Application of AHP-TOPSIS method to model copper mineral potencial in the Abhar 1:100000 geological map, NW Iran

Fulfillment of exploration activities is costly and time-consuming, which reveals the necessity of mineral prospectivity mapping (MPM) for identifying highly mineral potential zones. MPM as a key procedure, integrates multiple geoscience datasets performed across different scales. In this regard, Various methods have been developed over the past three decades that are basically categorized into either data-driven or knowledge-driven methods. This study aims to represent a new combinatorial MCDM method called AHP-TOPSIS to outline highly potential Cu zones in the Abhar area.

The Abhar area is located southeast of Zanjan province, NW Iran. From the structural and metallogenic point of view, the study area has been composed of two segregated sub-zones: Tarom sub-zone and Soltanieh sub-zone. Most of the known Cu occurrences are of vein type, the overwhelming of which have outspread in the northern part of the study area (Tarom sub-zone). A slight body of profoundly eroded leucogranite is partially manifested in the low topography circa Doran village, named Doran granite which affiliates to late Precambrian. The granite and some small sporadic Eocene granites have intruded into Kahar and Soltanieh dolomites, respectively. This could promise the skarn mineralization of iron and copper in this area.The research presented here endeavors to outline Cu potential zones based on subsequent investigations appertaining to the Abhar area using AHP-TOPSIS. This method is a hybrid method, in which the task of AHP is to determine the weights of criteria and sub-criteria, and thereinafter the TOPSIS method is concerned with order preference of decision alternatives. Despite the ability and competence of each of the foregoing methods, experience has affirmed that the combination of two MCDM methods and the concurrent use of their potentials gives better results and leads to eminent performance for the MPM. The first paramount step in mineral prospectivity mapping is to identify and collect geospatial data pertinent to the deposit type-sought. In the present study, aforesaid data is taken into consideration based on the data availability alongside theoretical and empirical analysis of the spatial association between Cu mineral deposit and various data. Hereupon, seven evidential layers, as predictor maps, were propounded to outline Cu potential zones. They consisted of multi-element geochemical signatures pertinent to Cu mineralization, closeness to late Eocene intrusive contacts, closeness to late Eocene volcanic rocks, proximity to the faults, and vicinity to argillic, phyllic and propylitic alterations.

The spatial evidence values in the obtained maps do not involve identical maximum and minimum values. Inevitably, the evidential values of mentioned maps were transformed into a new space. Then concentration-area (C-A) fractal model, was applied to classify spatial values of evidential maps. Next, AHP-TOPSIS MCDM method was applied to generate Cu prospectivity map. In this regard, AHP method was used to determine the weights of criteria and sub-criteria and TOPSIS method was used to rank the alternatives. For this, a decision matrix of 361638×7 was created that comprises 7 criteria (evidential maps) and 361,638 alternatives (each of which corresponds to a particular cell with a certain coordinate in the evidence layers). At the end, the Cu prospectivity map was generated. To evaluate the efficiency of prospectivity map obtained by AHP-TOPSIS method, the Predication-Area (P–A) plot was applied. According to the result of this P-A plot, the intersection point for this prospectivity model has been appeared in a high place, at about 78%. Thus, this map can be used as target map for subsequent detailed explorations.

The present study addresses the successful application of a new hybrid MCDM method called AHP-TOPSIS to generate a predictive model for Cu mineralization in the Abhar area. Despite the ability of two methods alone, experience has demonstrated that the combination of two or more MCDM methods eventuate to better results. Since the simultaneous use of these two methods will ultimately eventuate to the reduction of the target areas, this could indicate the high precision of the hybrid method in predicting target areas. Consequently, it was decided to use a combination of two methods, which are already mentioned. After generating the overlay prospectivity map, P–A plot was used to evaluate its efficiency in predicting favorable areas. Based on the results, the AHP-TOPSIS method can provide a potent tool for quantifying the specifications of geo-anomalies and delimiting target areas in mineral exploration programs