Multipulsed fluid evolution and tungsten prospecting potential of the giant Jiama skarn deposit, Tibet: Evidence from garnet geochemistry

The Jiama deposit, located in Tibet and recognized as the largest skarn-type Cu-polymetallic deposit in China, primarily hosts skarn orebodies within the structural zone between the Linbuzong Formation and Duodigou Formation. The importance of Jiama deposit has racently attracted many researchers on the studies of mineral composition, zoning, and the relationship with mineralization of the skarn. However, these studies argue for the evolution process based on garnet color variations to characterize the hydrothermal fluids of different skarn types. This approach may leave several critical questions unaddressed, such as the distinct features of fluid evolution in skarns with different structural types, how these fluids evolved, and how the evolution and ore-forming processes occurred. To address such problems, we adopt the major/trace elements composition analysis of garnets with different structures that aim to revealing the evolution of hydrothermal fliuds and the formation garnets of different structures in skarns. In specific, the Jiama skarn is characterized by a transition from proximal diopside-garnet skarn to distal garnet-wollastonite skarn, with four garnet types evolving from grossular to dominantly andraditic compositions as shown by petrography, EPMA, and LA-ICP-MS analyses. The results show that, T1 garnet in massive skarn exhibits enrichment in heavy rare earth elements (HREEs) and depletion in light rare earth elements (LREEs), T2 garnet shows weak fractionation between LREEs and HREEs, but both display negative Europium (Eu) anomalies. T3 and T4 garnets are distinctly different from the former types, exhibiting right-leaning patterns accompanied by pronounced positive Eu anomalies. The rare earth element (REE) patterns of vein-type garnets are more complex, and can be classified into three types: (1) Completely left-leaning type (LREE/HREE = 0.40–0.46) with distinct negative Eu anomalies; (2)Nearly flat type (LREE/HREE = 2.10–4.86) with almost no Eu anomaly; (3) Type with slight enrichment in LREEs and depletion in HREEs (LREE/HREE = 2.98–48.11), exhibiting no or weak positive Eu anomalies. REE patterns and paragenetic relationships indicate an early, relatively closed, neutral, low-fO₂, diffusion-dominated metasomatic stage followed by later fluid influx into an open, acidic, high-fO₂, advection-dominated environment, a shift also evidenced by compositional and REE variations in vein-type garnets. These significant differences in vein-type garnet features reflect significant changes in fluid physicochemical properties (e.g., pH, redox state, fluid composition) during their formation. It also shows that the geochemical characteristics of garnets in the Jiama deposit indicate a multi-stage evolution of skarn-forming fluids and the compositional variations in vein-type garnets are significant. Considering that in our recent skarn samples with a content of tungsten (W), these reulsts may imply that W-rich andradite garnets are often replaced by later minerals, a process that releases W from the garnet into the fluid, where it may subsequently concentrate to form ore deposits under favorable conditions.