Geochemical, fluid inclusion and O–H–S isotope constraints on the origin of the Rangraz copper deposit, Central Iran

Abstract

The Rangraz copper deposit (5 Mt @ 0.83% Cu) is located 20 km north of Saveh city, Markazi Province, Iran, in the central Uromieh-Dokhtar volcano-plutonic belt. Host rocks are mainly composed of Eocene volcanic and volcaniclastic rocks that have been intruded by late Eocene quartz monzodiorites and basaltic-andesitic dikes. The dominant copper-related hydrothermal alteration types include silicification, sericitization, and chloritization-kaolinitization. The ore body takes the form of fault-hosted, NW-SE-striking lodes comprising pyrite, chalcopyrite, bornite, specular hematite and magnetite as the principal hypogene ore minerals. Gold (maximum spot assays of up to ca. 0.37 ppm Au) and silver (up to 20.6 ppm Ag) have also been recorded. The ore minerals are mainly confined to veins or veinlets showing crustiform, colloform, open space-filling, dissemination and replacements characteristics in texture. The principal gangue minerals are quartz, barite and calcite. Most of the fluid inclusions identified in quartz-chalcopyrite ± specular hematite veins contain two liquid and vapour phases at room temperature. The primary and pseudo-secondary fluid inclusions have low to intermediate homogenization temperatures between 134 and 338 °C and salinities between 3.55 and 16.89 wt% NaCl equivalents. The fluid inclusion data are consistent with a two-stage evolution of the mineralizing system with initial metal precipitation at crustal depths of <2 km followed by a later pulse of metal deposition at shallower crustal levels of <1 km. The calculated 34SH2S values of sulphide mineral separates from the Rangraz copper deposit range from −9.1‰ to −7.3‰, whereas the calculated δD and δ18O values of the ore fluids range from −3.2 to 0.4‰ and −85 to −75‰, respectively. The δ34S values of the Rangraz samples are consistent with a magmatic sulphur source, implying a possible link between copper mineralization and the Rangraz quartz-monzodiorite. The H, S and O isotope data are consistent with extensive mixing of primary magmatic fluids with cooler, oxidizing meteoric waters occurred in the Rangraz deposit and was responsible for triggering the precipitation of ore minerals. Collectively, the fluid inclusion and isotope data point towards the copper-bearing fluids having been subjected to dilution and cooling, probably as a consequence of their mixing with meteoric waters and boiling. Taken as a whole, the geological setting, ore and gangue mineralogy, vein textures, hydrothermal alteration types, and geochemical, stable isotope, and fluid inclusion evidence may be taken to imply that the Rangraz copper deposit belongs to a class of vein-hosted low sulfidation epithermal mineral systems, several of which have been recorded in the Uromieh-Dokhtar magmatic arc.