Granitoids of Sanandaj-Sirjan Zone that are concurrent with Cimmerian Orogeny (178-160 Ma) belong to ilmenite series (S-type): investigation of reason for lacking the porphyry tin mineralization

The granitic rocks are divided into magnetite and ilmenite series (Ishihara 1977), coinciding spatially with the I-type (εNdi0; (87Sr/86Sr)i~0.704-0.706) and S-type granites (εNdi<0; (87Sr/86Sr)i~0.708-0.765), respectively (Takahashi et al., 1980). Most porphyry Sn deposits are associated with ilmenite (S-type) granitoids (Ishihara, 1977; Neiva, 2002). The published concepts on the origin and tectonomagmatic setting of Sanandaj-Sirjan Zone (SaSZ) Jurassic granitoids of 178-160 Ma are (1) metaluminous I-type granites formed in a magmatic arc of an Andean subduction system (Khalaji et al., 2007; Tahmasbi et al., 2010; Ahadnejad et al., 2011; Esna-Ashari et al., 2012), (2) subduction-related extensional basin (Shahbazi et al., 2010), (3) continental crust melting by roll-back of Neo-Tethys oceanic crust (Zhang et al., 2018), and (4) subduction-related S-type granites (Bayati et al., 2017). In this research, the origin and tectonomagmatic setting of Jurassic granitoids (from 178 to 160 Ma) in Sanandaj-Sirjan Zone and the tin mineralization potential are investigated based on the available geological, geophysical and isotopic geochemical data.

We used an integrated collection of published geochemical data (major, trace and rare earth elements of 102 samples), isotopic (e.g., (87Sr/86Sr)I of 50 samples, ƐNdi of 64 samples), geochronological (U-Pb dating of zircons), and geophysical data (airborne magnetic intensity) for the SaSZ granitoids of 178-160 Ma.

The SaSz Jurassic granitoids include granite, monzonite, diorite, Syenogranite, tonalite batholiths. The (87Sr/86Sr)I>0.707, N-MORB normalized patterns with enrichment in Low- and High Field Strength Elements (LFSE and HFSE) and depletion in P and Ti contents, chondrite-normalized patterns with flat heavy rare earth elements (HREE) patterns, magnetic susceptibility <10-5x100 (ilmenite series), no alteration, and no Sn, Cu, Pb, and Zn anomalies in whole rock composition of granitoids nor in the associated river sediments, absence of volcanic rocks, and occurrence of metamorphic rocks (slate and schist) during Cimmerian orogeny indicate the SaSZ granitoids are S-type granitoids formed in a continental collision zone.

Geological, geophysical and geochemical characteristics of granitoids in Sanandaj-Sirjan Zone (such as the absence of volcanic arc and volcanic rocks, continental crust thickening (56-52 km) and the formation of large-scale (batholith) granitoids at depths >4 km, regional metamorphism at ​​green schist facies (and amphibolite) following Cimmerian orogeny, low (Eu/Eu)N (reducing conditions), magnetic susceptibility <100x10-5 (ilmenite series), negative εNdi and (87Sr/86Sr)I >0.707) show that, unlike previous studies, these granitoids are S-type granitoids formed by melting the continental crust in a collisional zone. Therefore, tin mineralization might probably occurred in connection with them. However, there is ample evidence of the absence of tin mineralization by the magma that forms these S-type granitoids, that are including the lack of hydrothermal fluids and consequently mineralization potential (due to the absence of alteration minerals in ASTER satellite images), low content of tin, copper, lead and zinc elements in these granitoids Sanandaj-Sirjan Zone and associated river sediments, (Eu/Eu)N value > 0.2, Rb/Sr <3, low Y (10-75 ppm), Ba > 200 ppm, as well as the geological, geophysical and geochemical similarities to barren S-type (ilmenite series) granitoids in Lut block (in Najmabad, Sorkh kuh to Shah kuh areas) which have formed in a continental collision system during the Cimmerian orogeny.