Geology, Re-Os age, S and O isotopic composition of the Lar porphyry Cu-Mo deposit, southeast Iran

Abstract

The Lar porphyry Cu-Mo deposit is located 20 km northeast of Zahedan in the Sistan Suture Zone (SSZ), in the southeastern part of Iran. Lar is a sub-economic porphyry Cu-Mo deposit with average grades of 0.16% Cu, 0.01% Mo, and 0.66 ppb Au. The Cu-Mo mineralization developed in post-collisional shoshonitic syenitic to monzonitic intrusions in an Oligocene igneous complex known as the Lar Igneous Complex (LIC). The host and country rocks have undergone variable degrees of supergene and hypogene alteration including silicic, potassic, propylitic, phyllic, and argillic alteration. The Cu-Mo mineralization is mainly controlled by NNW striking faults and fractures, and occurs as quartz veins and veinlets, disseminated sulfides, and stockworks. The mineralization can be divided into two types: hypogene and supergene. Hypogene mineralization is characterized by chalcopyrite, pyrite, bornite, molybdenite and magnetite, whereas supergene mineralization is characterized by enargite, covellite, chalcocite, digenite, malachite, azurite, and iron hydroxides. Sulfur isotope values for hypogene sulfides range from 0.28 to −1.76‰ and suggest an igneous source/or mantle-derived origin for sulfur. The δ18O values for quartz in mineralized quartz veins range from 9.6 to 11.5‰. These values would be in equilibrium with waters having calculated δ18OH2O values of 9.3–11.2‰, suggesting magmatic water. Re-Os dating of two molybdenite samples from two different drill holes yields dates of 29.72 ± 0.11 and 31.95 ± 0.11, suggesting that the system was active for at least 2.0 m.y.