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ALTERATION MAPPING BY REMOTE SENSING: APPLICATION TO HASANDAG —
MELENDIZ VOLCANIC COMPLEX, CENTRAL TURKEY
E. Yetkin, V. Toprak, M. L. Süzen
RS-GIS Lab., Dept. of Geological Engineering, METU 06531 Ankara, Turkey
erdemyet@yahoo.com
PS WG
VIU3
KEY WORDS: Remote Sensing, Exploration, Classification, Landsat, Spectral.
ABSTRACT:
Certain alteration minerals are used to identify the hydrothermally altered rocks. In volcanic rocks, mainly potassic, phyllic (sericitic),
propylitic, argillic alteration and silicification are observed. The role of remote sensing in alteration mapping is the differentiation of the
minerals that are unique for different alteration types. In this study, Landsat TM 5 images are used. General alteration trend in the area is
mapped by conventional methods of color composite, band rationing, principal component analysis and multi-linear regression analysis.
Detailed mineral mapping is carried on by using the United States Geological Survey (USGS) spectral library data. Spectral reflectances
of selected minerals are analyzed according to the TM band intervals
and appropriate band ratios are selected. TM bands 1,2,3,4,5 and 7
are used. Outputs of mineral maps are investigated for zonal distribution. Mineral maps that are obtained by the mineral separation
method reveal that the youngest volcanic complex Hasandag is poorly altered. Instead, Keciboyduran, Melendiz and Tepekóy volcanic
complexes are found to be highly altered with the pattern of clay dominant in the center and increasing oxidation towards flanks. Also the
alteration along the previously mapped buried faults is a proof that the method can provide information about the alteration source.
1. INTRODUCTION
Hydrothermal alteration is defined as the reflection of response of
pre-existing, rock-forming minerals to physical and chemical
conditions different than those, under which they originally
formed, especially by the action of hydrothermal fluids (Beane,
1982).
The nature of the alteration products depends on 1) the character
of the wall rock, 2) the character of the invading fluid, which
defines such factors as Eh, pH, vapor pressure of various volatile
species, anion-cation composition, and degree of hydrolysis, and
3) the temperatures and pressures at which the reactions take place
(Guilbert and Park, 1986).
Alteration may result from 1) diagenesis in sediments, 2) regional
processes, such as metamorphism, 3) postmagmatic or post-
volcanic processes associated with cooling, and 4) direct
mineralization processes.
Potassic, phyllic (sericitic), propylitic, argillic alteration and
silicification are main alteration types observed in volcanic rocks.
Kaolinite, Illite, Montmorillonite, Pyrophyllite, Alunite,
Orthoclase, Quartz, Epidote, Chlorite, Hematite, Goethite and
larosite are alteration minerals that are mapped with techniques
described in this study.
Hydrothermally altered rocks are characterized by unusually
colorful rocks. The various colorful rocks are the host rocks of
those mineral deposits with the colors representing the results of
chemical interaction with the surrounding hydrothermal fluids.
The hydrothermal fluid processes altering the mineralogy and
chemistry of the host rocks can produce distinctive mineral
assemblages which vary according to the location, degree and
duration of those alteration processes. When these alteration
Products expose at the surface, it can be mapped at a zonal pattern,
theoretically concentric around a core of highest grade alteration
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and greatest economic interest. The importance of the recognition
of such spatial patterns of alteration makes the remote sensing
techniques as one of the standard procedure in exploration
geology, due to its speed and price.
One of the key idea of remote sensing techniques in exploration
geology is that it is applied to rocks, minerals, and structures
associated with a particular ore, and not the ore itself. There are
very logical reasons for this procedure. The ore is not always
exposed at the surface, and it often is not as spectrally unique or as
widely disseminated as the minerals and rocks that are associated
with the ore body (Vincent 1997).
1.1 Geological Setting
Cappadocian Volcanic Province (CVP) extends as a belt in NE-
SW direction for a length of 250-300 km situated in Central
Anatolia (Fig. 1). The volcanism of the CVP has been investigated
by several researchers who mainly concentrated on the
chronology, petrographical and geochemical characteristics, and
ignimbrite emplacement (Pasquare, 1968; Innocenti et al, 1975:
Besang et al, 1977: Pasquare et al., 1988; Ercan et al. 1990,
1992; Aydar et al., 1994; Le Pennec et al., 1994, Temel et al.,
1998; Schumacher and Mues-Schumacher, 1996). Accordingly,
the CVP is a calc-alkaline volcanic province whose formation is
attributed to the convergence between Eurasian and Afro-Arabian
plates occurring in the eastern Mediterranean.
Main geological units in CVP are the Pre-Mid.-Miocene basement
rocks, Lower Miocene-Pliocene continental clastics, Lower
Miocene-Quaternary volcaniclastics and volcanic complexes and
Quaternary fluvial deposits (Fig. 1) (Toprak, 1998). The study is
mainly concentrated on Hasandag, Keciboyduran, Melendiz and
Tepekóy volcanic complexes.
