Asadi, Hooshang 
  
in a local grid, and the analytical results. These data were captured in digital form using scanning and an optical 
Character recognition (OCR) program. The resultant data were saved in a spreadsheet file after careful validation of the 
data entry against the original data sheets. The soil samples had been collected on two separate rectangular grids, 
orientated at 46° (lines 280W-90E) and 63° (lines 100E-320E) to the North. In order to facilitate subsequent processing 
the UTM coordinates of all sample points were determined using a coordinate transformation option after the data had 
been transferred into a GIS. 
Geophysical data were available in the form of a contoured ground magnetic intensity hardcopy map at a scale of 
1:2000. The information contained on this map was converted into a digital format through digitising, a process that 
involved capturing the UTM coordinates (X and Y values) and ground magnetic total field intensity contour values (Z 
values) down the samples lines, which provided the closest approximation to the original data. The results were stored 
in an ASCII XYZ file format that allowed direct input for gridding and contouring processes. 
4 2-D GIS ANALYSIS 
4.1 Geophysical data 
Epithermal gold deposits are highly variable in form, ranging from thin quartz veins to large bodies of disseminated 
mineralisation, and occur in a variety of geological environments. Consequently, they exhibit a wide range of 
geophysical signatures. The hydrothermal alteration that accompanies these deposits causes pronounced changes in the 
physical properties of the rocks. Asadi and Hale (1999) attributed the high analytical signals of total magnetic 
intensities of aeromagnetic data in the Takab area to intermediate composition magmatic rocks both at surface and at 
depth. They demonstrated that the highest magnetic anomalies are located in the Zarshuran mining area, where a 
number of carbonate-hosted epithermal gold-arsenic and antimony deposits are present. In this study processing and 
interpretation of ground magnetic data over the Zarshuran gold deposit are used to identify magnetic signatures 
associated with surface and subsurface geological features controlling mineralisation. 
4.1.1 Qualitative analysis 
Paterson and Reeves (1985) point out that analytical signal analysis of total magnetic field is often used for mineral 
exploration. It displays magnetic information in a more interpretable manner than total magnetic intensity: anomalies 
correlate directly with causative bodies and are positioned symmetrically over them. The analytical technique assumes 
constant directions of both geomagnetic field and the crustal magnetization over a certain region. On the other hand the 
analytical signal is independent of the inclination of the magnetic field (Qin, 1994). 
An analytical signal image of total magnetic intensity was created for mapping surface and subsurface igneous rocks at 
Zarshuran (Figure 2a). The resulting analytical signal map with overlaid geology shows that the zone, which appears to 
have the highest amplitude signals (purple shade) is located in the marginal part of the Imam Khan schist. The strength 
of the magnetic intensity over the Iman Khan schist is very high in the order of 10,000nT. Such high large anomalies 
are seldom encountered (Reeves, personal communication) and are very typical of banded ironstones. From 
petrographic studies, in this case they can only be attributed to the serpentinized ophiolitic rocks. Such rocks have been 
recently mapped at the core of Iman-Khan anticline as metamorphosed ophiolitites in the 1:100,000 scale geological 
map of Takab area, published by Geological Survey of Iran. The low magnetic analytical signals (blue shade) are 
associated with Qaradash acidic volcanics and a highly altered Oligo-Miocene granitoid, which are both located in the 
SW of the area. 
Processing of the geophysical image also involved the use of a combination of various filtering techniques that assisted 
in the interpretation by highlighting various aspects of the magnetic data. In many cases such processing revealed 
information that is not recognisable in conventional contour maps. Three directional filters (edge enhancement filters in 
the X, Y and 45° directions), were applied to the gridded total magnetic field data of Zarshuran. The horizontal and 
diagonally filtered maps highlight several linear trends in a NW-SE direction, whereas the vertical gradient filter reveals 
the presence of several NE-SW striking linear features. The processed images assisted identification of several NW-SE 
lineaments which are locally crosscut by NE-SW structures (Figure 4). The interpreted lineaments which lie within the 
Imam Khan unit are not present on the available geological map. These are therefore interpreted as being an expression 
of fracturing and faulting within the greenschists, which may be related to the adjacent granitoid intrusion. 
  
84 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000.