Thermodynamic Model for the Galinge Fe Skarn Deposit in Qinghai

Abstract

The Galinge iron deposit in the Qiman Tagh orogen, western part of the East Kunlun, Qinghai province, occurs as lens-shaped magnesian skarn, with magnetite and base-metal sulfide orebodies, and is hosted in dolomitic limestone. It experienced a complete skarn and retrograde stage under varying fluid compositions resulting in thermodynamically controlled formation of magnesian skarn and mineralogical zonation. A series of Mg- and Ca-rich solid solutions were generated in the skarn stage, including forsterite-fayalite, spinel-hercynite-gahnite and diopside-hedenbergite solid solutions. A thermodynamic model setting pressure of 0.6 kbar and X(CO2)=0.3 was set up to trace the skarn evolution in the skarn stage. Magnetite is stabilized at fluid conditions of ca. 460 – 520°C and Δlog fO2 (HM) = -5 – -11 in the skarn stage, and co-precipitates with diopside and forsterite. Magnetite precipitation always shows strong relations with diopside and forsterite rather than fayalite and hedenbergite, which deplete iron from the fluid. The retrograde alteration stage is characterized by the formation of tremolite, chondrodite, phlogopite, clinochlorite, epidote, prehnite, serpentine, magnesiomagnetite and ludwigite. In the thermodynamic model of the retrograde alteration evolution, setting P= 0.6 kbar and X(CO2)=0.01. Most of the tremolite + diopside + magnetite and clinohumite + diopside + magnetite assemblages are stable at 360 – 460°C and Δlog fO2 (HM) = -16 – -5. The phlogopite is formed at a temperature range of ca. 360 – 420°C and Δlog fO2 (HM) = -11 – -6, and serpentine are stabilized below 460°C in the late retrograde stage. Their stability in the system are intensively effected by the Al2O3 activity of the fluid. The paragenetic sequence of retrograde minerals is most likely a result of internally buffered increasing oxidation state as the precipitation of magnetite. This suggests that oxidizing process is most important for understanding the major causes of skarn iron deposit formation in other areas.