Implications of garnet composition on metallogenic chronology and ore-forming fluid evolution of skarn deposits: A case study of the Kendekeke Fe-polymetallic deposit in East Kunlun

The skarn deposit formation process entails intricate fluid evolution processes, wherein the mineralization process is significantly influenced by the periodic pulsation and replenishment of hydrothermal fluids. However, there is a dearth of evident evidence supporting the periodic fluid replenishment viewpoint. One common mineral in skarn deposits is garnet. The objective of this study is to provide insights and evidence on fluid replenishment and to clarify the fluid evolution process by analyzing the crystal structures of the oscillatory zoning and epitaxial hyperplasia edges of garnet. The Kendekeke deposit is a skarn-iron polymetallic deposit located in the Qimantage metallogenic belt of the East Kunlun orogenic belt. The thick magnetite ore body developed in the deposit is located in the skarn belt, and garnet skarn is well developed throughout. Based on the major elements and trace elements of garnet, this paper clarifies the metallogenic age and evolution of ore-forming fluid in skarn iron polymetallic deposits. Through analysis of mineral zoning and cross-cutting relationships, four generations of garnets have been identified in the Kendekeke deposit. We conducted LA-ICP-MS U-Pb geochronological analysis on the nearest Grt-II to the ore body and obtained the ore-forming age. The major elements of garnet in four stages show the obvious changing trend. The first three stages change from Al-rich to Fe-rich, and the fourth stage shows the characteristics of grossularite-andradite. Through the correlation analysis of rare earth elements (REE) and Y elements, we determined that the whole evolution process did not have significant external fluid influence, and the REE patterns in the first three stages showed obvious inheritance characteristics, among which Grt-II showed some euhedral oscillatory zoned characteristics. The REE patterns from the core to the edge of the zone showed obvious fluid evolution process from Grt-I to Grt-III, while the Grt-IV REE patterns exhibit the characteristics of anti-inheritance, which is different from the first three stages. Among them, we observed that Grt-IV and Grt-II (epitaxy hyperplasia edge of Grt-I) have similar major element characteristics, but compared to Grt-II (epitaxy hyperplasia edge of Grt-I), Grt-IV has similar LREE and lower HREE patterns that we believe is the result of early fluid replenishment. At the same time, combining with trace elements such as U and Eu, we determined that the pH of the hydrothermal solution evolved from near-neutral to acidic, and the fO2 increased first and then decreased. In addition, through the analysis of the replacement elements (Sc, V, Co, and Ti) Grt-IV, the replenishment of fluid at this stage reduced fO2 again, promoted the further enrichment of ore-forming materials, prolonged the mineralization process, and thus formed a thick and iron-rich ore body.