Alteration Mapping by Remote Sensing
Techniques in South Iran - A Case Study
Iraj Navai & Simin MehdiZadej-Tehrani
Remote Sensing Group
Geological Survey of Iran (GSI)
P.O. BOX : 13185 - 1494 Tehran - Iran Fax : 60093378
Abstract
This study reports into mapping hydrothermal alterations as an aid to prospecting for epithermal mineral deposits in the Orumieh - Dokhtar volcanic belt of Iran. The Sarcheschmeh porphyry copper mine in the Pariz region was selected with a known alternation zone and ore deposite as training site for design of enhancement techniques for application in high lighting of hydrothermal alteration minerals.
The aim of this study was assess the effectiveness of TM data for detecting alteration zones. For this purpose different techniques from combined rock and vegetation enhancement. Well established method of algebraic operations among spectral bands, such as difference (Moore, et al., 1993), ratio and principal component analysis can be applied for the purpose. A hybrid P.C and difference colour composite achieved the most effective separation of hydrothermal alteration (high, medium and low) could be appeared much clearly.
1. Introduction
The Sarcheshmesh copper and molybdenium mine operate or porphyry copper within a belt of Palegene Neogene volcanics in the Pariz region of south-central Iran. The orebody is contained in hydrothermally-altered sub-volcanic and intrusive rocks. A landsat thematic mapper (TM) image (9/12/1990, orbit 161-39) has been used to identify area within Pariz region which show the same spectral features as hydrothermal alteration in and near the mine site. These newly located areas of hydrothermal alteration minerals field checked for prospecting potential.
1.1 Geological Setting
The study area is located in South Central Iran within Orumieh - Dokhtar volcanic belt. The district is mountainous, reaching altitudes of 3280 meters.
The Eocene volcano clastic and sedimentary rocks with thousand meters thicknes, including trachy-andesite, tracky-basaltic iavas, pyroclastic, sandstone, shale and limestone are the main our croppes in this area.
The volcanics overlie basement strata uncomformably and are themselves flanked and overlain by Quaternary deposits.
The platonic rocks form discordant massives and stocks in the Eocene complex and accompanies by numerous dykes. The intrusives are basically monzonite granodiorite with transition to syenite.
The intrusions have thermally metamorphosed adjacent host rocks and created swarms hydrothermal veins, some which contain bodies of minerals.
Hydrothermal activity has created extensive areas of aryillic alteration in both intrusions and surrounding volcanic.
1.2 Remote Sensing Image
Landsat Thematic Mapper imagery is made up of seven spatially co-registered spectral bands. Six bands (Tm bands 1,2,3,4,5 and 7) cover the spectral range from visible blue to the near infra red (0.4-2.5 m). spatial resolution (30 meter pixels) in the six viznir band images allows the imagery to be used at map scale as detailed as 1:50000. A seventh band TM6, representing the thermal infra-red spectral range (8-11 m) was not used in this study because of its lower spatial resolution (120 meter pixels).
2. Digital Image Enhancement and Interpretation
The first stage of the image enhancement programme was to chose the TM sab-scene of geologically well documented and mapped area with known alteration zones (Pariz region) and ore deposit (Sarcheshmesh mine) as "training site" for design of enhancement techniques for regional application in both geological mapping and high lighting of hydrothermal alteration minerals. For this purpose a sub-scene 36x36 km (1024 x 1024 lines/pixels) of Sarchehmesh copper mine area was selected from a cloud free, complete 185 x 185 KM Landsat TM Scene.
2.1 Fakse Colour composite images for Rock Type Enhancement
The most suitable imagery for discrimination and delineation of out cropping rock types in the study area are three bands colour composite images which produced by co-registered three bands display using red, green and blue "VDU" colour gunes. The choice of image bands must be on the basis of known spectral reflectance properties of the rocks and soils in relation to TM bands.
In order to achieve the optimised colour presentation of the image, the three ands used for colour composition must be carefully contract enhanced. In this study the best contrast enhancement results, were produced by the Balanced contrast enhancement Technique (BCET) (Liu 1991).
TM bands "BCET" colour composites were produced to illustrate the lithological variations, soils and vegetation in the Pariz area such as :
TM bands 2,3 and 4 displayed as blue, green and red to illustrate the variations of rock types.
The colour composite of TM bands 1,3 and 5 displayed as blue, green and red achieves the best discrimination of rock types and superficial deposits, in the test area. In this image the volcanic rock type and the 01igoscene granodioritediorite coomposition intrusive bodies are clearly highlighted.
