Episodic hydrothermal pulses controlling skarn copper mineralization in the Secuo deposit, Xizang: Insights from in situ garnet geochemistry and chronology

• Multi-stage garnets record distinct hydrothermal regimes (open vs. closed systems). • REE fractionation and Eu anomalies reflect redox-pH shifts during fluid evolution. • The Secuo skarn copper deposit shows multi-stage fluid evolution. The skarn formation process involves complex fluid evolution. The mineralization that occurs during this process is significantly influenced by periodic hydrothermal fluid pulsation and replenishment. However, the multistage hydrothermal evolutionary processes of skarn-type copper deposits are poorly researched. The Secuo deposit in the southern Yulong metallogenic belt is characterized by two distinct periods of garnet skarn development. This study integrates electron microprobe analysis, laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) U-Pb dating, in situ trace element analysis, and thermal infrared spectroscopy of multistage garnets in the Secuo deposit to reconstruct the evolution of hydrothermal fluids. In first-period garnet skarn, abundant chalcopyrite and malachite are distributed among the garnet grains (GrtⅠ). In contrast, second-period garnet skarn crosscuts the first-period garnet skarn, and no mineralization develops between the garnet grains (GrtⅡ) of this period. GrtⅠ is dark brown, well crystallized, and lacks oscillatory zoning, and it exhibits significant variations in its grain size (1 ∼ 100 mm). GrtⅡ is light brown, fine-grained (1 ∼ 5 mm), and well-crystallized, and it exhibits oscillatory zoning. Thermal infrared spectroscopy reveals that GrtⅠ has an absorption trough at 11.5 µm, whereas the aluminium-rich layer of GrtⅡ has an absorption trough at 11.2 µm. GrtⅠ has a compositional range from And 41.54 Gro 56.69 to And 94.25 Gro 4.83 , and it can be characterized by enrichment in LREEs, depletion in HREEs, positive Eu anomalies, high total rare earth element (∑REE) contents, and low LREE fractionation (D La /D Sm ). GrtⅡ exhibits a relatively broad compositional range of And 33.26 Gro 64.97 to And 100 Gro 0.00 , and it exhibits LREE enrichment, HREE depletion, positive Eu anomalies, lower ∑REE, and high LREE fractionation. The positive correlation between Fe 2+ and REEs and the negative correlation between Al 3+ and REEs suggest that both GrtⅠ and GrtⅡ form through menzerite substitution. The results of U–Pb dating via LA–ICP–MS show ages of 39.9 ∼ 39.2 Ma for GrtⅠ and 39.0 ∼ 38.6 Ma for GrtⅡ. These ages are contemporaneous with the emplacement of mineralized and mineralization-related granite porphyry (39.51 ∼ 38.19 Ma). The observed positive correlation between Y and REE concentrations, combined with the pronounced europium anomaly (δEu > 1), suggests that GrtⅠ occurs during the initial fluid activation phase (Fluid Ⅰ) under closed-system conditions featuring both weakly acidic pH and high f O 2 . The geochemical signature of GrtⅡ, marked by an Al-rich core and alternating Fe-enriched interlayers, indicates open-system crystallization during secondary fluid (Fluid Ⅱ) activity under low f O 2 and weakly acidic conditions. Diagnostic evidence includes a positive europium anomaly (δEu > 1), Y-REE anticorrelation, and divergent La/Ho-Y/Ho fractionation patterns. The formation of Fe-rich bands in GrtⅡ may have resulted from the dissolution and reformation of some GrtⅠ. Therefore, we propose that the formation of skarn in the Secuo deposit is controlled by episodic fluid pulses, and its copper mineralization is related to Fluid Ⅰ. This study suggests that the skarn-type copper deposits in the Yulong metallogenic belt exhibit characteristics of multi-stage fluid evolution.