The purpose of mineral prospectivity mapping is detecting new areas that have high potential for the occurrence of mineral deposits. There are two approaches to detect such areas: knowledge- and data-driven methods. While the knowledge-driven methods apply the knowledge and experience of geoscientists, the data-driven methods use the relationship between geospatial data and known occurrences in the study area.
In order to solve an MPM problem, it is necessary to collect, analyze, and integrate certain geospatial data. These data include geological, geophysical, geochemical, and remote sensing layers. In the final step, the association between known mineralization points and target areas should be surveyed and evaluated (Ghezelbash et al. 2023). Knowledge-driven methods are appropriate for poorly explored areas, whereas data-driven methods are appropriate for properly explored areas. Nowadays, Artificial Intelligence (AI) -based approaches, such as machine learning and deep learning methods, have proved to be highly effective in geoscience studies. For instance, complicated relationships between geospatial data and known mineralized points can be extracted using AI (Daviran et al. 2022).The application of machine learning methods saves time and budget in mineral exploration projects. Also, the models produced using AI-based methods are highly accurate and reliable.
An MPM method can be considered as a classification problem, for which the deposit points in the study area are considered as class one and non-deposit points as class zero (Daviran et al. 2021). The classification method in an MPM problem is carried out using training data, which are the deposit and non-deposit points. After finding the relationship between geodatabase and training points (deposit and non-deposit points), the relationship is generalized to the study area.
In recent years, different data-driven methods have been used for mineral prospectivity modelling. Random forest, support vector machine, artificial neural network, and adaptive neuro-fuzzy are some of the commonly used algorithms for this purpose. Although these algorithms were first developed by computer scientists, they have been widely used in other scientific studies, especially in geosciences.
In this paper, four machine learning methods, including artificial neural network, adaptive neuro fuzzy, generalized neural network, and random forest have been applied to the Share-Babak study area which is located on the Urumia-Dokhtar Magmatic zone in Iran. Afterward, these algorithms were compared and combined, based on which the final MPM models of the Shahr-e-Babak study area was constructed.
Study area and data used
Geology
This paper studies a part of the Urumieh–Dokhtar Magmatic Arc (UDMA) (Fig. 1), which is located on the Alpine–Himalayan orogenic belt. This belt resulted from the closure of the Neotethyan Ocean between Arabia and Eurasia. The UDMA, which is considered to be one of the most important Cu-bearing regions of the world, contains three giant and ten intermediate porphyry Cu ± Mo ± Au deposits with a total reserve of > 40 Mt Cu (Raesi et al. 2021).
The study area is the Shahr-e-Babak 1:100K geologic map (640 km2), which is located in South Eastern Iran (Fig. 1). The western parts of the area are covered with small outcrops of colored mélange units. A 150 m thick Eocene conglomerate unit along with a well-bedded and fine-grained sandstone unit and massive limestone units cover the eastern parts of the area. The Eocene flysch and volcanic units cover the study area in the north. The volcanic complex consists of andesitic basalt with high pyroxene and low olivine contents, as well as a 60 m thick red tuff, and trachy-andesite and trachy-basalt in lower parts.
The Eocene quartz monzonite and granodiorite rocks are outcropped in the South eastern. These rocks are similar in composition but quartz monzonite has more k-feldspar than plagioclase. The younger andesitic agglomerates cover volcanic rocks. The Neogene volcanic units outcrop around the Kuh-e-Masahim volcano in the north. This volcano forms the highest altitudes of the study area, with its peak having an elevation of 3500 m. An altered micro dioritic unit, which is the youngest subvolcanic unit in the area, forms the northern part of caldera of this volcano. The mafic units of the complex are also altered and contain sulfide mineralization. Figure 2 shows geology map of Shahr-e-Babak study area.
Dataset
In this study, we have used nine datasets to build the evidential maps: Geology, Geophysics, Geochemistry (Cu and PC 3), phyllic alteration, Argillic alteration, iron oxide alteration, lineaments density, and elevation data.
Geology
The 1:250K Shar-e-Babak geologic map was used as the basis for the lithological layer. This geologic map was digitized using the GIS software, and its Eocene and Neogene subvolcanic stocks, including diorite and granodiorite, were extracted as the heat sources (Fig. 3a).
Lineaments were extracted from the Geologic map and the ASTER dataset. Lineament density map was prepared from lineament structures (Fig. 3b). The density of lineaments in the north is more than other parts of the study area. The density map of these structures was prepared.
Geophysics
The airborne magnetic data acquired by the Atomic Energy Organization of Iran (AEOI) during 1977 and 1978 was used to prepare the total magnetic intensity map of the study area. These data were obtained at a flight spacing of about 500 m and an altitude of about 120 m. The RTP filter was performed on the total magnetic intensity map, and the anomalies were extracted from the resulted map (Fig. 3c).
Geochemistry
The data of stream sediment geochemical explorations can be used in Cu porphyry mineral potential modelling. A dataset of 604 samples from the geochemical surveys conducted by the GSI was used for geochemical processing. These samples were analyzed for Cu, Pb, Zn, Sb. Ni, Co, Cr, B and B. The boxplot method was applied to replace outlier data and the logarithmic method to normalize the remaining dataset. The histogram and QQ diagrams were plotted for the normalized data. The Cu anomaly map was created as an evidential layer for porphyry copper deposit prospectivity (Fig. 3d). The high Cu anomaly signatures are located in the north western and eastern parts of the study area. Also, a multivariate geochemical analysis (Principal Component analysis – PCA) was carried out on geochemical data. After preprocessing the geochemical data, PCA was applied on normalized data. PC3 that shows higher values of Cu, Pb, and Zn was determined to be associated with porphyry mineralization (Table 1). The map for this factor was also created as another evidential layer. Multivariate geochemical map is shown in Fig. 3e.
Table 1
Principal component analysis for multi-element geochemical data, where component 3 corresponds to Cu mineralization.
| Component |
---|
1 | 2 | 3 |
---|
Ni | .883 | − .047 | .136 |
Ba | − .016 | .818 | − .056 |
Pb | .153 | .468 | .695 |
B | .714 | − .080 | .258 |
Co | .842 | − .065 | − .080 |
Sb | − .110 | .685 | .098 |
Cr | .615 | .557 | .157 |
Cu | .521 | .295 | .515 |
Zn | .059 | − .150 | .808 |
ASTER data
The most significant advantages of using remote sensing in porphyry Cu mineralization exploration studies is the close relationship between alteration and mineralization areas (Riahi et al. 2022). Phyllic, argillic, and iron oxide alterations are common zones in cu porphyry systems. In this study, the mentioned alterations were obtained using the band ratio method from ASTER data. Band ratio is a widely used approach in improving the spectral characteristics of the alteration zones depending on the absorption bands of their altered minerals (Elhusseiny 2023).
The ASTER dataset covers a wide spectral range of 14 spectral bands, measuring reflected radiation in three bands between 0.52 and 0.86 µm (visible-near infrared, VNIR) with 15-m resolution, and six bands from 1.6 to 2.43 µm (shortwave Infrared, SWIR) with 30-m resolution. The emitted radiation is measured at 90-m resolution in five bands through the 8.125–11.65-µm wavelength region (thermal infrared; TIR) (Zoheir et al. 2019).
Band ratio 3/1 was applied for detecting the iron oxide alteration zone in the study area. Band ratios 7/6 and (5 + 7)/6 were applied for extracting argillic and phyllic alteration zones, respectively (Fig. 3f, 3g, 3h).
Elevation data
Volcanic rocks host the porphyry mineralization in the Shahr-e-Babak study area. They are used as dimension stones and have a rough topography. Therefore, elevation data were used as an effective evidential layer in modelling the host rocks of the porphyry mineralization in this area. The elevation data were extracted from ASTER images to prepare the Digital Elevation Model (DEM) (Fig. 3i).
A raster file with a cell size of 100×100 was generated for each evidential layer. Every evidential layer was then transferred into a [0, 1] interval using a linear fuzzy function. All of these raster files were transformed into an ASCII format to be used in MATLAB.
Target variable
In order to create target variables in the study area, we used 37 known mineralized points. Copper mines and indications were simply considered as known mineralized points, and a value of one was attributed to these points. On the other hand, non- deposit points refer to the points where there is no mineralization, and it is very complicated to specify whether a point is non-depositor not. Three conditions should be applied to create non-deposit points (Ghezelbash et al. 2021).
(1) The number of non-deposit and deposit points should be equal, which is 37 in this study.
(2) Non-deposit and deposit points should have an appropriate distance from each other.
(3) The deposit points usually occur in clusters or follow a specific form. Non-deposit points should be far from these clusters and should be chosen randomly.
The point pattern analysis was applied to delineate adequate distances between deposit and non-deposit points. Based on the point pattern analysis (Fig. 3j), all of the known mineral occurrences are located in a radius of 5247 m. As a result, with 100% probability, another occurrence can situate in this distance. Therefore, this distance was applied to known mineralized points to create the buffer distance. Also, in order to add accuracy to non-deposit points, a buffer of 500 m was applied to subvolcanic units as the heat sources and volcanic units as the host rocks of porphyry mineralization in the study area.