Chemical composition of oscillatory zoned garnets from the large-scale Mengya'a Pb-Zn skarn deposit: implications for fluid physicochemical conditions and formation

The Mengya'a Lead-zinc deposit is a large skarn deposit in the north of the eastern segment of Gangdese metallogenic belt. The garnet is the main altered mineral in the Mengya'a area. The color of the garnet varies from chartreuse to dark yellow brown and to russet. The brown garnet (Grt1) is related to pyrrhotite and chalcopyrite, and the green garnet (Grt2) is associated with lead-zinc mineralization. LA-ICP-MS is the induced coupled plasma mass spectrometry. This paper has used this technique to investigate Grt1 and Grt2. Grt1 develops core-rim textures with strong oscillation zone occurring in rim, whereas Grt2 lacks core-rim textures and featured by oscillation zone. LA-ICP-MS analysis shows that garnets of Mengya'a are rich in CaO (29.90-37.52 %) and FeO (21.17-33.35 %), but low in Al2O3 (0.05-4.85 %). The calculated end members belong to grandite (grossular-andradite) garnets andradite. The negative Al (IV) versus Fe3+, positive Al (IV) versus total Al stoichiometric number, the positive Al (IV) versus Fe3+, and the negative Al (IV) versus total REE, all indicate that the substitution of REEs in garnets is controlled by YAG. All Garnets are depleted in large lithophile elements (e.g., Rb = 0.00-4.01 ppm, Sr = 0.03-8.56 ppm). The total REE in Grt1 core is high ( n-ary sumation REE = 233-625 ppm), with HREE enriched pattern (LREE/HREE = 0.33-1.69) and weak negative Eu anomalies (delta Eu = 0.21-0.47). In contrast, the total REEs in the Grt1 rim and Grt2 are low (sigma REE = 12.4-354 ppm; sigma REE = 21.0-65.3 ppm), with LREE enriched pattern (LREE/HREE = 0.54-34.4; LREE/HREE = 11.4-682) and positive Eu anomalies (delta Eu = 0.35-27.2; delta Eu = 1.02-30.7). After data compilation of garnet chemicals, we found that the early fluid responsible for the core of Grt1 was a relatively closed and chloride-depleted fluid system. It was close-to-neutral, with a low water-rock ratio. The core of garnet was formed by fluid diffusion in metasomatic processes. The fluid was changed into a relatively open system with reduced, chloride-rich, and weak-acid fluid. It was fluid infiltration and metasomatism that resulted in the formation of Grt1 rim and Grt2.