Enhancement of Hydrothermal Alteration in ASTER Images of Sar Cheshmeh Area, Using Logical Operators

Summary: Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) data and Interactive Data Language (IDL) logical operator algorithms with calculating threshold based on the land, and experimental laboratory analysis, were used to map hydrothermally altered rocks in the Sar Cheshmeh, Darrehzar, Nowchoon, Seridune and Sarkuh areas. Hydrothermal alteration enhanced by this method include silicic (hydrous quartz, chalcedony, opal), propylitic (calcite and epidote- chlorite), argillic (alunite-pyrophyllite-kaolinite) and phyllic (sericite-illite). By use of this method, different types of hydrothermal alteration in and around mineral deposits such as Sar Cheshmeh and Darrehzar, based on field observations and sampling, confirmed. Calcite and epidote- chlorite minerals were separated in the propylitic zone based on the logical operator algorithms.
Band ratios are based on the absorption and reflection parts of the electromagnetic spectra of minerals. In order to minimize the interferences from other sources such as vegetation cover, combined use of band ratios are suggested by different workers. Logical operators suggested by Mars and Rowan (2006) and Mars (2013) are used here for mapping different minerals associated with hydrothermal alterations. The important point in using these logical operators is the proper selections of the threshold for each alteration type. This point is discussed in this paper.
Methodology and Approaches: ASTER data used for hydrothermal alteration mapping using the combined band ratio method. The combination of different band ratios with specified thresholds, is used to map hydrothermally altered areas. The threshold values are calculated based on both statistical method and field/laboratory observations. Finally, logical operators suggested by Mars and Rowan (2006) and Mars (2013) used for this purpose. The result will be a binary image based on true (presence of alteration, which is one) or false (absence of alteration, which is zero) for each pixel of the image. The alteration maps obtained are evaluated using field and laboratory observations.
Results and By use of this method, different types of hydrothermal alteration in and around mineral deposits such as Sar Cheshmeh and Darrehzar, based on field sampling/laboratory observations, confirmed. In areas such as Seridune due to the weathering of host rocks, phyllic and argillic alterations are observed together. Calcite and epidote- chlorite minerals were separated in the propylitic zone based on the logical operator algorithms. Sampling results and field observations have confirmed the alteration maps.