relatively isotropic cores with chaotic anisotropism (Grt 2a) or oscillatory zoned rims (Grt 2b). The Grt 2 cores have low REE contents, are LREE-enriched, HREE-depleted, with positive Eu anomalies, and formed under high W/R ratios and acidic conditions. The Grt 2 rims have higher REE contents, are LREE-depleted, HREE-enriched, with weak negative Eu anomalies, and formed under low W/R ratios and acidic condition. Crystals of third-generation garnet (Grt 3) coexist with pyroxene and are light yellow–brown, zoned or optically homogeneous. They are relatively Fe-rich grossular (Adr49-51Grs48-50), compared with Grt 1. They have the lowest REE contents of the garnets at Haobugao, and a REE pattern similar to that of the Grt 1, which indicates they formed under low W/R ratios, reduced to oxidized and neutral pH conditions. The REE content and fractionation of garnets indicate periodic fluctuations in fluid composition occurred in the Haobugao Zn-Fe-Sn skarn system, likely caused by the joint effects of magmatic and meteoric water. Diverse mechanisms contributed to the %Sigma;REE variation in garnets.