Fracture Analysis of the Changarzeh Pb-Ag Deposit in the Malayer-Isfahan Metallogenic Belt, Northeastern Isfahan

Articles in Press

Document Type : Research Article

Authors

• Seyedeh Zahra Hajiloo ¹
• Reza Alipoor ²
• Ahmad Hosseinkhani ³

¹ M.Sc., Department of Geology, Faculty of Basic Science, Bu-Ali Sina University, Hamedan, Iran

² Assistant Professor, Department of Geology, Faculty of Basic Science, Bu-Ali Sina University, Hamedan, Iran

³ Ph.D., Sormak mining company, Tehran, Iran

10.22067/econg.2025.1126

Abstract

The Changarzeh Pb-Ag deposit is located in the southeastern part of the Malayer-Isfahan metallogenic belt, approximately 75 km northeast of Isfahan city. The host rock of this deposit consists of Middle Triassic dolostones, and mineralization occurs in two forms: hypogene sulfide and supergene non-sulfide. This study aims to investigate the fault system of the Changarzeh deposit using fractal analysis and field surveys. So, surface and subsurface field measurements were conducted along exploration and extraction tunnels. The results indicate that the Changarzeh fault divides the study area into two zones: Abanbar and Takhtchal, and it represents the most significant structure affecting the overall deformation of the region. The northwest-southeast trending faults control the mineralization within the Changarzeh deposit and are generally intersected by northeast-southwest trending faults. Fractal analysis of the faults reveals that an increase in fault density corresponds with a higher Pb ore grade. The left-lateral strike-slip motion of the Changarzeh fault has led to the formation of secondary fractures, including R, X, and T fractures, and also bending along the fault trace has led to the formation of duplex structures.
 
Introduction
The Malayer-Isfahan metallogenic belt is one of the most important Pb-Zn (Ag) mineralization zones in Iran, trending northwest-southeast and located within the Sanandaj-Sirjan structural zone (Karimpour and Sadeghi, 2018; Rajabi et al., 2012). Most of these deposits are formed in siltstones and carbonate rocks from the Lower to Upper Cretaceous, as well as sandstones and shales of Jurassic age. The tectonic setting for the formation of sediment-hosted Pb-Zn deposits in the Malayer-Isfahan metallogenic belt was a back-arc extensional basin between the Central Iran and Sanandaj-Sirjan zones during the Early Cretaceous (Niroomand et al., 2019). The oblique convergence between the Arabian and Eurasian plates, which led to the closure of the Neo-Tethys Ocean, resulted in significant shortening, ductile and brittle deformation, accompanied by the formation of deep basement faults within an extensional setting (Mohajjel and Fergusson, 2014). These faults were later reactivated during subsequent compressional phases, forming imbricate thrust systems and duplex structures, leading to further deformation (Agard et al., 2005). The Changarzeh deposit is structurally located in the central part of the Zagros orogen and within the Sanandaj-Sirjan zone. The Triassic rock units are the most prevalent in the Changarzeh deposit area, primarily consisting of the yellow and brown dolomites of the Shotori Formation and the calcareous shale-siltstone of the Nayband Formation. Olive and gray shales of the Early Jurassic Shemshak Formation overlie the Triassic sequences. Geographically, the Changarzeh Pb-Ag deposit is located in the southeastern part of the Malayer-Isfahan metallogenic belt, approximately 75 kilometers northeast of Isfahan city and about 26 kilometers southeast of Natanz city. Therefore, in this study, the fault and fracture system of the Changarzeh deposit has been investigated through fractal analysis and field surveys.
 
Materials and methods
In this study, to accurately investigate the fractures of the Changarzeh Pb-Ag deposit, fault maps of the mining area were initially prepared using existing geological maps and Google Earth satellite images. Subsequently, fractal analysis was conducted using the box-counting method (Log-Log), where the study area was divided into smaller boxes, and the fractal dimension was calculated for each box. The results of the fractal dimension calculations for the faults and their density were then compared with Pb grade data from exploratory trenches. Additionally, field surveys were carried out in multiple stages to examine the geometry and kinematics of the faults. In this context, the mine tunnels and the trenches significantly contributed to identifying faults geometry.
 
Result
Fractal analysis results indicate that the highest frequency of fault trends in the study area is observed for the NW-SE (N30W to N35W) and NE-SW (N25 to N30E and N70E to N75E) trends. The plotted diagrams for the fractal dimensions of the faults in six boxes of the study area indicate that F and E boxes have the highest fractal dimensions. Comparing the fractal dimensions and the distribution of Pb content across different sections of the study area reveals that zones F, E, and D, which have the highest fractal dimensions for the faults, also exhibit high Pb percent grades. In fact, the southern half of the study area shows the highest Pb percent, while the northern half, which has the lowest fractal dimensions for the faults, displays lower Pb percent grades. The most important fault structure influencing the Changarzeh deposit is the Changarzeh fault with NE-SW trend. This fault divides the study area into the Abanbar and Takhtchal zones. The attitude of the Changarzeh fault is N30E/75SE and a 30° rake angle, exhibiting a left-lateral strike-slip movement.
 
Discussion
The most important mineralized faults in the Abanbar zone have a trend ranging from N10W to N65W and exhibit a steep dip of 70 to 80 degrees toward the NE. In this zone, some faults also have the same trend but with a gentler dip of approximately 40 to 65 degrees toward the SW and do not exhibit mineralization. In the Takhtchal zone, the faults with a NW-SE trend are the primary structures controlling mineralization. These faults generally have a strike between N70W and N80W and, similar to the Abanbar zone, exhibit a dip toward the NE. It seems that in both zones, the NW-SE trending faults are the primary structures controlling mineralization, and these have been displaced by NE-SW trending faults. Additionally, the NW-SE trending joints have been displaced by joints with a NE-SW trend. The fault system in the study area is generally related to secondary or lateral fractures of the main strike-slip Changarzeh fault, which exhibits a left-lateral shear mechanism. In the study area, most of the NW-SE trending faults are R-type fractures related to the Changarzeh fault zone, which likely formed during the final phase of the deformation of this fault. X-type fractures related to the Changarzeh fault zone have also formed, leading to main structural complexities. Additionally, T-type fractures have developed in some parts of the study area. Also, along the Changarzeh main fault, duplex structures have formed in the T2 dolostone unit. These compressional structures developed at the bend of the main resulted from the left-lateral shear movement.
 
Acknowledgements
The authors gratefully acknowledge the support from Sormak Mining Company for their assistance with field studies. They also extend their sincere thanks to the editor and reviewers of the journal Economic Geology for their valuable contributions.
 

Keywords

• Fault
• Fractal
• Left-Lateral Strike-Slip
• Mineralization
• Isfahan
• Sanandaj-Sirjan

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