hydrothermal alteration

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;

140

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 absorption 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

141


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.

142


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


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

143

Iron oxide and hydroxyl enhancement JAG l Volume 2 -Issue 2 - 2000

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


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

144


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

145


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 composants 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 dhydroxyle, 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 indica 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 transformados 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 probable de mineral de hierro en el area de Mashayekh-Nowdan, al oeste de Shiraz, se ensayo un analisis de componentes principales con seis y cuatro bandas de Landsat TM mediante el metodo Crosta, con fines de realzar y discriminar las areas impregnadas

146

Documents

hydrothermal alteration