Recently, specific relation of known mineral deposits with different structural
features, such as faults, is a factor in some exploration programs (Derakhshani
and Mehrabi, 2009a) specially for the regions where geology is known but
geoexploration data are lacking. Porphyry copper deposits genetically could
associate with faults and plutons according to plate tectonic theory (Shahabpour
and Doorandish, 2008; Hezarkhani, 2006a, b).
Recently, curvy-linear structural features are studied from mineral deposit
concentration viewpoint (Derakhshani and Mehrabi, 2009b;
Mayer and Sausse, 2007; Carranza 2002,
2009; Carter and Cheng, 2008).
The specific relation of some mineral deposits and curvy-linear structural
features is due to the localizing mineralization by those structures. For example,
faults can canalize mineralizing fluids whilst igneous intrusions cause chemical
reactions with the intruded rocks by providing heat sources which may cause
mineral deposition there (Niemeyer and Munizaga, 2008).
Since, faulting could play a close relation with copper mineralization, specific association of them is quantitatively calculated in Chahargonbad area, taking advantages of GIS, RS and weight of evidence modeling.
MATERIALS AND METHODS
This research is studied the specific relation of faults and porphyry copper deposits in Chahargonbad area that is situated at the Southern part of Central Iranian Volcanic Belt. Remotely sensed data from Landsat 7 are used for deriving lineaments of the study area by applying some filters, such as Laplacian and Sobel and also some directional filters in 8 different directions: N, N45E, E, S45E, S, S45W, W and N45W. Since, this lineament map may include some non-geological lineament, such as roads and rivers, so fault map of the area is provided from lineament map after field checking. Porphyry copper deposits of the area are mapped and finally rastrised using GIS advantages. Weight of evidence method is applied finally to find a quantitative specific relation between faults and copper deposits.
Measuring of specific relations of faults and copper deposits involved the
||Digitizing faults as lines
||Rasterizing fault maps and generating buffers around rastered fault lines
||Digitizing porphyry copper deposits as points
||Rasterizing porphyry copper deposit points
||Calculating number of copper deposit points in and out of a binary domain
by crossing or overlaying each of the raster maps of the binary domains
with the raster map of copper deposits
||Determining the weights base on Eq. 1 and 2
||Measuring C and Studentised C
RESULTS AND DISCUSSION
Calculating of weights to estimate specific relation requires polygonal domains. Chahargonbad area, have both curvy-linear structural features (such as faults) and points (such as porphyry copper deposits). Distance buffering is performed to convert faults to polygonal domains.
These faults were detected by studding of Landsat satellite images, aerial
photographs and field checking (Fig. 1). Finally 122 faults
identified in Chahargonbad area. Fault map converted to raster from vector form.
Buffer zones of 500 m interval are provided around each faults using ArcGIS
software. After mapping and rasterizing of porphyry copper deposits of the area,
it is overlaid on the buffered fault map by using spatial data modeler software.
W+ and W-factors of each buffer zone are calculated by counting of the porphyry copper deposits in and of it. After doing this process for each buffer zone, it is resulted that the number of Studentised C in 3500 m buffer zone is 1.9021 which is the minimum one among buffer calculated numbers. According to weight of evidence method, this number show that the specific relation of faults and copper mineralization is in its minimum state in 3500 m wide buffer zone. The spatial association of faults and copper deposits is positive in 500 to 2000 m wide buffer zones. Studentised C is cached to its maximum level, 4.3662, in 1000 m wide buffer zone. So, the most relation of faults and copper deposits could be found in 1000 m wide buffer zone around faults.
The general basis of area selection in many mineral exploration programs is
quantitative knowledge about the specific relation between faults and mineral
deposits. Here, the specific relation between faults and porphyry copper deposits
in Chahargonbad area are quantified using weight of evidence modeling. Chahargonbad
area located at the Southern part of Central Iranian Volcanic Belt, between
29°30′-29°45′N latitudes and 55°55′-56°15′E longitudes (Fig.
2). There are 17 porphyry copper deposits in this area while Chahargonbad
deposit with 530 km2 area is an active cupper mine there.
||Strike-slip faults and fractures map and porphyry copper deposits
in the study area
The Central Iranian Volcanic Belt with the NW-SE direction extends as the same
direction of Zagros Mountain Range (Derakhshani and Farhoudi,
2005) and mostly, includes volcanic-alluvial parts of Eocene period (Fig.
2). One of major features of the region is great amount of volcanic Eocene
rocks which have been considered as the main ore creating part (Derakhshani
and Abdolzadeh, 2009a). Porphyry coppers of this Volcanic Belt are created
by injection of diorite to granodiorit bodies from middle Oligocene to Miocene.
(Derakhshani and Abdolzadeh, 2009b). Weight of evidence
method which was developed by Carter and Cheng (2008)
helped us to find a quantitative relation between faults and copper deposits
in the southern part of this volcanic belt.
Given the absence or presence of a domain, e.g., a buffer zone around a fault,
a weighting performed between that domain and a set of points, e.g., porphyry
copper deposits. The weighting yields (1) W+ weights in the domain (DP) and
(2) W- weights out of the domain (DA). T = DP + DA; where, T is the total area
being studied. Because the aria of a deposit is much smaller than the domain
being considered, the weights of evidence are estimated by taking the natural
logarithms of the probability ratios.
||Location and geological map of study area
W+ and W- present measures of the specific relation between the deposits and
a fault buffer zone. (W+) >0 and (W-) <0 present positive specific association,
(W+) <0 and (W-) >0, imply negative spatial relation and (W+) = (W-) =
0 present no spatial association. The variances of the weights can be calculated
Representation of optimum distance between porphyry copper
deposits and strike-slip fault fractures
where, mDP and mDA are, respectively the number of pixels with copper deposits in and out of a given buffer zone; bDP and bDA are, respectively the number of pixels without copper deposits in and out of a given fault buffer zone and DP = mDP + bDP and DA = mDA + bDA.
The contrast C combines these weights into an overall measure of the specific
relation with a set of copper deposit points for each binary domain and is defined
where, C will have negative values in the case of negative specific association
while for a positive spatial relation, C will have positive values. Usually
the maximum C gives the cutoff distance buffer at which there is optimal specific
relation between a fault domain and a set of porphyry copper points. C would
be meaningless in cases of a small number of copper deposit points such as the
study area, because of the uncertainty of the weights which could be large.
In such areas with small number of copper deposit points, a useful measure to
determine the most significant cutoff distance is to calculate the studentised
value of C. The Studentised C, measured as the ratio of C to its standard deviation,
C/s(C), shows the statistical significance of the specific relation. The standard
deviation of C is measured as:
Studentised C more than 2 presents a statistically significant specific association.
Due to the small number of porphyry copper deposits in the study areas, the
cutoff distance within which their specific relation with a given buffer is
optimal was chosen according to the maximum amount of Studentised C (Fig.
||Variation of weights and contrasts for cumulative distances
from strike-slip fault fractures respect to porphyry copper deposits
According to the above discussion, it is obvious that calculation of weights to estimate specific relation requires polygonal domains. Present study domains, however, are either point (porphyry copper deposit) or linear geological features (faults). Distance buffering was performed to convert these point and linear domains into polygonal domains.
By calculating the Studentised C, the specific relation between faults and
porphyry copper deposits in the study area is obvious (Table 1).
This specific relation between faults and copper deposits is due to the role
of faults in canalizing mineralization. This finding is supported by some researches
in other areas (Carranza, 2009; Meyer and Sausse, 2007;
Derakhshani and Mehrabi, 2009a) and findings of this
research correlate with the aim of the project. The positive specific relation
is statistically significant within 500 to 2000 m; however, based on the highest
Studentised C, it is optimal within 1000 m. There is very little published work
in the study area on the large scale analysis of specific relation of copper
mineralization and faults in Central Iranian Volcanic Belt and to the best of
our knowledge, this is of the first studies of its kind that concentrates on
the quantitatively interpretation of this specific relation by using of weights
of evidence modeling, remote sensing and GIS.
Porphyry copper deposits in Chahargonbad area are associated spatially within 1 km buffer zones.
According to these quantifications of specific relations, it could be concluded that fault zones can canalize magmatic fluids and are favorable for the emplacement of porphyry copper deposits. Consequently, these fault zones are favorable loci for copper exploration.
The researchers greatly appreciate the scientific comments of Dr. Ansari. Also, the author would like to thank Dr. Raoof and Naji for their helpful comments and suggestions. This study has greatly benefited from the changes suggested by two anonymous reviewers.