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Δελτίο της Ελληνικής Γεωλογικής Εταιρίας, τόμος L, 2016
Πρακτικά 14ου Διεθνούς Συνεδρίου, Θεσσαλονίκη, Μάι ος 2016 Bulletin of the Geological Society of Greece, vol. L, 2016
Proceedings of the 14th Intern. Congress, Thessaloniki, May 2016
HYDROTHERMAL ALTERATION ZONES DETECTION IN
LIMNOS ISLAND, THROUGH THE APPLICATION OF
REMOTE SENSING
Anifadi A.1, Parcharidis Is.1 and Sykioti O.2
1Harokopeio University of Athens, Department of Geography, Εl. Venizelou 70, 176 71, Athens,
Greece, parchar@hua.gr, alexandra.tragana@gmail.com
2Institute for Astronomy, Astrophysics, Space Applications & Remote Sensing Vas. Pavlou & I. Metaxa,
GR-15 236 Penteli, Greece, sykioti@noa.gr
Abstract
In this study we use Landsat 8 OLI satellite imagery in order to identify and map alteration
zones in Limnos island (N. Aegean, Greece). Pre-processing included sea and vegetation
masking. In order to enhance spatial resolution, data fusion to 15m is performed. A
lineament map is extracted from the panchromatic image that gives the general tectonic
view of the island. The detection and mapping of alteration minerals is performed using
specific band ratios and consequent composite images. The colour composite using bands
10, 11, 7 (RGB) show the spectral signature and general distribution of silica. Band ratios
6/7, 4/2, 6/5, reveal alteration zones containing iron oxides, clay alteration and ferrous
minerals correspondingly. The aforementioned analysis has shown that hydrothermally
alteration areas in Limnos are located in the west part of the island and at the Fakos
Peninsula, Sardes, Roussopouli and Paradeisi hill. These areas are compared and
validated with the reported field work. We conclude that hydrothermal alteration zones
can indeed be detected and mapped using medium resolution satellite multispectral data.
However, for the identification and mapping of specific types of rocks and minerals, a
sensor with high spectral resolution is required.
Keywords: Satellite data, Landsat 8 OLI, Miocene Volcanism.
Περίληψη
Στην παρούσα εργασία αναλύθηκαν τα δεδομένα του θεματικού χαρτογράφου Landsat 8, με
σκοπό την διάκριση και αναγνώριση ζωνών υδροθερμικής εξαλλοίωσης στην ευρύτερη
περιοχή της νήσου Λήμνου. Η προ-επεξεργασία των δορυφορικών δεδομένων αφορούσε την
δημιουργία μάσκας της βλάστησης και της θάλασσας. Για να βελτιωθεί η χωρική διακριτική
ικανότητα στα 15 m έγινε συγχώνευση δεδομένων. Ένας χάρτης γραμμώσεων παρήχθη από
την πανγχρωματική εικόνα προσδίδοντας τη γενική τεκτονική άποψη του νησιού. Η
ανίχνευση και η χαρτογράφηση των εξαλλοιωμένων πετρωμάτων πραγματοποιήθηκε
χρησιμοποιώντας λόγους καναλιών και ακολούθως σύνθετων ψευδέγχρωμων εικόνων. Η
ψευδέγχρωμη εικόνa 10, 11, 7 (RGB) δείχνει την φασματική υπογραφή και την κατανομή
των πυριτικών ορυκτών. Οι λόγοι καναλιών 6/7, 4/2, 6/5, αποκαλύπτουν ζώνες εξαλλοίωσης
που περιέχουν οξείδια του σιδήρου, αργιλική εξαλλοίωση, και σιδρούχα (Fe 2+) ορυκτά. Η
ανάλυση έδειξε ότι οι υδροθερμικά εξαλλοιωμένες περιοχές στη νήσο Λήμνο τοποθετούνται
στα δυτικά της Λήμνου, στη χερσόνησο του Φακού, στις Σάρδες, στο Ρουσσοπούλι και
περιμετρικά στο λόφο Παραδείσι. Αυτές οι περιοχές συγκρίθηκαν και τεκμηριώθηκαν με
εργασίες πεδίου που πραγματοποιήθηκαν από προηγούμενους ερευνητές. Συνοψίζοντας, οι
υδροθερμικές ζώνες εξαλλοίωσης μπορούν να εντοπιστούν και να χαρτογραφηθούν
χρησιμοποιώντας μέτριας ανάλυσης δορυφορικά πολυφασματικά δεδομένα. Ωστόσο, για την
αναγνώριση και χαρτογράφηση συγκεκριμένων τύπων ορυκτών και πετρωμάτων, απαιτείται
ένας αισθητήρας υψηλής φασματικής ανάλυσης.
Λέξεις κλειδιά: Δορυφορικά δεδομένα, Landsat 8 OLI, Μειοκαινική ηφαιστειότητα.
1. Introduction
The goal of the present paper is to detect the hydrothermal alteration zones in Limnos Island, N. Aegean,
through the application of Landsat 8 OLI band ratios. The key elements in mineral exploration are to
gain understanding of geologic area through lithological mapping and to assist in defining target areas of
potential mineral interest. Remote sensing can assist and provide valuable information in bedrock
mapping, detection, identification and estimating affluence of specific minerals at a specific scale
(Ahmed and Beiranvand Pour, 2014; Sabins, 1999; Parcharidis et al., 1998; Hunt, 1977). In Limnos
Island, field geological studies have referred the detection of hydrothermal alteration zones in several
sites like Sardes, Roussopouli and Fakos peninsula (Papoulis et al., 2014; Fornadel, 2010; Papoulis et
al., 2009; Skarpelis and Voudouris, 1998). Positive results can be obtained using band rationing and false
colour using these ratios. Limnos island is located at the North Aegean sea in Greece. The island occupies
476 Κm2 with a coastal line of 260 Km.To sum up the hydrothermal alteration zones can indeed be
detected and mapped using medium resolution satellite multispectral data but it is not possible to identify
and map specific types of rocks and minerals.
2. Geologic Settings
Figure 1- Digitized geologic map of Limnos island (after IGME scale 1:50000).
2.1 Stratigraphy
Limnos is an island of Greece in the northern part of the Aegean Sea. The principal town of the island
and seat of the municipality is Myrina. The island is mostly flat (hence its more than 30 sand beaches),
but the west, and especially the northwest part, is rough and mountainous. The main gulfs are Moundros
and Pournia, but the rock weathering creates many coves. The areas with high frequency, a dense
hydrographical system and big slope use to be faults with direction NW-SE, NE-SW. Generally, at the
center and east of the island the rocks are permeable (Quaternary) so the hydrographical system is poor.
On the other hand, at the rest of the island the hydrographical system is dense (volcano rocks).
2.2 Geology
The geology of Limnos Island is characterized by a sedimentary background which is a basin-fill
succession. Hydrothermal alteration in the island is linked to early Miocene volcanism that occurred in
the broader north-eastern Aegean Sea and Western Turkey. The remnants of large stratovolcanoes are
present in specific areas in the islands of Lesvos, Limnos and Samothraki and in western Turkey. The
sedimentary rocks are flysch and molasse. They were deposited in NE-SW trending postorogenic basin
that formed as a result of normal faulting and extension during postorogenic collapse of the Rhodope-
Sakarya zone and were slightly folded prior to igneous activity. The Tertiary sedimentary basement rocks
can be delineated into two discrete units, the Upper Unit and the Lower Unit. The Upper Eocene to lower
Oligocene Lower Unit covers the majority of the island and is composed of siliclastic continental slope
deposits including conglomerates, sandstones, mudstones, claystones and turbidites. The lower
Oligocene Upper Unit has been deposited in a shallower environment than the Lower Unit. Lower in its
section, the Upper Unit is composed of marine and brackish fluviodeltaic sediments including
interbedded claystones and sandstones , sandstones and sandy limestones. Towards the top of its
exposure, the Upper Unit is composed of terrestrial fluvial sediments including conglomerates and
sandstones. The volcanic centers are located in the western and southwestern portions of the island where
volcanic rocks overlie the sedimentary basement. The presence of the volcanic centers is delineated by
domes and lava flows that are accompanied by lesser agglomerate. The sedimentary basement is exposed
at the surface in the east and northeast of the island, distal to the volcanic centers. The volcanic rocks
are divided into three units: Katakolon, Romanou and Myrina. These rocks are early Miocene (21-18
Ma) and demonstrate a calc-alkaline to shoshonitic affinity. The lower-most Katakolon unit consists of
NW-SE trending K-rich andesitic to dacitic lavas. In places, it is interbedded with or is crosscut by
andesitic lava flows monomineralic breccias, sills and E-W trending dikes. Andesite and dacite in the
Katakolon unit yielded a K-Ar age of 20-21 Ma. The Katakolon unit is overlain by the Romanou unit
that is composed of K-rich dacites and latites. At its base, the Romanou unit is dominated by ligh- colored
lithic and pumice-rich pyroclastic flows that are up to 160 m thick. To the west and upsection, the
pyroclastic flows of the Romanou unit are intercalated with volcanic breccias, banakitic lavas, airfall
tuffs and terrigenous sediments. Ignimbrites and andesites from the Romanou unit yielded K-Ar ages of
19.8 Ma. The uppermost Myrina unit, overlies Romanou unit and is composed of K-rich dacite with
lesser amounts of andesite and trachyte, which are associates with monomineralic breccias, lava lows,
and lahars. Lavas, dackites and andesites of the Myrina unit yielded K-Ar ages of 19.3 to 18.2 Ma. Both
volcanic and sedimentary basements of Limnos Island are overlain by a Pliocene to recent alluvial
sedimentary unit that is composed of conglomerates, calc-arenites and sandstones. Faults and joints cut-
cross both sedimentary and igneous rocks. The axes trend E-W and WSW-ENE to the WSW. Folding
does not affect the Miocene volcanic rocks. Limnos Island is located in the area of Aegean Sea that is
characterized by a moderate positive heat flow anomaly. This anomaly, in conjunction with active hot
springs found on the island, indicate that a steepened thermal gradient has persisted even after the end of
observable igneous activity on the island (Skarpelis and Voudouris, 1998; Fornadel, 2010). The presence
of hot springs on the island implies that brittle structures on the island play a role in conducting fluids
(Fornadel, 2010).
2.3 Metal bearing
The area of Fakos peninsula is dominated by two large hills, the western Tourlida Hill and the Petrospitos
Hill both of which are 300 m in elevation. Their prominence is controlled by subvolcanic intrusions that
were emplaced into the sedimentary host rocks. The sedimentary basement on Fakos peninsula is
composed largely of medium -grained quartz - rich sandstones that were subjected to, and cemented by
a hydrothermal silicification event. Finely disseminated sulfides are pervasive in these sandstones. The
extrusive rocks on Fakos Peninsula range from shoshonitic andesites (latites) to trachyandesites and
trachytes. Towards the central portion of Fakos peninsula, the extrusive sedimentary basement rocks
were intruded by subvolcanic microporphyritic quartz monzonite. Much of the southwestern portion of
Limnos Island was subject to hydrothermal alteration along fault zones. Four discrete zones of
hydrothermal alteration based on clay mineralogy were defined as smectite, illite, hallousite and
kaolinite-dickite zones. The Fakos Peninsula like the rest of Limnos island is crosscut by many major
NE-SW and ENE-WSW-trending faults. These structures controlled the emplacement of the subvolcanic
bodies and facilitated the flow of hydrothermal –magmatic fluids that were responsible for metallic
mineralization. Fakos quartz monzonite and adjacent host rocks were locally subjected to intense
hydrothermal alteration. In silica zones there is a metal bearing, to the south of the area and especially to
the quartz veins within the sandstones and to the quartz monzonite. At the western metal bearing zone in
the quartz veins with direction N55ο W και N70οE. The east metal bearing zone, large 1 Km and wide
10m, is located about 600 m SE of the central zone and it is characterized by the major concentration of
gold of the Fakos Peninsula. In the Sardes area, a system of quartz is developed inside the subvolcanic
and sandstones and is connected with faults systems of direction Ν55ο W, Ν70ο Α (like in Fakos area).
In the Roussopouli area: three zones of black silification are observed in the volcanic breccias. The
silification is opaline and is connected with sericitic alteration of the adjacent rock. The zone of alunite
is developed up from the silicification zone. The metal-bearing includes pyrite, marcasite and veinlets of
silica (Voudouris and Skarpelis, 1998).
3. Materials and Methods
3.1. Remote sensing data
Landsat 8 is the new product from NASA under Landsat open source series which has been launched in
February 2013. Landsat 8 data consist of 11 bands; 5 in the visible and Near-Infrared (VNIR), 2 in the
Thermal Infrared (TIR) region of the electromagnetic spectrum, 2 in the Shortwave Infrared (SWIR)
region, and 1 panchromatic band (band 8). The spatial resolution is 15 m for the panchromatic band, 30
m for VNIR and SWIR bands, and 100 m for the TIR bands. Two additional bands represent the
difference between Landsat8 and the previous product (Landsat ETM+), a deep blue coastal / aerosol
band and a shortwave-infrared cirrus band (table 1).
Table 1 Bands of Landsat 8.
BAND
WAVELENGHT
(micrometers)
RESOLUTION
(meters)
Band 1 - Coastal
aerosol
0.43 - 0.45
30
Band 2 - Blue
0.45 - 0.51
30
Band 3 - Green
0.53 - 0.59
30
Band 4 - Red
0.64 - 0.67
30
Band 5 - Near
Infrared (NIR)
0.85 - 0.88
30
Band 6 - SWIR 1
1.57 - 1.65
30
Band 7 - SWIR 2
2.11 - 2.29
30
Band 8 -
Panchromatic
0.50 - 0.68
15
Band 9 - Cirrus
1.36 - 1.38
30
Band 10 -
Thermal Infrared
(TIRS) 1
10.60 - 11.19
100
Band 11 -
Thermal Infrared
(TIRS) 2
11.50 - 12.51
100
The Landsat 8 Oli image which has been used in this study cloud free acquired on August 16 2014. The
sun azimuth is about Β139ο and the sun elevation about 57ο.
3.2. Image processing
The initial DN values were converted to reflectance through atmospheric correction of the initial image.
In Fig. 1 a flowchart showing with the processing steps is presented.
Figure - Flow chart: processing steps.
The following four images were then produced.
A colour composite image with the spectral bands 7, 5, 3 (RGB) in order to distinguish the lithological
units from the vegetation (Fig. 2)
Image Landsat 8 Oli
L1
pre-processing of the image: converting the digital numbers of the image to
reflectance through atmospheric correction of the initial image.
Lineaments
(faults)
Data fusion (Panchromatic and
multispectral bands)
Detect silica and no silica
False composite color image
(FCC) 10,11,7 (RGB)
Color Composite image of
Band Rationing (CCR)
6/7, 4/2, 6/5 (RGB)
Analyse the landscape
False composite color image
(FCC) 5,7,3 (RGB)
Sea mask
Mask NDVI
Band Ratio Spectral Bands
6/7, 4/2, 6/5
Detection of hydrothermally altered zones
Figure 2- colour composite image 7, 5, 3 (RGB).
The Normalized Difference Vegetation Index (NDVI) in order to discern the vegetation distribution
and consequently mask of vegetated areas.
A colour composite image using the thermal bands tir1-tir2-swir2 (10-11-7) (RGB) for silicate
mapping (Fig.3).
Figure 3 - Silica distribution. Silica rocks yellow colour and no silica purple (10-11-7) (RGB).
The colour composite ratio image 6/7, 4/2, 6/5 (RGB). The ratio 6/7 reveals clays, the 4/2 iron oxides
and the 6/5 the ferrous (Fe2+) minerals. This colour composite was produced after data fusion data
(15m) (Fig. 4) and without data fusion data (30m) (Fig. 5).
Figure 4 - composite ratio image 6/7 (clays) 4/2 (FeO) 6/5 (Ferrous) (RGB). After data fusion
(15m)
Figure 5 - composite ratio image 6/7 (clays) 4/2 (FeO) 6/5 (Ferrous) (RGB). Without data fusion.
(30 m) the black area is the vegetation mask and the sea mask.
Figure 6 - Left: the main hydrothermal alteration zone in Fakos Peninsula. Right: Limits of
hydrothermally alteration zones in Fakos Penisnula by Fornadel, 2010.
The lineaments that were extracted from the panchromatic image (band 8) provided the general tectonic
view of the island.
Figure 7 – Left: Lineaments issued from the panchromatic image of Landsat 8 OLI. Right: The
lineament directions are shown in the rose diagram (by demo surfer 11 golden).
4. Results and Discussion
The colour composite image 7, 5, 3 (RGB) can give an analysis of the landcover. The red and brownish
areas correspond to vegetation, while the white colour corresponds to rocks and soil. Generally, the
vegetation in the island is very low, and as it is shown in fig. 2 the vegetation extends mainly in drainage
systems and hills. The colour composite image 10,11,7 (RGB) shows the spectral signature and
distribution of silicate. Yellowish and yellow colours correspond to areas with silica presence and purple
to areas of absence of silica as it is shown in Fig. 2. The Figures 4, 5 and 6 show the areas with
hydrothermal alteration zones.
• Alteration in Fakos Cape: A zone about 16 km at the south of Fakos cape which separates the tints of
red-yellow from tints of blue-cyan. Inside the alteration zone the tints of yellow shows the high
reflectance at bands 6 and 4 and low reflectance at other bands 2, 7, 5. The different tints of yellow
indicates the predominance one of the two kind of rock (iron oxides-clay minerals) (Parcharidis et al.,
1998). The tints of blue-cyan perimetricaly external of the alteration zone are referring to ferrous-iron
oxide (high reflectance to band 4). The geological bedrock is characterized by ploutonian rocks that have
intruded in the sediments age Upper Eocene-Oligocene and present strong alteration (granites, syenites,
diorites, biotite). The alteration products of trachiandesites are clays and iron oxides (serikite, chlorite,
calcite, iron oxides). Also, a smaller alteration zone (~3 km) exists at NE of Fakos cape and the geological
bedrock consists of trachiandesites. Unfortunately because of the low spectral analysis of Landsat 8 OLI
it is impossible to detect and map specific rocks. Furthermore, the mask of vegetation does not allow the
possibility of vegetation existence where there is a high reflectance in 5 and 6 bands. Also in that area
the rocks have impregnated with iron oxides (geological map of IGME) which means high reflectance at
band 4. Fornadel, 2010 has mentioned the existence of metallic minerals in the western ore zone that
have been oxidized, as denoted by the abundance of iron oxides (i.e. limonite, goethite) in the vein
material, and largely disseminated in the vein matrix, although locally it forms ≥1 cm wide aggregates
or infillings. The metallic minerals include pyrite, chalcopyrite, sphalerite, galena, arsenopyrite,
tetrahedrite, bournonite, hessite, altaite, and native gold. These minerals can be detected with
hyperspectral data. At the NE in Fakos cape there is cyan colour (width ~ 300m and length ~ 2 km) due
to the fact that there are iron oxides according to the geological map of IGME. The geological bedrock
consists of silificated volcanic rocks whose initial composition has change from the influence of
hydrothermal fluid which are rich to SiO2 through faults. There are reddish lavas because of the abidance
of iron oxides. Fornadel, 2010 mentioned about a veins system which penetrates the silica alteration zone
and the adjacent rocks. The veins consist of small quantities of tourmaline, barite and sericite. Ore
minerals in the eastern ore zone include galena, sphalerite, arsenopyrite, and bournonite. The silica
distinguishment can be confirmed at the map in fig. 3. The alunitic alteration zone, as well as the
topographically higher silicic alteration zone, is crosscut by hydrothermal breccias in which alunitized
rock fragments are surrounded and cemented by iron oxides. Alunite also occurs in veins, which consist
of alunite, sulfur, and tridymite/cristobalite that crosscut sericitized rock in the northern part of the study
area.
• Other alteration in the island: At the NW of the island near Sardes and at the west coast there are small
areas with hydrothermal alteration, as well as, at the east of the island in Roussopouli near to the contact
with the tuffs. Finally, parametrically of Paradeisi hill, is seemed to be an alteration zone around the
vegetation (black color) and at the west Fig.5. At the geological map of IGME this area is characterized
of sediments (Up. Heocene-Oligocene) and at the west there is a contact with trachiandesites. At
Paradeisi hill, geological map shows impregnation of iron oxides and sulfides.
5. Conclusion
The aim of this study was to conduct an investigation using Landsat 8 data and remote sensing techniques
to map the alteration zones in Limnos Island. The outcome of the remote sensing techniques such as
colour composite and band ratios are promising in mapping lithological and altered rocks. Band ratio
technique showed the distribution of the alteration zones. The results showed that Landsat 8 data have
the potential to detect and map hydrothermal alteration zones at a regional scale.
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