Genesis of the Mamuniyeh copper deposit in the central Urumieh-Dokhtar Magmatic Arc, Iran: Constraints from geology, geochemistry, fluid inclusions, and H–O–S isotopes

Abstract

The Mamuniyeh Cu deposit is located in the central part of the Urumieh-Dokhtar Magmatic Arc (UDMA), 10 km south of the city of Mamuniyeh, Iran. Mineralization is controlled by faults with a NW-SE trend and hosted within an Eocene volcanic sequence and Oligo-Miocene hypabyssal calc-alkaline monzonitic and gabbroic bodies. Quartz + chalcopyrite veins are most abundant and high-grade ore containing up to 5 wt% Cu, although quartz + pyrite veins have the most abundant sulphide content. In addition, quartz + chalcopyrite + specular hematite ± pyrite veins/veinlets are another common mineralized assemblage in the Mamuniyeh copper deposit, with pyrite, chalcopyrite, bornite, and oxide minerals (specular hematite, titanomagnetite, and magnetite) typical of the hypogene stage. Chalcocite, covellite, and dignite also formed at the margins of primary sulphides in the supergene (paleoweathering) stage. The mineralized veins exhibit colloform, crustiform, open space-fillings, replacements, and dissemination textural characteristics associated with mineralizing assemblages with silicification, argillization, chloritization, and sericitization assemblages. The salinity for L > V fluid inclusions is between 1.74 to 11.7 wt% NaCl and for (V > L) inclusions between 1.7 to 11.4 wt% NaCl. The average homogenization temperature and salinity for quartz + chalcopyrite + pyrite veins is 186 °C and 4.9 wt% NaCl. In the quartz + chalcopyrite assemblage an average of 185 °C and 4.5 wt% NaCl and for quartz + chalcopyrite + specularite ± pyrite (QCSP) an average of 195 °C and 5.59 wt% NaCl was determined. In these three vein types, the fluid density has almost identical values ranging from 0.8 to 1.0 g/cm³. The mineralizing system evolved in two-stages; the first metal precipitation occurred at less than 1 km of crustal depths and second metal deposition stage at shallower crustal levels (less than 500 m). Although it appears that the boiling process occurred within the fluids of the area, the primary factor contributing to Cu mineralization was influenced by fluidmixing processes. The δ¹⁸O and δD values of ore fluids computed vary from + 6.08 to −0.50 ‰ and −92 to −71 ‰, respectively, indicative of the blending of oxidizing and cooler meteoric waters with primary magmatic fluids. Calculated values of δ³⁴S of H₂S in equilibrium with chalcopyrite ranges from −7.6 to −1.9 ‰ and H₂S in equilibrium with pyrite ranges from −7.1 to −3.8 ‰, respectively; this is consistent with monzodiorite to gabbro as the magmatic sulphur source for copper mineralizing fluids. Furthermore, the QCSP vein data align more closely with primary magmatic water compared to other veins, suggesting that precipitation occurred mainly from magmatic fluids, which experienced depletion in δ¹⁸O due to mixing with meteoric waters (shallow oxygenated ground waters), which caused sulphide deposition. The geochemical features for these magmas show that contamination with crustal materials occurred during the ascent of the parent magma, as well as the role of suprasubduction fluids released from the subducting plate in mantle metasomatism. Based on all evidence, Cu mineralization in the Mamuniyeh deposit has been categorized as a low-sulphidation epithermal-type system, which formed during active magmatism in the central part of UDMA.