hydrothermal alteration
DESCRIPTION
mapping hydrothermalTRANSCRIPT
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JAG l Volume 2 - Issue 2 - 2000
COMMUNICATION
Iron oxide and hydroxyl enhancement using the Crosta Method: a case study from the Zagros Belt, Fars Province, Iran
Majid Hashemi Tangestani and Farid Moore
Department of Geology, College of Sciences, Shiraz University, 71454 Shiraz, Iran
KEYWORDS: alteration mapping, principal component
analysis, Landsat-TM, eigenvalue, eigenvector, color
composite
ABSTRACT
Following preliminary reports on the probable occurrence of iron
ore in the Mashayekh-Nowdan area, west of Shiraz, principal com-
ponents analysis on 6 and 4 Landsat-TM bands was tested by the
Crosta method for the enhancement and discrimination of iron
oxide stained and hydroxyl-bearing areas in the region. Eigenvector
loadings of visible and infrared bands of TM bands 1, 3, 4, 5, and 7
show that in each case the first principal component (PCI) indicates
albedo, PC2 indicates the difference between visible and infrared
bands, and PC3 indicates vegetation. Features with lower impor-
tance such as iron oxide or hydroxyl-bearing minerals are concen-
trated in subsequent principal components. PC4 of unstretched
data transformation on bands 1, 4, 5, and 7 indicates the hydroxyl-
bearing and carbonate exposures; and on bands 1, 3, 4, and 5, it
indicates iron oxides. Color composites of hydroxyl and iron oxide
images enhance the iron oxide exposures, but not as clearly in the
case of hydroxyls, because of some spectral behavior similarities
with carbonates.
INTRODUCTION
Landsat data have been used for a number of years in arid
and semi-arid environments to locate areas of iron oxides
and/or hydrous minerals [Abrams et a/, 1983; Kaufman,
1988; Ranjbar & Roonwall, 19971 which might be associ-
ated with hydrothermal alteration zones. However, iron
oxides have a wide range of occurrences that are often
unrelated to alteration phenomena; these include sedi-
mentary red beds, volcanic rocks, and weathered alluvium.
In addition, there are types of alteration which are iron-
oxide free, such as advanced argillic and siliceous rocks
that are highly leached. These leached areas are character-
ized by the presence of hydrous minerals such as kaolinite,
sericite, montmorillonite, and alunite.
The second generation Landsats, launched in 1982, carry
a multispectral scanner called the Thematic Mapper (TM).
This instrument has seven channels and provides data
with 30-m spatial resolution. Spectral bands 5 and 7 of
the Thematic Mapper are located beyond 1 .O pm and are
situated in spectral regions that contain characteristic fea-
tures of hydrous minerals, and hence many hydrothermal-
ly altered rocks. The 1.65~ym band is located where
altered rocks have their highest reflectance; the 2.2~ym
band spans the region where hydrous minerals have a
strong absorption feature ( Figure 1).
The principal component transformation is a multivariate
statistical technique that selects uncorrelated linear com-
binations (eigenvector loadings) of variables in such a way
that each successively extracted linear combination, or
principal component (PC), has a smaller variance [Singh &
Harrison, 19851. The statistical variance in multispectral
images is related to the spectral response of various surfi-
cial materials such as rocks, soils, and vegetation. The
methodology of this paper, which follows an earlier study
in the Mashayekh-Nowdan area [Hashemi Tangestani &
Moore, 19971, relies specifically on the Crosta & McM.
Moore methodology [Crosta & McM. Moore, 19891 and
also on the selective input of only four image bands for
PCA [Loughlin, 19911.
A 1878 x 2034-pixel subscene of the Landsat TM
163/039/3 quarter image covers the Mashayekh-Nowdan
area, west of Shiraz and north of Kazerun. The image
was acquired on 10 September 1990. The area is semi-
arid; the vegetation type and amount are influenced by
elevation, aspect, and availability of soil moisture. The
results of the examinations are illustrated for an area that
covers the northern part of the subscene, called
Mashayekh-Nowdan.
GEOLOGY
The Mashayekh-Nowdan area, which is about 50 km long
and 40 km wide, lies within the southern margin of the
Zagros Mountain Range (29 36 - 30 03 N , 51 31 -
51 56 E) in an area generally known as Simply Folded
Belt. The detailed geology of the Zagros Mountain Range
has already been described in the literature [Alavi, 1980;
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Iron oxide and hydroxyl enhancement JAG l Volume 2 - Issue 2 - 2000
a4 a6 0.6 IO u I.4 lk 1;s 2:o i2 2:4 wad~lh h microns
-.._.._.. 9resn w9stdon -.-.-. carbonale- bwlnp roil or rock
km- bearing soil or rock _--_ hydmxyl- bearing soil or rock
Darvishzadeh, 1992; Falcon, 1974; James & Wynd, 19651.
The most prominent structural feature of the area is the
presence of three anticlines, namely the Dashtak anti-
cline, the Nowdan anticline, and the Anar anticline,
which trend parallel to the general trend of the Zagros
Mountain Range, that is, NW-SE (Figure 2).
The exposed formations, in order of oldest to youngest,
are the marly Kazhdumi Formation (Albian), the calcare-
ous Sarvak Formation (Cenomanian-Turonian), the shaly
Gurpi and Pabdeh Formations (Santonian-Campanian),
the calcareous Asmari Formation (Oligocene-Miocene),
and the evaporitic Gachsaran Formation (Miocene). The
contacts between all these formations are conformable.
FIGURE 2 Geological map of the western part of the Mashayekh- Nowdan Area (29 36 - 30 03N, 51 31 - 51 56E modified from NIOC. Map No.2051 2, 1:250,000); the area covered by the subscene of the Landsat TM 163/039/3 quarter image extends some 8 km further eastwood
FIGURE 1: Diagrammatic SpeCtra illus- trating the position of diagnostic iron, clay, carbonate and chlorophyll absorp- tion bands. (after Kaufman, 1988)
Structurally, the Mashayekh-Nowdan area has hardly
been disturbed and several small normal faults of local
importance occur in the anticlines. These small local faults
and some huge slides in the Asmari Formation can be
related to the Kazerun lineament activity. This lineament
is part of the N-S Qatar-Kazerun lineament and passes to
the west of the study area.
Carbonatic formations comprise a large part of the areal
surface, and among these, exposures of Asmari
Formation are distinctly prominent. This carbonatic for-
mation also exerts an influence on the morphology of the
anticlines. Despite the widespread distribution of Asmari
exposures, it is only on the northern flank of the Anar
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Iron oxide and hydroxyl enhancement JAG l Volume 2 - Issue 2 - 2000
anticline that anomalous quantities of iron oxides are cent of the total variance for the unstretched data PCA. reported. Close field observations have revealed that iron Overall scene brightness, or albedo, is responsible for the oxides occur mainly as a thin veneer of Quaternary sedi- strong correlation between multispectral image channels. ments on top of Asmari limestone, giving the impression PCA has effectively mapped this into PC1 of the transfor-
of an authochthonous origin. mation (Figure 3).
PRINCIPAL COMPONENTS ANALYSIS OF SIX
TM BANDS
Table 1 lists the image eigenvalues (which give an indica-
tion of decreasing variance in successive principal compo-
nents) and eigenvector loadings (linear combinations of
weighted input images in the principal components) for a
principal components transformation, using the covari-
ante matrix, on all six reflective bands of TM on the
Mashayekh-Nowdan subscene. The transformation was
carried out on unstretched data.
In this transformation, the first principal component (PC 1)
is composed of a positive weighting of all total bands. As
indicated by the eigenvalues, PC1 accounts for 91.65 per-
Eigenvector loadings for PC2 in Table 1 indicate that PC2
describes the difference between the visible channels
(TMI, 2, and 3) and the infrared (IR) channels (TM5 and 7).
Eigenvector loading for PC2 of TM4 is not considered
because it is very close to zero.
Eigenvector loadings for PC3 (in Table 1) indicate that
PC3 is dominated by vegetation, which is highly reflective
in TM4; the positive loading of TM4 in this PC (0.9091)
also indicates that strongly vegetated pixels will be bright
in this PC image (Figure 4). The percentage of variance
mapped into this vegetation PC is only 2.55 percent,
which is not a measure of vegetation abundance in the
Mashayekh-Nowdan area, where most pixels will contain
some vegetation.
TABLE I: Principal components analysis on 6 TM bands of Mashayekh-Nowdan area.
input bands TM1 TM2
PC1 0.3433 0.2536 PC2 0.6778 0.3063 PC3 -0.2427 -0.0562 PC4 -0.5411 0.1349 PC5 -0.0498 -0.0265 PC6 -0.2614 0.9053
TM3 TM4 Eigenvector matrix
0.3883 0.2844 0.2984 0.0913 -0.0386 0.9091 0.8013 -0.1234 -0.0490 0.2610 -0.3333 -0.0280
TM5
0.6533 -0.5089 -0.0215 -0.1621 -0.5359 -0.0037
TM7 Eigenvalues (%)
0.3988 91.65 -0.3003 4.83 -0.3307 2.55 -0.0483 0.53 0.7993 0.40 0.0000 0.04
FIGURE 3 PC1 image (albedo image) from 6-band PCA, FIGURE 4 PC3 image from 6-band PCA. Vegetated areas are MashayekhNowdan area. enhanced in bright pixels.
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Having mapped albedo to PC1 and visible to IR differ-
ences, and vegetation to PCs 2 and 3, respectively, the
remaining three PCs can be expected to contain informa-
tion due to the varying spectral response of iron oxides
(absorption in visible bands 1 and 2 and higher reflectance in TM3) and hydroxyl-bearing minerals
(absorption in TM7, higher reflectance in TM5) ( Figure
1). By looking for moderate or large eigenvector loadings
for TM1 and TM3 in PCs where these loadings are also
opposite in sign, we can predict that iron oxides will be
distinguished by bright pixels in PC4 of Table 1.
(0.7924) and moderate negative loading for TM5 (-
0.5467) can be considered as an H image for the
Mashayekh-Nowdan area.
Hydroxyl-bearing minerals are mapped as drak pixels in
PC5 due to the fact that the contribution is negative
from TM5 and positive from TM7 in this PC (Table 1). If
the number of input channels is reduced to avoid a par-
ticular spectral contrast, the chances of defining a
unique PC for a specific mineral class will be increased
[Loughlin, 19911.
Table 3 describes the principal components transforma-
tion on unstretched TM bands 1, 3, 4 and 5 of the
Mashayekh-Nowdan subscene. TM7 could be substituted
for TM5 in this analysis with little effect on the result;
one SWIR band is omitted deliberately to avoid hydroxyl
mapping. The PCs can be interpreted as albedo in PCI, IR
versus visible in PC2, vegetation in PC3, and iron oxide as
dark pixels in PC4 (eigenvector loading for TM3 = -
0.8457 and for TM1 = +0.4825). This PC image (F) can
be negated to show iron oxide stained areas as bright
pixels ( Figure 6).
The rules for iron oxide mapping are similar to those for
hydroxyl mapping. The magnitude of eigenvector load-
ings for TM1 and TM3 in either PC3 or PC4 should be
moderate or strong and opposite in sign.
PCA FOR HYDROXYL AND IRON OXIDE MAPPING Table 2 describes the principal components transforma-
tion on unstretched TM bands 1, 4, 5, and 7 of the
Mashayekh-Nowdan subscene. TM bands 2 and 3 have
been deliberately omitted to avoid mapping iron oxides,
and it should be noted that TM2 or TM3 could substi-
tute for TM1 in this transformation. Following the rea-
soning process described above, we can predict that
PC1 is the albedo image, PC2 describes the contrast
between the short wave infrared (SWIR) and the visible
region, PC3 is brightest for vegetation, and PC4 high-
lights hydroxyl-bearing minerals as dark pixels. This
Hydroxyl (H) image is therefore negated in Figure 5
to show anomalous concentrations of H as brightest
zones.
The methodology for hydroxyl mapping by PCA on TM
bands 1, 4, 5 and 7 is to examine the eigenvector load-
ings for bands 5 and 7, in the PC3 and PC4 images. The
PC image that best discriminates hydroxyl-bearing miner-
als is that with a high or moderate eignvector loading,
irrespective of sign, for TM7 and a high or moderate
eignevector loading of opposite sign for TM5. Negation
of those PCs in which the TM7 loading is positive makes
the anomalous pixels brightest in all cases. PC4 in Table
2 with a relatively strong positive loading for TM7 FIGURE 5 PC4 image from 4-band (1, 4, 5, 7) PCA. Hydroxyl- bearing exposures are in bright pixels (after negation).
TABLE 2 Principal components analysis for hydroxyl mapping of Mashayekh-Nowdan area.
Input bands TM1
PC1 0.3767 PC2 0.8509 PC3 -0.3549 PC4 -0.0888
TM4 TM5 Eigenvector matrix
0.3195 0.7428 0.2510 -0.3863 0.8772 -0.0008 0.2554 -0.5467
TM7
0.4517 -0.2520 -0.3232 0.7924
Eigenvalues (%)
91.51 4.77 3.21 0.51
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TABLE 3 Principal components analysis for iron oxide mapping of Mashayekh- Nowdan area.
Input bands TM1
PC1 0.3893 PC2 0.7615 PC3 -0.1886 PC4 0.4825
TM3 TM4 Eigenvector matrix
0.4401 0.3278 0.2929 0.0064 -0.0720 0.9401 -0.8457 0.0928
TM5 Eigenvalues (%)
0.7397 91.34 -0.5780 5.15 -0.2745 2.82 0.2081 0.68
FIGURE 6 PC4 image from 4-band (1, 3, 4, 5) PCA. Iron oxide stained areas are in bright pixels ( after negation).
DISCUSSION AND RESULTS
The monochrome hydroxyl and iron oxide images produced
by PCA on four bands (such as those in Figures 5 and 6)
are easy to interpret in that anomalous concentrations of
each mineral category are represented by the brightest pix-
els on each image (after negation in some cases). There is
no need to consult the eigenvector matrices after the
images have been created to understand and interpret
these images; this would be necessary for PC images from
a six-band principal component transformation.
The Crosta images have another advantage in that they
can be added together to produce an image (an H+F
image ) on which pixels with anomalous concentrations
of both hydroxyls and iron oxides are the brightest. The
H+F image is produced simply by adding the H and F
images and resealing the resultant image to 256 gray lev-
els. An alternative can be a pairwise PCA using the H and
F images as the two input bands. One of the two PCs
from this is the H+F image. Care should be taken during
this transformation to equalize the statistics of the input
images such that the eigenvector loadings are approxi-
mately equal in the output PCs [Loughlin, 19911.
The color composite image is created by stretching the H,
H+F, and F images so that the brightest pixels in each are
favorably enhanced, and the darkest portion of each dis-
tribution is clipped to a certain extent. These three
images can then be combined in various ways to suit the
personal preferences of individual photogeologists.
Different combinations of Crosta images have been empir-
ically assessed, and the combination of H, H+F, and F in
red-green-blue (RGB) has already been suggested in the
literature [Loughlin, 19911. The iron oxide stained areas in
Mashayekh-Nowdan and in some alluvial deposits to the
south are dark blue in this color composite ( Figure 7); the
hydroxyl-bearing areas in central-south and west of the
area are sharp yellow. The combination of H, H+F, and F
images as green, blue, and red, respectively, enhances the
iron oxide stained areas in magenta to reddish and the
hydroxyl-bearing areas in light blue ( Figure 8). All the
rock and soil materials in the area are affected to some
degree by iron oxide staining, hence, the F images are
merely a measure of the intensity of iron staining. Field
observations indicate that the iron oxide stained areas
coincide with alluvial deposits of the Asmari Formation,
mostly on the northern flank of the Anar anticline.
In the study area, the Asmari Formation contains up to 1
wt. percent iron oxide. The weathered products of Asmari
carbonatic rocks are accumulated in the form of alluvial
deposits covering the low angle slopes of Anar anticline.
Considering the rather high iron oxide content of the par-
ent carbonate, it is not surprising to find that the alluvi-
um is further enriched to about 5 wt. percent [Hashemi
Tangestani & Moore, 19971. However, despite distinct
enhancement of iron-oxide-rich exposures in the Anar
anticline and previous reports of probable iron ore occur-
rence, the present study indicates that the iron content is
too low to be considered. Field observation further
reveals that hydroxide enhancements both in PC images
and color composites coincide mainly with Pabdeh-Gurpi
shales and Asmari carbonate. Figure 1 shows that hydrox-
ide and carbonate minerals both have relatively strong
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FIGURE 7 Color composite on H, H+F, and F images as Red, Green, and Blue, respectively (more details in the text).
absorption in band 7 of TM, so the likelihood of their
enhancement in principal component transformations and
even in ratioing processes is high. Considering the similar
spectral behavior of these compounds, their co-appear-
ance in enhanced images is not unusual. In conclusion,
although good enhancements were obtained for iron
oxide and hydroxide exposures using PC transformations,
field observations and laboratory analyses refute the pres-
ence of ore grade anomalies in this area.
The comparison of principal component analyses on 6
and 4 TM bands of the Mashayekh-Nowdan area indi-
cates that use of 4 bands in each analysis is more suitable
because enhancement of hydroxyl or iron oxide exposures
can be obtained by direct reference to the fourth princi-
pal component.
ACKNOWLEDGEMENT
The authors would like to thank the Research Council of
Shiraz University for Grant No. 77-X-l 131- 638, without
which this research could not have been conducted.
REFERENCES
Abrams, M.J., D. Brown, L. Lepley & R. Sadowski, 1983. Remote sensing of porphyry copper deposits in Southern Arizona. Economic Geology 78: 591-604.
Alavi, M., 1980. Tectonostratigraphic evolution to the Zagros sides of Iran. Geology 8: 144-149.
Crosta, A.P. & J. McM. Moore, 1989. Enhancement of Landsat Thematic Mapper imagery for residual soil mapping in SW
FIGURE 8 Color composite on F, H, and H+F images as Red, Green, and Blue, respectively (more details in the text).
Minais Gerais State, Brazil: A prospecting case history in Greenstone belt terrain. Proceedings of the 7th (ERIM) Thematic Conference; Remote Sensing for Exploration Geology, pp. 1173- 1187.
Darvishzadeh, A., 1992. Geology of Iran. Amir Kabir Publications, Iran, pp. 901. [In Persian].
Falcon, N.L., 1974. Southern Iran: Zagros Mountains, in Mesozoic- Cenozoic erogenic belt. Geological Society of London, Special Publication, pp. 199-2 1 1.
Hashemi Tangestani, M. & F. Moore, 1997. Application of TM data for discrimination of lithologic and weathered units in Mammassani-Kazerun area, Fars Province, Iran. Proceedings of the Second Annual Conference of the Geological Society of Iran, Meshed, Iran, pp. 546-550.
James, G.A. & J.C. Wynd, 1965. Stratigraphic nomenclature of Iranian oil consortium agreement area. American Association of Petroleum Geologists Bulletin 49(12): 2182-2245.
Kaufman, H., 1988. Mineral exploration along the Aqaba-Levant structure by use of TM-data concepts, processing and results. International Journal of Remote Sensing 9(10 & 11): 1630-l 658.
Loughlin, W.P., 1991. Principal Component Analysis for alteration mapping. Photogrammetric Engineering & Remote Sensing 57(g): 1163-I 169.
Ranjbar, H. & G.S. Roonwall, 1997. Integrated mineral exploration for porphyry copper mineralization in Pariz area, Kerman, Iran: a case study of the Darrehzar porphyry copper deposit. In: Heikki Papunen (Ed.), Mineral Deposits: Research and Exploration; Where Do They Meet? Balkema, Rotterdam, pp. 677-879.
Singh, A. & A. Harrison, 1985. Standardized principal components. International Journal of Remote Sensing 6(6): 883-896.
R,ESUME
Suivant des rapports preliminaires sur la presence probable de minerai de fer dans le Mashayekh-Nowdan, a Iouest de Shiraz, des analyses de composants principaux dans les bandes 6 et 4 de Landsat-TM ont et& test&es par la methode Crosta pour
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Iaccentuation et la discrimination de zones teintees doxyde de fer et porteur dhydroxyle dans la region. Les vecteurs propres dans le visible et infrarouge des bandes TM 1, 3, 4, 5 et 7 montrent que dans chaque cas le premier composant principal (PCI) indique Ialbedo, PC2 indique la difference entre les bandes visible et infrarouge et PC3 indique la vegetation. Des details de moindre importance tels que Ioxyde de fer ou des mineraux porteurs dhydroxyle sont concentres dans les compo- sants principaux suivants. PC4 issu de la transformation de don- nees non etirees sur les bandes 1, 4, 5 et 7 indique de Ihydroxy- le et des affleurements de carbonate ; et sur les bandes 1, 3, 4 et 5 il indique des oxydes de fer. Des couleurs composees dimages dhydroxyle et doxyde de fer accentuent les traces doxyde de fer, mais pas aussi clairement dans le cas dhydroxy- le, a cause de certaines similarites de comportement spectral avec les carbonates.
de oxides de hierro y productores de oxhidrilos en la region. Los valores de eigenvectores de las radiaciones visibles e infrarrojas de las bandas TM 1, 3, 4, 5, y 7 muestran que, en cada case, el primer principal componente (PCI) indica el albedo, el PC2 indi- ca la diferencia entre las bandas visibles e infrarrojas, y el PC3 indica la vegetaci6n. Rasgos de menor importancia coma bxidos de hierro o minerales portadores de oxhidrilos se concentran en 10s principales componentes siguientes. El PC4 de datos trans- formados en las bandas 1, 4, 5, y 7 indica 10s afloramientos de oxhidrilos y carbonates; el PC4 de datos transformados en las bandas 1, 3, 4, y 5 indica oxides de hierro. Composiciones de colores a partir de las imagenes representando oxhidrilos y 6xi- dos de hierro realzan 10s afloramientos de oxides de hierro, pero no tan claramente en el case de 10s oxhidrilos debido a algunas semejanzas de comportamiento espectral con 10s carbonates.
RESUMEN
A continuation de informes preliminares sobre la ocurrencia pro- bable de mineral de hierro en el area de Mashayekh-Nowdan, al oeste de Shiraz, se ensayo un analisis de componentes principa- les con seis y cuatro bandas de Landsat TM mediante el metodo Crosta, con fines de realzar y discriminar las areas impregnadas
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