Ilvaite trace mineral chemistry as a thermodynamic recorder of retrograde alteration and metallogenic indicator for discrimination of skarn deposits: an example from the iron deposit in North Sanandaj-Sirjan Zone

Ilvaite, CaFe22+Fe3+(Si2O7)O(OH), is one of the important trace minerals in the retrograde alteration stage of skarn deposits. In this contribution, by using microscopic evidence, X-ray diffraction (XRD) and electron microprobe analysis (EPMA), the crystallographic characteristics, crystal chemistry and thermodynamic conditions of ilvaite have been investigated in the formation of skarn deposits. The Gholamabad iron skarn deposit (1.3 Mt with an average grade of 37.2% FeO) is located 30 km SW of Dehgolan, North Sanandaj-Sirjan Zone (N-SaSZ). Skarn formation has occurred in this deposit at the contact of alkali granite porphyry (Upper Jurassic) and tuffaceous siltstone sequence (Triassic-Jurassic), which was accompanied by progressive alteration of brown andradite-hedenbergite near the intrusive body and retrograde alteration of epidote-actinolite-calcite in the vicinity of volcanic rocks. Ilvaite with empirical formula, Ca0.90Fe2+1.88Fe3+0.98Al0.04Mn0.04Mg0.02Si2.14O8(OH), formed from the reaction of andradite, magnetite, and quartz minerals during retrograde alteration stage (temperature 400 to 470 °C and ∆logfO2(HM) between -4.2 and -4.0). Then, in the final stage of retrograde alteration, with increasing XCO2 activity (0.05 to 0.005) and decreasing oxygen content (∆logfO2(HM) < -4.5), ilvaite has been decomposed to the magnetite, calcite, and ferroactinolite assemblage at temperature 270 to 350 °C. According to the results of EPMA, the high ratio of Fe2+/ Fe3+ (avg.= 1.90) and low content of Mn2+ (<0.05 assumed six cation sum) in the ilvaite of Gholamabad deposit indicates that it belongs to Fe skarn deposits, which is different from other skarn systems. In general, although ilvaite is a trace mineral in most skarn systems, the results of this approach show that the composition and thermodynamics of ilvaite can be used to distinguish different types of skarn deposits and the evolution of geochemical-skarnification processes.