Geology and genesis of the North Narbaghi Cu-Ag deposit in the Urumieh-Dokhtar magmatic arc, Iran: Fluid inclusion and stable isotope constraints

Abstract

The North Narbaghi Cu-Ag deposit in the central part of the Urumieh-Dokhtar magmatic arc (UDMA) of Iran is hosted by Eocene volcanic sequences and Oligo-Miocene intrusions of arc affiliation. The mineralization can be divided into five stages that are represented by barren hydrothermal breccias, Cu-Fe sulfides cementing in the breccias, sulfosalt and sulfide hydrothermal breccias, barren post-ore quartz veinlets, and barren post-ore calcite veinlets, respectively. The metallic minerals consist of pyrite, chalcopyrite, bornite, sphalerite, and tennantite-tetrahedrite, which are variably associated with gangue minerals including quartz, calcite, and sericite. The upper parts of the ores have been oxidized to form supergene minerals including malachite, azurite, chrysocolla, chalcocite, covellite, hematite, and goethite. The ore-related hydrothermal alteration includes silicification, pyritization, carbonatization, intermediate argillitization, and propylitic alteration. Primary fluid inclusion assemblages in quartz associated with main-stage sulfides and sulfosalts have homogenization temperatures ranging from 184° to 385 °C and calculated salinities between 12.0 and 29.7 wt% NaCl equiv. (avg. 21.4 wt% NaCl equiv.). The coexistence of fluid inclusion assemblages with highly variable volume percentages of vapor phase indicates that boiling commonly occurred in the hydrothermal system, contributing to Cu saturation. Sulfide minerals range in δ34S from −1.5 to 3.7 ‰, indicating a probable magmatic sulfur source. This view is further supported by the carbon and oxygen isotopes of hydrothermal carbonates, with δ13CPDB of −2.2 to −1.5 ‰ and δ18OSMOW between 5.30 and 11.75 ‰. Stable isotope and fluid inclusion data illustrate that boiling of the ore fluid and its mixing with meteoric water facilitated mineralization at the North Narbaghi. We suggest that the North Narbaghi Cu-Ag deposit can be best interpreted as an intermediate-sulfidation epithermal deposit formed due to post-Eocene magmatic-hydrothermal activity. The intersections of post-Eocene subvolcanic rocks with regional faults were the most favorable sites for focusing fluids responsible for epithermal mineralization along the UDMA.