6. Formation and Breakdown of Ilvaite in the Large Galinge Skarn Fe Deposit, Western China: A Record of Multistage Retrograde Alteration

The Galinge deposit, the largest Fe skarn deposit in the Qiman Tagh porphyry-skarn metallogenic belt (western China), is noteworthy for its well-developed Ca-rich retrograde alteration. The ilvaite-bearing skarn associations were studied to determine their physicochemical formation conditions. Petrographic evidence for replacement of garnet and magnetite by ilvaite in the early retrograde stage (Stage I) combined with thermodynamic modeling suggests that the alteration may have occurred at ca. 400°C – 470°C under moderately high oxygen fugacity (ΔlogfO₂(HM): ca. -4 − -4.2). The model is based on a maximum pressure of 0.5 kbar calculated from magmatic amphibole geobarometer. The continuous breakdown of ilvaite with quartz to form ferro-actinolite and magnetite occur in the late retrograde stage (Stage II). The reactions occurred at about 400°C – 440°C under moderate fO₂ (ΔlogfO₂(HM): ca. -4 − -4.4). In Stage III, the breakdown of ilvaite to form calcite, pyrite and ferroactinolite depends on X(CO₂) which is unknown but can be estimated to be a range of 0.005 to 0.05.Under these conditions the breakdown occurs at ca. 270–350°C and low fO₂ (up to -5.2 log units below HM), but the reaction would occur at higher temperatures with increasing X(CO₂). The thermodynamic model for continuous evolution from Stage I to Stage III completely records the conditions of the retrograde alteration. Although Mn is absent, the presence of substantial Fe and Mg strongly affects the stability field of ilvaite in the skarn system. Therefore, the petrography and phase relations of ilvaite are useful indicators of reaction conditions in various skarn deposit types.