Deciphering the ‘Purge of Impurities' Effect: Tin Speciation Evolution and Selective Dissolution During Hydrothermal Cassiterite Recrystallization

Multi-stage superimposed tin （Sn） mineralizations in hydrothermal systems are commonly accompanied by the dissolution and reprecipitation process （DRP） of cassiterite. However， the role of cassiterite DRP in Sn enrichment remains ambiguous. To investigate this， a natural cassiterite （Yongde cassiterite） crystallized in a single metallogenic event with pure geochemical compositions， was selected for crystallization experiments in SnCl₂-HCl-H₂O and SnCl₄-HCl-H₂O systems using both hydrothermal diamond anvil cell （HDAC） and cold seal pressure vessel （CSPV）. HDAC experiments show that the Yongde cassiterite （Cst 1） was soluble in both the SnCl₂-HCl-H₂O and SnCl₄-HCl-H₂O systems during heating， while secondary cassiterite （Cst 2） crystallized in both systems during cooling. Notably， Cst 1 exhibited higher solubility in the SnCl₂-HCl-H₂O system， evidenced by its lower initial dissolution temperature （430 ℃） compared to the SnCl₄-HCl-H₂O system （500 ℃）. Raman spectra collected on the Cl-bearing aqueous solution in the HDAC sample chamber suggests that Sn（II）-Cl is always significant in the hydrothermal transport of Sn， whereas Sn（IV）-Cl is significant only for relatively oxidized conditions. Major and trace element analysis of the cassiterite recovered after CSPV experiments demonstrates the enrichment and depletion of elements during cassiterite DRP. In the SnCl₂-HCl-H₂O system， SnO₂ （avg. 99.28 to 99.79 wt.%） is enriched， while Ti （avg. 1755 to 1568 ppm）， Fe （avg. 189 to 162 ppm）， and Zr （avg. 32.2 to 22.1 ppm） are depleted in Cst 2. In the SnCl₄-HCl-H₂O system， Al （avg. 1.83 to 481 ppm）， Sb （avg. 0.41 to 2.91 ppm）， and W （avg. 0.31 to 24.0 ppm） are enriched， while Ti （avg. 1762 to 417 ppm）， Fe （avg. 121 to 46.0 ppm）， Zr （avg. 48.2 to 17.6 ppm）， Hf （avg. 3.59 to 1.25 ppm）， and Ta （avg. 3.40 to 1.06 ppm） are depleted in Cst 2. The content variations of redox-sensitive elements （e.g.， Al， Sb， W） and temperature-sensitive elements （e.g.， Ti， Fe， Zr） reflect dynamic redox and temperature-driven purification during cassiterite DRP， wherein impurity elements （e.g.， Ti， Fe， Zr） are expelled， enabling progressive Sn concentration. Our findings establish that cassiterite DRP contributes to remobilization and local enrichment of Sn in multi-stage hydrothermal systems. This perspective enhances our understanding on Sn enrichment during superimposed Sn metallogenic events， with implications for analogous deposits involving oxides and sulfides.