Geochronology and geochemistry of zircons from fertile and barren intrusions in the Sangan mining area (NE Iran): Implications for tectonic setting and mineral exploration

Abstract

The Sangan mining area is the largest Cenozoic skarn iron ore district in the far eastern part of the Alborz Magmatic Arc in Iran. In this study, we evaluate the zircon composition as an important exploration tool and pathfinder for skarn mineralization to elucidate distinctive zircon signatures diagnostic of metallogenic fertility of the parent magma. Granitoid intrusions in the Sangan mining area are subdivided into the fertile Sarnowsar intrusion composed of syenite, syenogranite, and granite and the barren Sarkhar and Bermani intrusions composited of monzogranite and syenogranite. The Eocene Sarnowsar intrusion was produced by the flare-up magmatism and is an oxidized I-type granitoid suite. Fourteen iron ore bodies occur along the contact of this intrusion with the carbonate country rocks. In the barren Sarkhar and Bermani intrusions, monzogranites have zircon U-Pb ages of 41.7 ± 0.6 Ma and 41.9 ± 0.3 Ma, whereas syenogranites have ages of 37.8 ± 0.3 Ma, 37.9 ± 1.7 Ma, and 37.4 ± 1.8 Ma. These ages span the same time interval as the Eocene flare-up occurrences throughout Iran. Zircons of fertile intrusions have collectively higher Eu (>0.3 ppm), Ce (>100 ppm), 10,000*(Eu/Eu*)/Y (>1) and (Ce/Nd)/Y (>0.001) anomalies, slightly lower Dy/Yb ratios (0.21 to 0.38) compared to the barren intrusions with Dy/Yb ratios of 0.2 to 0.51 and lower Eu and Ce anomalies. In fertile intrusions, zircon 10,000*(Eu/Eu*)/Y ratios are positively correlated with (Ce/Nd)/Y ratios, but this correlation is absent in barren intrusions. The fertile intrusions are characterized by high zircon Ce4+/Ce3+ ratios (>150), higher crystallization temperatures (average 817 °C), and high fO2 value (average △FMQ > 7), whereas the barren intrusions are characterized by much lower zircon Ce4+/Ce3+ ratios (average below 50), lower crystallization temperatures (average 720 °C), and low fO2 values (average △FMQ = -2 to −5). Zircons of the rhyolite are similar to these fertile intrusions. We suggest that oxidized and I-type magmas are more favorable for porphyry-skarn mineralization within arc tectonic settings. The chemical characteristics of zircons potentially can be used in regional exploration for porphyry-skarn mineralization in arc-related magmatic suites in Iran and elsewhere.