Metallogeny of Manto-copper deposits, special view in Nasim copper deposit, northwest of Bardaskan, Khorasan Razavi

The most important Manto-type deposits are located in the Coastal and Central Cordillera, northern Chile. Manto-type copper deposits have been reported in Iran in Saveh-Jiroft zone, the volcanoes of eastern Iran, the volcanic-intrusive complex of Alborz-Azerbaijan, Sabzevar zone, and Sanandaj-Sirjan zone. Nasim copper deposit is located in the northwest of Bardaskan, northeastern Iran. The deposit is part of the Iranian Plateau, Sabzevar Subzone, and Oryan region, located at the end part of the Khaf-Kashmar-Bardaskan magmatic belt. The geological units in the area include the Late Tertiary (Paleocene-Eocene), volcanic rocks, and sedimentary rocks including conglomerate and limestone. In Nasim deposit, mineralization has been done in conglomerate unit as a particular horizon. This unit is composed of volcanic fragments with carbonate and volcanic cements. Chalcocite is the most important and main sulphide mineral in the study area. Alteration can be divided into pre-mineralization and syn-mineralization stages. Pre-mineralization includes Celadonite, Carbonate, Silicified, and Propylitic alteration. Syn-mineralization consists of small amounts of Chlorite, Zeolite, and Calcite. In this system, the chemistry of solution is different from those of other systems such as IOCG, massive sulfide, porphyry etc., due to completely reduced and iron and silica-deficient solution.

Nasim deposit is located in northeastern Iran, 50 kilometers northwest of Bardaskan. The study area is part of the Iranian Plateau, Sabzevar subzone, and Oryan region, which is located in the end part of the Khaf-Kashmar-Bardaskan magmatic belt. The most important Manto-type deposits of Iran are located in Bardaskan region. The study area includes Paleocene-Eocene volcanic rocks, major volcanic and minor intrusive fragments. In this study, the chemistry of solution was investigated based on alteration and paragenesis in Bardaskan region.

This study was done in two parts: field studies and laboratory works. Sampling and structural studies were done while doing field studies. Logging drill cores was done for about 1000 meters in 20 boreholes. After field work, a total of 150 thin sections and 50 polished sections were prepared and studied to investigate petrography and mineralogy and to prepare geological map.
 

Geology of the area includes sequence of volcanic rocks of basalt, basaltic-andesite and andesite, respectively, formed in a non-marine environment. Conglomerate and limestone units formed after volcanic activity. Mineralization occurred only within the conglomerate unit due to useful porosity. Mineralization formed clearly post-dates conglomerate and limestone in the region. Chalcocite is the most important primary copper mineral in Manto Chalcocite systems, which has a high amount of copper and lacks iron. Mainly in the conglomerate unit, due to the good porosity of the conglomerate, the solution has risen up through the faults and penetrated into the conglomerate. Limited mineralization is observed in the carbonate rock. The mineralizations have no time and origin relationship with the volcanic cycle and the time of conglomerate formation. The porosity in the conglomerate and the fault structure in the region have played an important role in the mineralization.
Manto copper solution is a solution poor in iron and poor in silica, and it is completely reducing. This solution cannot react with lime under any conditions; so the mineralization rate is very low in lime, and this shows that the solution has a special chemistry.
Some exploration consultants have used the word agglomerate instead of conglomerate in Bardaskan. This is not acceptable because agglomerate is created during volcanic activity and its fragments consist of only one type of composition and have a rounded state (volcanic bomb). But conglomerate is formed during the erosion cycle and all the pieces are rounded when transported by river water or on the seashore. The use of the term agglomerate becomes a fundamental problem regarding the time and manner of formation of copper mineralization. Not knowing the exact time of Qata mineralization challenges the exploration.
 Alteration assemblage do not consist of epidote and quartz due to the lack of iron and silica in the solution. Moreover, the reducing solution is due to the presence of organic substances. Some researchers (e.g., Wilson and Zentilli, 2006; Tosdal and Munizaga, 2003) suggest a volcanic origin for Manto-type deposits. The volcanic rocks contain at least 5% primary magnetite. If the rocks are the origin, the solution will be rich in iron, but there is no evidence of iron-bearing minerals such as chalcopyrite and pyrite in the area.
Other researchers (e.g., Palacios, 1986; Oliveros et al., 2008) see intrusive rocks as the origin of these deposits. If the rocks are the origin, solution will be definitely rich in silica, iron, and aluminum, but there is no evidence of quartz-chalcocite mineral assemblage. Therefore, the determination of the origin of the deposits requires further studies and consideration of other factors.

We would like to thank Ferdowsi University of Mashhad and Kome Madan Pars Company for cooperating and supporting this research. We thanks the reviewers and editor(s) for their thoughtful contributions.