Abstract

The Pahnavar calcic Fe-bearing skarn zone is located in the Eastern Azarbaijan (NW Iran). This skarn zone occurs along the contact between Upper Cretaceous impure carbonates and an Oligocene granodioritic Batholith. Skarnification process can be categorized into two discrete prograde and retrograde stages. Prograde stage began immediately after the initial emplacement of the granodioritic magma into the enclosing impure carbonate rocks. The effect of heat flow from the batholith caused the enclosing rocks to become isochemically marmorized in almost homogeneous limestone layers and bimetasomatized in thin interlayers of clay-rich carbonates. Segregation and evolution of a fluid phase in the batholith and its invasion into fractures and micro-fractures of the marmorized and bimetasomatized rocks developed during this stage. The influx of considerable amounts of Fe, Si and Mg from the granodiorite to the metasomatic aureole led to development of anhydrous calc-silicates. The retrograde metasomatic alteration can be divided into two distinct early-retrograde and late-retrograde sub-stages. During early-retrograde sub-stage, the previously formed skarn zones were affected by intense multiple hydro-fracturing phases. In addition to Fe, Si and Mg, nearly substantial amounts of Cu, Pb, Zn, along with volatile components such as H2S and CO2 were added to the skarn system. Consequently, considerable amounts of hydrous calc-silicates (epidote and tremolite–actinolite), sulfides (pyrite, chalcopyrite, galena and sphalerite), oxides (magnetite and hematite) and carbonates (calcite) replaced the anhydrous calc-silicates. Late-retrograde sub-stage was concurrent with the incursion of relatively low temperature, more highly oxidizing fluids into skarn system, caused to partial alteration of the early-formed calc-silicates and developing fine-grained aggregates of chlorite, clay, hematite and calcite. Lack of wollastonite in the mineral assemblage, intergrowth of garnet and clinopyroxene , lack of any reaction rim and emplacement texture between these crystals demonstrate that they formed contemporaneously within the temperature and ƒO2 ranges of 430–550ºC and 10-26–10-23, respectively.