Dogan copper deposit (south of Shahroud): copper-molybdenum porphyry mineralization in the Toroud-Chah Shirin magmatic arc

The Dogan copper-molybdenum deposit is located in the northern Central Iranian magmatic arc, south of Shahrood. Mineralization in this area is caused by the injection of a microdioritic subvolcanic intrusion into Eocene volcanic rocks. Mineralization occurs frequently as veins, veinlets, and disseminated ores and is mineralogically composed of primary minerals like pyrite, chalcopyrite, bornite, and molybdenite as well as secondary minerals like chalcocite, iron oxides and hydroxides, and malachite. The alteration zoning in the Dogan deposit is circular and concentric and changes from potassic in the central part to phyllic alteration and then propylitic alteration in the margins. Some parts of argillic alteration are observed in the upper and surface parts of the phyllic zone. The fluids producing potassic alteration were rich in liquid and a small amount of steam (L + V), had a high temperature (398 to 513ºC), and a high salinity (50 wt% NaCl), according to fluid inclusions studies. These fluids were most likely magmatic in origin and were responsible for the formation of the V1 and V2 veins. The activity of meteoric fluids containing liquid vapor phases (V + L) with lower temperature (210 to 360°C) and salinity less than 10 wt % NaCl causes phyllic alteration (V3 veins). In terms of geochemistry, the studied igneous samples are adakititc in nature and are located in the domain of calc-alkaline magmas of the active continental margin. The presence of potassic alteration in surface and deep cores, the high salinity and temperature of hydrothermal fluids, the type of mineralization (disseminated and vein-veinlet), the high potential of copper and molybdenum, and the zonation of existing alterations are all indicative of a porphyry system. The Dogan mining area is similar to copper-molybdenum porphyry deposits in terms of tectonic environment of formation, host rock type, texture and structure, mineralogy, and alteration zonation. Introduction One of the ore-bearing magmatic arcs in the northern structural zone of Central Iran is the Toroud-Chah-Shirin magmatic arc (TCMA) (Fig. 1A). It hosts a significant volume of Eocene volcanic and pyroclastic rocks and equivalent subvolcanic and intrusive bodies. According to the distribution of mineralized systems in the aforementioned magmatic arc, the majority of the ore deposits under investigation are epithermal (see, for instance, Sheibi and Mousivand, 2018; Mehrabi and Ghasemi Siani, 2012; Tale Fazel et al., 2019). This study has shown the geological proof of a typical Cu-Mo porphyry ore deposit at Dogan, which is 130 km southeast of Shahrood (in the province of Semnan) and 18 km north of the village of Toroud. Materials and methods Precise microscopic investigations of mineralogy, texture, and mineralography were made on 10 thin and 27 thin-polished sections. In the Vancouver (ACME) laboratory in Canada and the TU Clausthal laboratory (IELF) in Germany, whole rock geochemistry of microdioritic samples with the least alteration was examined by XRF, ICP-OES and ICP-MS methods. Fluid inclusion thermometry is measured using the Linkham MDSG600 heating/freezing stage at the economic geology laboratory of the Shahrood University of Technology. At the Clausthal Laboratory (IELF) in Germany, the Linkham MDSG600 heating/freezing stage and the XRD methods have been used to identify some very fine fluid inclusion analyses and clay minerals, respectively. Results A subvolcanic intrusion was introduced into Eocene volcanic rocks, resulting in the development of the Dogan Cu-Mo deposit. The microdiorites have a distinct LREE/HREE fractionation and are enriched in large ion lithophile elements (LILE) and depleted in high field-strength elements (HFSE). In addition, the Nb and Ti negative anomalies indicate a magmatic arc signature. However, they differ from typical volcanic arc magmas geochemically due to having SiO2 ≥40 wt.%, Al2O3≥10 wt%, 1 18 ppm, along with the depletion of HREE have adakititc affinities. Based on field and laboratory studies, potassic alteration, propylitic, phyllic, argillic alterations have been detected in the Dogan deposit. Potassic alteration is located in the central part of the system and varies from abundant potassium feldspar veins in the superficial parts to microdiorite containing abundant hydrothermal biotites and potassium feldspar in boreholes. For the samples that have undergone potassic alteration, the fluid inclusion homogenization temperature is greater than 590°C and is similar to values found in other porphyry deposits. This alteration also led to the formation of two known mineralized veins, namely V1: quartz + potassium feldspar + biotite + pyrite + magnetite + chalcopyrite, and V2: potassium feldspar + anhydrite/gypsum + pyrite + molybdenite + chalcopyrite. Sericitic (phyllic) alteration in Dogan is frequently restricted to the fractures where quartz, sericite, and pyrite have been produced as a result of hydrolysis of the potassic- altered rocks. In phyllic altered rocks, the majority of third type veins (V3) containing quartz and trace amounts of pyrite + chalcopyrite ± bornite have been observed. Like many copper and copper-molybdenum porphyry systems (for example: Lepanto Far Southeast deposit in Hedenquist et al., 1998), advanced argillic alteration is observed exactly in the upper part of the Dogan deposit. Significant amounts of Na, Ca, and Mg are removed from the structure of pre-existing minerals during this process due to the low pH (Clark et al., 2003). Propylitic alteration is found at the periphery, from the surface to medium depths, and close to phyllic and argillic alteration in the Dogan deposit. Discussion The microdirotic intrusion has formed at an active continental margin with an adakitic nature. The hypogene sulfide mineralization occurs mainly as disseminated chalcopyrite and pyrite, typically in the matrix or associated stockworks containing potassium feldspar-gypsum/anhydrite, especially in the rocks affected by potassic and phylic processes. The fluids producing potassic alteration are rich in liquid and less vapor (LV); they have high temperatures (398 to 513°C) and high salinity (more than 50 wt% NaCl). These fluids have a magmatic origin and are considered to be the cause of mineralized veins. Phyllic alteration is caused by the activity of fluids containing vapor and liquid phases at lower temperatures (210 to 360°C) and less than 10 wt% NaCl salinity. The low temperature homogenization of the fluid inclusions in phyllic altered rocks (210°C) indicates that the thermal gradients have decreased and the meteoric fluids have flowed. In the next stage with decreasing temperature, the addition of significant amounts of meteoric fluids causes Na-Mg-Ca metasomatism and a mineral assemblage of propylitic alteration, i.e., epidote, chlorite and calcite. The subvolcanic nature of the host rocks (microdiorite) and their formation in the magmatic arc, the presence of potassic alteration evidence in surface and drilled cores, the salinity and high temperature of hydrothermal fluids, the type of mineralization (disseminate and vein-veinlet), the high potential of copper and molybdenum, and zonation of existing alterations all indicate the occurrence of a porphyry system. Geologists should be motivated by the supplied information to look for undiscovered porphyry systems in the Toroud-Chah-Shirin and other Iranian magmatic arcs.

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