Genetic relationships between skarn ore deposits and magmatic activity in the Ahar region, Western Alborz, NW Iran

Abstract

Paleocene to Oligocene tectonic processes in northwest Iran resulted in extensive I-type calc-alkaline and alkaline magmatic activity in the Ahar region. Numerous skarn deposits formed in the contact between Upper Cretaceous impure carbonate rocks and Oligocene-Miocene plutonic rocks. This study presents new field observations of skarns in the western Alborz range and is based on geochemistry of igneous rocks, mineralogy of the important skarn deposits, and electron microprobe analyses of skarn minerals. These data are used to interpret the metasomatism during sequential skarn formation and the geotectonic setting of the skarn ore deposit related igneous rocks. The skarns were classified into exoskarn, endoskarn and ore skarn. Andraditic garnet is the main skarn mineral; the pyroxene belongs to the diopside-hedenbergite series. The skarnification started with pluton emplacement and metamorphism of carbonate rocks followed by prograde metasomatism and the formation of anhydrous minerals like garnet and pyroxene. The next stage resulted in retro gradation of anhydrous minerals along with the formation of oxide minerals (magnetite and hematite) followed by the formation of hydrosilicate minerals like epidote, actinolite, chlorite, quartz, sericite and sulfide mineralization. In addition to Fe, Si and Mg, substantial amounts of Cu, along with volatile components such as H 2 S and CO 2 were added to the skarn system. Skarn mineralogy and geochemistry of the igneous rocks indicate an island arc or subduction-related origin of the Fe-Cu skarn deposit.

Interaction between felsic and mafic magmas in the Salmas intrusive complex, Northwestern Iran: Constraints from petrography and geochemistry

“…Previous studies on mafic-intermediate rocks have been suggested that the parental mafic gabbroic magmas were generated from a spinel-peridotite source via 5-20% degrees of fractional melting at a depth of ∼52 km (Mollai et al, 2014). According to Mollai et al (2014), fractionation modeling involving the major elements reveals that the diorites are the products of fractional crystallization of gabbroic rocks. The produced dioritic melts ponded at a level of neutral buoyancy probably located at the crust-mantle boundary and would have transferred both heat and mass to the overlying lower crust.…”

“…Trace element ratio modeling indicates that the gabbroic magmas were generated from a spinelperidotite source via 5-20% degrees of fractional melting at 52 Km depth (Mollai et al, 2014).…”

Magma mixing/mingling in Salmas granodiorite, NW Iran: evidence from mafic microgranular enclaves

“…The mixed rocks (granodiorites) cover an area of approximately 20 km 2 and are exposed at the contact of granitic and maficintermediate stocks which all of them are emplaced into Precambrian metamorphic complex (Khodabandeh et al 2002;Ghaffari 2008;Ghaffari et al 2013). Marble, recrystallized limestone and a skarn zone have developed at the contact of mafic-intermediate rocks with the limestone which is a common phenomenon in North West of Iran (Mollaei et al 2009;Mollai et al 2014). The Quaternary basaltic rocks which occurred after Late Miocene calc-alkaline magmatism are the last phase of magmatism activity (Dabiri et al 2011).…”

Geochemistry of skarn and porphyry deposits in relation to epithermal mineralization in the Arasbaran metallogenic zone, NE Tabriz, Iran

“…The lat ter, as the name im plies, are the skarns that con tain min er ali sa tion, and are formed as a re sult of in fil tra tion of flu ids de rived from ig ne ous in tru sions. In the Ahar re gion the pro cesses that lead to the for ma tion of skarn de pos its in clude three stages as follows: (1) Em place ment of plutonic magma which leads to isochemical con tact meta mor phism (cal cite, pyroxene, plagioclase); (2) prograde metasomatic skarn for ma tion as the pluton cools and an ore fluid de vel ops, (epidote, tremolite/actinolite, chlorite) and; (3) ret ro grade al ter ation of ear lier formed min eral as sem blages, lead ing to the for ma tion of hydrosilicate min er als along with ore de po si tion (Mollai et al, 2014). In gen eral, skarns are zoned from endoskarns in the caus ative pluton, to prox i mal gar net and dis tal pyroxene, a pat tern which also in di cates the over all ox i da tion state of the skarn sys tem (Meinert et al, 2005).…”