Discrimination of fertile and barren skarn-related magmatism from Sangan Fe Skarn, NE Iran: Constrains from whole-rock geochemistry, zircon chemistry and crustal thickness

The Sangan mining district consists of 14 iron ore deposits at the contact of Eocene granitoids to Mesozoic sedimentary rocks. Syenite, syenogranite and granite represent the fertile Sarnowsar intrusion and monzogranite to syenogranite barren Sarkhar and Bermani intrusions. LA-ICP-MS U-Pb zircon ages of fertile syenogranite samples are between 39.6 ± 0.7 and 39.1 ± 0.4 Ma corresponding to lithospheric thinning by Eocene flare-up magmatism. Barren Sarkhar and Bermani intrusions (monzogranites at 41.7 ± 0.6 and 41.9 ± 0.3 Ma; syenogranites between 37.4 ± 1.8 and 37.9 ± 1.7 Ma were emplaced contemporaneous with flare-up magmatism.

Plots of Nb vs. Ta, Y vs. Yb/Sm and Y vs. Ce/Ce* classify studied zircons as granitoid-type as parental magma. Uniformly high Hf contents of zircons indicate crystallization from a more evolved felsic magma, especially the Sarkhar and Bermani intrusions. In the U/Yb versus Hf and U versus Yb discrimination diagrams, all data plot in the continental-series and are clearly distinguishable from ocean crust zircons. In the U/Yb versus Nb/Yb diagram, both the whole-rock and zircon compositions show the characteristics of a magmatic-arc array. The Nb content of arc magmas is depleted relative to within-plate settings. As such, arc zircons possess lower Nb/Hf and higher Th/Nb ratios with a similar degree of magmatic fractionation. Bivariate discrimination diagrams such as Th/U versus Nb/Hf and Th/Nb versus Hf/Th indicate that most zircons plot in the orogenic field, signifying a magmatic arc or orogenic setting and a calc-alkaline parent magma. Fertile intrusions are characterized by a higher zircon Eu (>0.3) and Ce (>100) anomalies, 10,000*(Eu/Eu*)/Y (>1), (Ce/Nd)/Y (>0.001), and slightly lower Dy/Yb ratios (0.21 to 0.38) than zircons in barren intrusions with Dy/Yb ratios (0.2 to 0.51) and lower Eu and Ce anomalies. The chondrite normalized zircon Eu/Eu* ratio correlates with whole rock La/Yb and has been used to estimate the crustal thickness. Our results reveal that fertile Sarnowsar granitoids formed in thicker crust (48 to 66 km) than barren Sarkhar and Bermani (25 to 38 km) granitoids. As the granitoids formed in adjacent areas, these differences in crustal thickness imply either a heterogeneously composed crust, or the iuxaposition of units across a major fault, or hitherto unknown complications with the method. The zircon/rock partition coefficients of REEs, and Y, Nb, Ta, Th, and U contents indicate that the trace element patterns of the studied granitoids are controlled by the liquid composition at the magmatic crystallization. Compared to Sarkhar and Bermani magmatism, Sarnowsar magmatism has a higher temperature (736 to 915 °C), higher zircon/rock partition coefficient of REEs, Y and Th (up to 2770 for Zr), and it was formed in higher oxidant conditions (△FMQ values between -0.06 to 17.01). This indicates that the fertile intrusions are interpreted to indicate extremely high magmatic water content and oxidized like fertile porphyry systems. We suggest that oxidized and I-type magmas are more favorable to porphyry-skarn mineralization under arc tectonic settings. Such a conclusion could be used in regional exploration for porphyry-skarn mineralizations in the Cenozoic arc-related magmatism of Iran and elsewhere.