Integration and analysis of airborne geophysics and remote sensing data for exploration of porphyry copper deposits in the Central Iranian Volcanic Belt

Integration and analysis of airborne geophysics and remote sensing data for exploration of porphyry copper deposits in the Central Iranian Volcanic Belt

Although Crosta method is very useful for hydroxyl mapping; nevertheless, there are some areas which are altered but are not enhanced by this technique. Such area is located in the northern part of the Sar Cheshmeh mine. At the same time there are altered areas which have a good signature in satellite data but they do not have a good signature in the geophysical data(e. g. Hoseynabad deposit in Figure 3). For mapping such areas integration of satellite and geophysical data can be helpful.

Figure 4: The flow chart that shows the steps for preparation of alteration image.
Data integration and analysis
As geophysical data have more ground penetration than the satellite images, both satellite and geophysical data were integrated, in order to map the altered areas. To do this a control area is chosen first. The data is subdivided into target and explanatory variables(Pan, 1993). The geophysical, geochemical and remote sensing data are georeferenced and given a common UTM coordinate system. The explanatory data includes airborne geophysical data( K, Th, U counts and total magnetic intensity) and remote sensing data(hydroxyl and iron oxide images which are generated by Crosta method). The explanatory data covers 1: 50000 scale topographic sheet of Sar Cheshmeh with an area of 640 km2 . The geophysical data for the target area is k, Th, U counts and total magnetic intensity(Helicopter born). Other data for the target area include geochemical(Cu and Mo), ground geophysics(IP and resistivity) and alteration data(phyllic and propylitic).

The steps taken for the integration and analysis of the data are shown in figure 4. The geophysical data for the target area was imported directly from the source file. The geochemical and alteration data were digitized and then made into raster format. In order to reduce scale difference, all variables were converted into 8-bit integer format. The variables were subjected to statistical analysis (Table 1). Data analysis is performed in two steps. (1) Integration and analysis of the target and explanatory variables in the control area, (2) transformation of explanatory variables by using the eigenvector loadings of the first step.

Table 1: Principal component analysis for the target and explanatory variables over Darrehzar porphyry copper deposit. Eigenvector loadings less than 0.2 are excluded.

The first PC highlights the areas with phyllic and argillic alteration. Higher negative loadings of resistivity, magnetic intensity and propylitic zone indicate that the propylitic zone is more resistive and have more magnetite content. Higher loadings of K counts , Th counts , Cu and Mo and hydroxyl, iron oxide are associated with the phyllic alteration zone. The explanatory variables(hydroxyl and iron oxide images, airborne geophysics) were integrated and transformed by using the eigenvector loadings of the first PC using following equation(Figure 5).

PC1=0.35(K_counts)+0.37(Th_counts)+0.31(U_counts)-0.46(Magneticintensity)+0.87(iron oxide)+ 0.85(hydroxyl)

The image shown in figure 5 is highlighting the areas with intense alteration. There are altered areas which are not enhanced with Crosta method(e.g. north of Sar Cheshmeh mine). The reason may be a thick soil and vegetation cover over this area. At the same time the boundaries of the altered areas are not clear in the airborne geophysical data (e.g. Darrehzar area). After applying the above method this area is also shown as an altered one.

Figure 5: Combination of remote sensing and geophysical data. The altered areas are shown in bright pixels.
The higher K and Th counts over the altered areas are due to the presence of abundant sericite, clay and K-feldspar minerals. The thin section study from the propylitic zone revealed that the observed higher total magnetic intensity over this zone is mainly due to the presence of abundant magnetite. Similar result has been obtained over the Sar Cheshmeh area, located just north of the Darrehzar area (Ranjbar et al., 2002).

4. Summary and conclusions
Airborne magnetic/radiometric data sets have been integrated with ETM+ data and analyzed using directed principal component analysis (Figure 5). Darrehzar porphyry copper deposit is chosen as a control area for determining the correlation between airborne geophysical data and satellite images, and geochemical /alteration data. The data analysis is performed in two steps. (1) integration and analysis of the target and explanatory variables in the control area, (2) transformation of explanatory variables by using the eigenvector loadings of explanatory variables from the first step.

This technique is found useful for delineation of hydrothermally altered zones which in turn provides some information about the mineralization type. The combination of satellite images with geophysical data can enhance the alteration zones with more confidence. This technique can reduce the enhancement of the false anomalies (e.g. salt encrustation). Unfortunately the authors do not have permission to publish the results of Helicopter borne geophysical data. As these data have better spatial resolution and collected at lower height, integration of remote sensing and Helicopter born geophysical data results in better alteration mapping.

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