The Permian - Triassic metamorphosed limestone rocks in the South-East of
Cheshin, Hamadan area, is located at the vicinity of various metamorphosed clay rocks such as
schist and hornfels. Intrusion of Jurassic Alvand batholith into the clay and limestone rocks
lead to the formation of contact metamorphic rocks in areas of Hamedan (Cheshin village).
Based on the modal mineralogy (calcite, dolomite and silicate minerals), the metamorphosed
calcareous rocks of Cheshin consist of (a) marbles and calc- silicates, and (b) skarns. Ore
minerals formed in the border of calc-silicates and silimanit-bearing hornfleses, which lead to
formation of skarn rocks. According to the mineralogical investigations, skarns in the area
contain diopsied, garnet, tremolite, idocrase (vesuvianite), epidote and ore minerals like
hematite and magnetite. Skarnization processes occurred in two metamorphic stages :1)
progressive metamorphism and 2) retrograde metamorphism. The first stage involves a
progressive transformation and formation of minerals such as garnet and pyroxene. The second
step is retrograde that considered to be main parameter in the development of skarn. In
addition of Fe, Si and Mg, large amount of Fe were added to the skarn system by fluids.
Finally, anhydrous minerals in the carbonate host rock were replaced by considerable amount
of hydrated minerals (such as epidote + chlorite + amphibole), oxides, and carbonates. Using
multiple curves of thermodynamic equilibrium (with the help of the THERMOCALC
program), the temperature, pressure and fluid composition of calk-silicate and skarn rocks
calculated to be 620 ° C, 4 Kb and >0.17 XCO
, respectively. Oxygen fugacity in these rocks
is intermediate. Mineralogical studies of Cheshin skarns confirm the accuracy of calculations.
Skarn deposits have a direct relationship with magmatic activity and there is a significant
correlation between the chemical composition of parental plutonic rocks and metal content of
2
associated skarns. Petrographic and geochemical studies show that Alvand granitoid range in
composition from granite to quartz diorite. Like many of the granitoid skarns along the world,
Cheshin skarns belong to acidic, calc-alkaline, per aluminous S-type granite series.
Comparison of geochemical characteristics of major and minor elements of intrusive rocks
with other granitic skarns in the world suggest that this mass is analogous to that of Au-Cu
granitites of Ryvnarsa gold belt, Fe-Cu-Au Skarn in Selby, Turkey and Cu-Fe skarn of
Mylstrym Canada. Therefore, Cheshin skarn could be important for Fe-Cu-Au mineralization.
Comparing the samples with barren and productive granites, it seems that the Alvand intrusion
is barren