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hydrothermalalterationzones-170106110532

publicité
Omer M. Ahmed
MSc, 2016
University of Kerala, India.
Digital Image Processing and Analysis
Techniques for Detection Of
Hydrothermal Alteration Zones
A Case Study in Siah-Jangal Area, South
Eastern Iran
By: Zahra Mokhtari & Mohammad Boomeri &
Sasan Bagheri
Journal: Indian Society of Remote Sensing
Keywords
Alteration zones .
ETM+ .
Band ratio .
Color composite .
Principal Component Analysis .
Iran.
Geology of Iran
Iran comprised of different structural and petrological
features, according to Berberian and King (1981) there are
nine major lithotectonic units in Iran
1) Zagros Fold Belt
2) Sanandaj–Sirjan Zone
3) Sahand Bazman Belt
4) Central Iran
5) Lut Block
6) Alborz Zone
7) Kopeh Dagh zone
8) Eastern Iran suture zone
9) Makran zone
The Siah-Jangal area is part of Makran zone
Fig:1 Major lithotectonic structural zones of Iran and location of
Siah-Jangal area (modified after Berberian and King 1981)
 The Siah-Jangal area is located in south eastern Iran within the
Makran Quaternary volcanic belt. Which is extends from Iran to
Pakistan and made magmatic arc known as Northern Makran
magmatic Arc (NMA).
 In Iranian part of NMA (where in the Siah-Jangal area), there is a
wide range of Hydrothermal alteration and mineralization related
to magmatic event and great potential for mineral exploration
such as Cu and Au.
Fig.2 Regional geotectonic setting of the Makran arc-trench system and Neogen
Baluchistan volcanic arc (Based on Grando and McClay 2007; Ellouz- Zimmermann etal.
2007)
 There is a thick mineralized quartz vein in central part of the
study Area surrounded by
 Hydroxyl bearing minerals such as
• Phyllic (Quartz serisite and pyrite)
• Argillic (alteration of clay minerals)
• Propylitic (alteration zones causing by iron and magnesium)
 The iron oxide minerals that recognized in the area are hematite,
goethite and limonite.
Fig.3
(a): extending of Hydrothermal
alteration zones around the
central quartz vein
(b):occurrence of iron oxide
minerals.
Geology of the Area
 The oldest rocks in this area are mainly siliciclastic deposits and
serpentines rocks intercalated with basaltic lava, shales and
carbonate rocks.
 The second unit is Eocene fossiliferous limestone and
greywacke.
 Quaternary volcanic rocks consist of andesitic rocks and dacite
they are unconformable with the flysch deposits.
 The youngest exposures in the area are the Alluvial deposits.
 There is structural control in the ore deposit types in the area.
B
D
(B): Fossiliferous limestone
(C): Sandstone
(D): Flysch sediments and lava flow of basalt
Other color Quaternary Alluvial plain
C
Image Processing Techniques
The main purpose of the methodology is to extract the
mineral alteration zones from the ETM+ data in the area.
Why Landsat 7 ETM+ images ?
It is consist of eight bands with a spatial resolution 30 m
for bands 1 to 7.
Band eight is panchromatic band with 15 m spatial
resolution.
 Three processes techniques were done for the image
• False Color Composition (FCC)
• Band Ratio
• Principal component analysis (PCA)
 The techniques applied
ArcGIS 9.3 programs.
by using ENVI 4.3 and
Color Composite of Images
 The rule of color composites is to set the most
informative band for a particular purpose in the red
gun, the next in green gun and the least informative
band in blue gun 3 2 1 in R G B Respectively known
as true color composite.
 All other band combinations are termed as False Color
Composite (FCC).
 Two FCCs were done for the area,
FCC (bands 7, 4, 1), FCC (bands 3, 7, 5).
the best one is 7, 4, 1.
 Three rock units are identified in the area (Volcanic
rocks, Sedimentary rocks and alluvial sediments).
Fig :5. FCC
(7:4:1); of area
Q Quaternary
Alluvial plains
ES
ES
Q
ES
ES
Q
ES Eocene
Sedimentary rocks.
ES
CV Cretaceous
Volcanic rocks
white and
yellowish ellipses
and circles:
hydrothermal
alteration areas.
ES
ES
CV
CV
Fig :6.
FCC
(RGB 3:7:5)
of the area
there is no
obvious variation
between the rock
units
Band Ratio Images
 Enhancements resulting from the division of brightness
values in one band by the corresponding values in
another band.
 To reduce the effects of environment (i.e. topographic
slope and shadows).
 The ratio image provides unique information that are
often difficult to detect in a standard image.
Two band ratio were done for the image
• Band 5 to band 7 was used for hydroxyl absorption and
the ratio appears in light tone due to the high content of
hydroxyl-bearing minerals.
• band 3/1 ratio for iron oxides
and the areas with
abundant of these minerals indicated as bright pixels.
Fig : 9
5/7 band ratio
image for the
area.
The areas
with abundant
hydroxyl
bearing
minerals are
shown as
bright pixels.
(Fig. 10)
Band 3/1 ratio
the areas with
abundant of the
iron oxide
minerals
indicated as
bright pixels
The combinations of 5/7, 5/4 and 3/1 were created as (FCC)
image for the detection of hydrous minerals and iron-rich
zones in RGB
(Fig. 11)
The areas with
hydroxyl and iron
oxide minerals are
shown with bright
pixels
Principal Components Analysis
o PCA is linear transformation technique used to
emphasize variation and bring out strong patterns in
data set.
o Creates a new set of images with reduced spectral
redundancy and it maybe more interpretable than the
original data.
o Also used to compress the information content of a
number of bands of images in to just two or three
transformed principal component images.
 According to (Crosta Et al. 2003) there are six band
(1, 2, 3, 4, 5 and 7) can be used for principal
component analysis.
 Table1 shows the values of the principal components
for six bands of ETM+ in the area.
• Iron oxides minerals have low reflectance in band 1
and higher reflectance in band 3 based on this, we can
predict that iron oxides can be distinguished by dark
pixels in the PC3.
Fig:12.
PC3 image of the iron oxides
minerals. The altered areas
are shown as bright pixels.
 hydroxyl bearing zones are shown as bright pixels in
gray scale image of PC4.
Fig:13
PC4 image of the
hydroxyl minerals.
The altered areas are
shown as bright pixel.
• The false color composite Image obtained using
combination of hydroxyl image and iron oxide image
in the RGB. The bright pixels are hydrothermal altered
zones in study area.
Fig.14
FCC image (RGB) of
hydroxyl and iron oxide
image.
Summary and Conclusions
 The major types of alteration found in the SiahJangal area are clay minerals and Quartz vein.
 The mineral assemblages are iron-bearing minerals and
hydroxyl bearing minerals.
 Image processing techniques are applied to the digital
subset ETM+ such as false color composite, band ratio
and principal component analysis (PCA).
 The hydrothermal alterations in the area are Hydroxyl
minerals detected by band ratio 5/7 and iron oxide
minerals detected by band ratio 3/1.
 RGB false color composites of 5/7, 5/4, 3/1 indicated
the similar altered mineral areas as white color.
 PC4 shows the distribution of hydroxyl minerals.
 PC3 images reveals the distribution of iron oxide
minerals.
 There is no obvious different in the hydroxyl and iron
oxides images in the Band ratio and PC methods.
 The spectral transformations on the ETM+ data can
provide effective methods in the identification of
hydrothermal alteration minerals in arid and semi-arid
areas.
References:
Zahra Mokhtari & Mohammad Boomeri & SasanBagheri. Digital Image Processing and
Analysis Techniques for Detection of Hydrothermal Alteration Zones: A Case Study in SiahJangal Area, North of Taftan Volcano, South eastern Iran. Indian Society of Remote Sensing
(ISRS), 43(2):363–377 (June 2015).
Omer M. Ahmed [email protected]
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