Remote sensing and Aeromagnetic investigations in porphyry copper deposits for identification of areas with high concentration of gold: a case study from the central part of Dehaj-Sarduiyeh belt, Kerman, Iran

Remote sensing has shown tremendous potential in the identification of alteration zones. The importance of this science for mineral exploration and recognition of alteration zones with lower cost, time, and manpower is confirmed in many studies (Amer et al., 2012; Hosseinjani Zadeh et al., 2014; Tayebi and Tangestani, 2015; Shahriari et al., 2015). Gold is one of the byproducts in most of the porphyry copper deposits (PCDs). Although the gold assay is partly low and reaches between 0.012- 0.38 g/t in these deposits, the high tonnage of copper deposits provides a considerable source of gold which has an important economic value (Kerrich et al., 2000). Extension, intensity of alteration, assays and the type of mineralization vary in different deposits. For instance, many Au-poor porphyry copper deposits in southwest USA, Central Asia, and west of South America are associated with widespread phyllic alteration (Kesler et al., 2002). In addition, there is a positive correlation between gold and magnetite in PCDs (Kesler et al., 2002; Shafiei and Shahabpour, 2008; Sillitoe, 1979). Therefore, aeromagnetic investigation could be useful in identification of these deposits. The aim of this research is discrimination of alteration zones and investigation areas with high concentration of gold through processing of remote sensing and aeromagnetic data.
A number of prone areas with different concentrations of gold in Dehaj-Sarduiyeh copper belt including Sar Kuh, Abdar, Meiduk, Sarcheshmeh, Darrehzar, Sara, Iju and Seridune were investigated using the processing of Advanced space borne thermal emission and reflection radiometer (ASTER), and aeromagnetic data. Pre-processing acts such as crosstalk correction and Internal Average Relative Reflection (IARR) calibration were implemented on the ASTER data in order to remove noise and acquire surface reflectance. The alteration minerals were discriminated by implementation of appropriate algorithms such as color composite and partial sub-pixel method, and Mixture tuned matched filtering (MTMF) on a pre-processed and calibrated ASTER data. The results were verified by field surveys and laboratory analyses such as spectroscopic studies, optical microscopy, and XRD. The boundary of each deposit was determined by the results obtained from ASTER data. The aeromagnetic data were also processed using different filters like the reduced to the pole first and second vertical derivatives. Then the aeromagnetic data were clipped according to the boundaries determined with ASTER data and were exported into the GIS environment along with the determined abundances of altered minerals for the investigation of the characteristics of areas with a high concentration of gold.
The results of ASTER image processing revealed that the distribution of phyllic alteration is high in the area. It is shown that most of the poor-Gold porphyry copper deposits are associated with widespread phyllic (Kesler et al., 2002). Therefore, the high exposure of phyllic confirms the low amount of gold in the study area. According to aeromagnetic results, the maximum and minimum differences in magnetic intensity were observed at Abdar- Sarkuh and Iju- Seridun, respectively which have high and low concentrations of gold in these deposits. In addition, the results which were obtained from reduced to pole transform revealed most correspondence with gold differences in the deposits.
ASTER datasets were conducted on the eight porphyry copper deposits (PCDs) of Urumieh–Dokhtar magmatic belt including Sar Kuh, Abdar, Meiduk, Sarcheshmeh, Darrehzar, Sara, Iju and Seridune to investigate and detect the high potential areas for gold mineralization. ASTER false color composite image of bands 4, 6, and 8 in red green, and blue determined argillic and sericite altered rocks, as light red to pink, and propylitic altered rocks, as dark to light green. The results obtained from MTMF revealed that sericite is the dominant alteration mineral in the area. The discriminated minerals at most occurrences, including Sarcheshmeh, Meiduk, Sereidun, Darrehzar, Abdar and Iju showed a circular to elliptical pattern with sericite as dominant zone, scattered kaolinite and sparse alunite–pyrophyllite, surrounded by a combination of epidote, chlorite, and calcite. Investigation of aeromagnetic data by applying different filters showed that the magnetic intensity is high in areas with a high concentration of gold. For example, the maximum differences in magnetic intensity were observed at the Sar Kuh and Abdar which contain a higher concentration of gold.
Acknowledgements : The authors would like to acknowledge the Institute of science and high technology and environmental sciences, Graduate University of Advanced Technology, Kerman, Iran for their financial support during this study under the contract number 7.395.