Thermodynamic conditions of ore-forming fluid and migration and enrich-ment mechanism of iron in the Tieshan Fe-Cu deposit, southeastern Hubei Province

Based on petrographic observations of garnet and tremolite collected from the Tieshan Fe-Cu deposit, the authors found that the primary fluid inclusions are generally of three types:gaseous inclusions, liquid inclusions and halite daughter mineral-bearing three-phase inclusion. According to temperature measurement results, the homogenization temperatures at the prograde skarn stage range from 499.2℃ to 594.8℃, the fluid salinity peak values range from 17.3%NaCl to 19.5%NaCl, the densities range from 0.45g/cm³ to 0.62g/cm³, and the pressure ranges from 58.0×10⁶ Pa to 90.6×10⁶ Pa; the homogenization temperatures at the retrograde alteration stage range from 356.2℃ to 428.6℃, the fluid salinity peak values range from 7.2% Nacl to 15.5%_NaCl, the densities range from 0.52g/cm 3 to 0.83g/cm³, the metallogenic pressure ranges from 23.8×10⁶ Pa to 29.7×10⁶ Pa, and the metallogenic depth ranges from 0.90km to 1.12km with an average of 0.95km. These data indicate that the ore-forming fluid of the Tieshan Fe-Cu deposit has the characteristics of high temperature, moderate-low salinity, and low density. Therefore, the Tieshan Fe-Cu deposit is an epithermal skarn deposit. Composition analysis of grouped fluid inclusions indicates that gaseous composition is dominated by H₂O and CO₂, fol-lowed by small amounts of CH₄, C₂H₆, N₂ and H₂S, the cations of liquid composition are dominated by Na⁺, Ca²⁺ and K⁺, and the an-ions are dominated by SO₄^2- and Cl^-. The results show that Cl^-, SO₄^2- and carbonate complexes played an important role in migration and enrichment of iron. The mixing of fluids of different origins and the evolution of pH might have been the dominant enrichment and precipitation mechanism of the Tieshan Fe-Cu deposit.