Original paper

Geochemistry and economic potential of the Samen Granitoid Intrusion, Northwestern Iran: Implications for skarn mineralization

Ahmadnejad, Farhad; Zamanian, Hassan; Sameti, Mona

Abstract

The Samen Granitoid Intrusion is located along the northwestern part of the Sanandaj–Sirjan metamorphic belt in the southwestern part of Malayer County, Iran. Based on mineralogical and geochemical characteristics, three main rock types are distinguished in the study area, which are (1) granodiorite, (2) quartz monzodiorite and (3) monzogranite units. Geochemistry of major elements indicates that the Samen Igneous Suite is metaluminous (A/CNK = 0.75) to peraluminous (A/CNK = 1.21). The suite displays the geochemical characteristics of I-type granites related to active continental margin magmatism and petrochemically, belongs to the shoshonite and high-K calc-alkaline series. Depletion in Nb, Zr, Hf, Y, Ti and HREE and enrichment in K, Rb, Cs, Th and LREE are related to the subduction of Neo-Tethyan oceanic crust beneath the Sanandaj-Sirjan Zone. Chondrite- normalized REE patterns of the quartz monzodiorite unit indicate overall REE depletion and light over heavy REE enrichment relative to the other units. In the Samen district both, calcic exoskarn (andradite-grossular/ hedenbergite-diopside) and endoskarn (tremolite-actinolite/epidote) occur along the contacts between granitoid and marble. Composition of calc-silicate minerals in the Samen skarn show that the clinopyroxenes mainly have an hedenbergitic to diopsidic composition (Hd77.4 – 52.1 Di45.9 –18.3 Joh4.2 –0.3), with low abundances of Na2 O, TiO2, Al2 O3, and Cr2 O3, and medium to high Fe/(Fe + Mg) ratios (between 0.53 and 0.81). In addition, the chemical composition of the analyzed garnet shows the presence of a grossular–andradite solid solution series (And70.4–46.7Gro44.1–27.2 Alm6.2 –1.3) with andradite as the dominant end-member, low abundances of TiO2 and medium to high Fe/(Fe + Al) ratios (between 0.55 and 0.72). Therefore, the composition of calc-silicate minerals in the Samen skarn is similar to those of calc-silicate minerals from Fe, Cu and Au skarn deposits. The present study indicates that the geochemical characteristics (e.g.Rb vs. Sc and Zr vs. Rb/Sr diagrams) of the granodioritic and quartz monzodioritic units are similar to the averages of Au-Cu- and Fe- skarn granitoids. These data probably indicate that some other element enrichment, e.g., Sn and Mo in the Samen district, could be associated with monzogranitic units. Therefore, given the extent of intrusive suites in the area and only limited exploration thus far, this suggests new exploration possibilities for the Samen district. Based on oxidizing conditions and magma evolution, the Samen Granitoid Intrusion is characterized by relatively unevolved to moderately evolved and oxidized suites, as in most Au-Cu core metal associations globally. Field studies, composition of calc-silicate minerals and petrological-geochemical characteristics of the analyzed pluton could provide powerful tools for the study of particular metallic associations near granitoids of the Sanandaj Sirjan Zone which, until now, have not been explored to its full potential.