Mineralogical Specifications, Metasomatic Alteration and Conditions of Fe-Skarn Formation at Pahnavar, East of Siahrood, East-Azarbaidjan, Iran

Pahnavar skarn, located in the east of Siahrood, is a typical magnetite-rich skarn developed along the contact of Qolan granitoid (quartz diorite) I-type intrusive body (Oligo-Miocene) with carbonate (principally limestone) rocks. Field evidence and mineralogical study show that Pahnavar skarn is of calcic-type and includes two distinct zones: 1) endoskarn and 2) exoskarn. Exoskarn is the major skarn zone and by itself includes two separate zones: a) garnet skarn in igneous contact side and b) epidote skarn in parts farther from the contact. Two general stages of skarnification were recognized in Pahnavar: 1) prograde and 2) retrograde. Prograde stage was developed within the temperature ranges of 470-550 ºC and includes two discrete stages: a) isochemical formation of hornfelsic skarnoid (bimetasomatic) and recrystallized limestone (marble) and b) formation of metasomatic skarn during which a series of medium to coarse-grained anhydrous calc-silicates were developed. Retrograde stage commenced at temperatures <470 ºC and includes two distinct stages: 1) early retrograde stage during which anhydrous calc-silicate assemblages (formed during prograde stage) were replaced by a series of hydrous calc-silicates (epidote, tremolite-actinolite), silicates (quartz), carbonates (calcite), and opaques (magnetite, hematite, and pyrite). Magnetite was principally developed during this stage. 2) Late retrograde stage during which most of minerals formed during previous stages were altered into a series of fine-grained aggregates of chlorite, calcite, quartz, hematite, and clay minerals.