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Hydrothermal Alteration zones detection in Limnos Island, through the application of Remote Sensing


Abstract and Figures

In this study we use Landsat 8 OLI satellite imagery in order to identify and map altera-tion zones in Limnos island (N. Aegean, Greece). Pre-processing included sea and vege-tation 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 tec-tonic 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
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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
Anifadi A.1, Parcharidis Is.1 and Sykioti O.2
1Harokopeio University of Athens, Department of Geography, Εl. Venizelou 70, 176 71, Athens,
2Institute for Astronomy, Astrophysics, Space Applications & Remote Sensing Vas. Pavlou & I. Metaxa,
GR-15 236 Penteli, Greece,
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.
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 1 - Coastal
0.43 - 0.45
Band 2 - Blue
0.45 - 0.51
Band 3 - Green
0.53 - 0.59
Band 4 - Red
0.64 - 0.67
Band 5 - Near
Infrared (NIR)
0.85 - 0.88
Band 6 - SWIR 1
1.57 - 1.65
Band 7 - SWIR 2
2.11 - 2.29
Band 8 -
0.50 - 0.68
Band 9 - Cirrus
1.36 - 1.38
Band 10 -
Thermal Infrared
(TIRS) 1
10.60 - 11.19
Band 11 -
Thermal Infrared
(TIRS) 2
11.50 - 12.51
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
reflectance through atmospheric correction of the initial image.
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
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
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
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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The European Union (EU) is highly dependent on critical and rare metals which are very important for a sustainable development. However, European industry is not able to cover its demands from native sources and it imports commodities from third countries. Greece is one of the EU countries with the most potential for supplying these strategic metallic raw materials in the future, since it hosts a large number of ore deposits. The epithermal-and porphyry-type deposits and the reduced intrusion related systems of the Serbomacedonian and the Rhodope metallogenic provinces in Northeastern Greece are promising targets for a future exploitation and exploration in Sb, Te, Mo, Re, Ga, In, REE and PGE. Greece is the leading producer of Ni and Al in the EU from laterites and bauxites of central and northern Greece. These deposits also contain significant amounts of Co or REE which should be considered in the future plans of the processing industries. REE are found in high contents at the placer deposits between Chalkidiki and Kavala (North Greece) and elevated PGE concentrations are associated with the chromitites of northwestern Greece. Therefore, the mineral wealth of Greece can contribute significantly to a sustainable and a competitive economy of Europe.
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The Kehdolan area is located at 20 kilometers to the south-east of Dozdozan Town (Eastern Azarbaijan Province). According to structural geology, volconic rocks are situated in Alborz-Azarbyjan zone, and faults are observed in the same direction to this system with SE-NW trend. The results show that kaolinite alteration trend with Argilic and propylitic veins is the same direction with SW-NE faults in this area. Therefore, these faults with these trends can be considered as the mineralization control for determination of the alterations. Different image processing techniques, such as false color composite (FCC), band ratios, color ratio composite (CRC), principal component analysis (PCA), Crosta technique, supervised spectral angle mapping (SAM), are used for identification of the alteration zones associated with copper mineralization. In this project ASTER data are process and spectral analysis to fit for recognizing intensity and kind of argillic, propylitic, philic, and ETM+ data which are process and to fit for iron oxide and relation to metal mineralization of the area. For recognizing different alterations of the study area, some chemical and mineralogical analysis data from the samples showed that ASTER data and ETM+ data were capable of hydrothermal alteration mapping with copper mineralization. Copper mineralization in the region is in agreement with argillic alteration. SW-NE trending faults controlled the mineralization process.
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The Kahramanmaraş Foreland Basin (KFB) margins are formed by Paleozoic-Mesozoic limestone, metamorphic and ophiolite. This basin contains sandstone and claystone alternations with lesser amounts of reefal limestone, channelized conglomerates, debrites and slump deposits. An irregular topography and absence of path obscured the accurate and complete mapping of the basin. Thus, Landsat TM images were used. The RGB 751 false colour composites were chosen due to abundant claystone and limestone content of the KFB. The RGB PCA 145 was preferred due to high loading of TM bands 5 (clay and iron oxide mineral), 7 (carbonate mineral), 3 and 1 (albedo and topography). The RGB 5/7-5/1-4 (light green dots in 5/7 indicate enhanced claystone area and OH- bearing alteration zone; red dots in 5/1 indicate enhanced iron oxide bearing alteration zone; fourth band for water bearing minerals) shows the transportation way of sediments. The OH- and iron oxide bearing alteration zones are found on older source rocks and younger deposition area. Main lithological boundary, geometry and provenance properties of the KFB were satisfactorily obtained from the Landsat TM images. However, low spatial resolution (30 m) prevents detail facies discrimination that still requires hard field working and/or detailed satellite images.
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Occurrences of halloysite-rich material in altered volcanic rocks, principally trachyandesites, dacites, and tuffs, extend over an area of ∼1 km2 in the southwestern part of Limnos, Island, northeast Aegean Sea, Greece. The present study was designed to investigate the alteration processes which acted on the biotite in these volcanic rocks, to describe in detail the mechanism of formation of the halloysite, and to specify the mechanisms of formation of anatase during the alteration processes. Samples were examined using polarized-light microscopy, X-ray powder diffract ion, scanning electron microscopy, scanning electron microscopy-energy dispersive spectroscopy, and Fourier-transform-Raman techniques. The extensive alteration of the parent rocks, triggered by the circulation of hydrothermal fluids through faults and fractures, resulted in the alteration of biotite to halloysite. Six stages of alteration were recognized. Nanoparticles of halloysite were initially formed on the mica layers, which progressively grew through short-tubular to well formed tubular halloysite, with increasing alteration. In the most altered samples, laths and interconnected laths with the composition (Al3.96Fe0.04)Si4O10(OH)8, were the dominant halloysite morphologies. Anatase was encountered as an alteration product of both ilmenite and biotite. Ilmenite was altered to anatase and Fe oxides. The altered ilmenite crystals constrained most of the newly formed anatase within the space occupied previously by ilmenite, leading to the formation of skeletal anatase. The layered structure of the micas was the main factor governing the morphology of newly formed anatase developed outside ilmenite margins in the form of layers parallel to those of mica. An unusual ring-like structure of anatase was thought to be the result of the uncommon alteration of inner parts of mica folia to tubular halloysite oriented perpendicular to the mica layers. The detachment of the halloysite tubes by circulating hydrothermal fluids was considered to be the reason for the creation of holes which were subsequently surrounded by the anatase ring forms.
We studied the applicability of data from the recently launched Landsat-8 for mapping hydrothermal alteration areas and lithological units associated with porphyry copper exploration in arid and semi-arid regions. Sar Cheshmeh copper mining district in the Urumieh-Dokhtar volcanic belt in south-eastern Iran was selected for a case study. Several red–green–blue colour combination images and specialized band ratios were prepared from Landsat-8 bands. Band ratios derived from image spectra (4/2, 6/7, 5 and 10 in red–green–blue) allow identification of altered rocks, lithological units and vegetation at regional scale. Analytical imaging and geophysical hyperspectral analysis processing methods and mixture tuned matched filtering were applied to Landsat-8 bands to identify alteration zones associated with known porphyry copper deposits. Fieldwork, previous remote sensing studies and laboratory analysis were used to verify the image processing results. We conclude that Landsat-8 bands, especially bands 2 and 4 in the visible and near-infrared, 6 and 7 in the shortwave infrared and 10 in the thermal infrared spectra, contain useful information for porphyry copper exploration. The thermal infrared bands of Landsat-8 significantly improved the quality and availability of remote-sensing data for lithological mapping. The results of this investigation should encourage geologists to use Landsat-8 operational land imager and thermal infrared sensor data for porphyry copper exploration and geological purposes. Keywords: Landsat-8; Operational land imager; Thermal infrared sensor; Hydrothermal alteration mapping; Porphyry copper exploration
This study used the ability of remote sensing technology to identify and map the lithological units and alteration zones in a gold mining area in North-eastern Sudan by using Landsat 8 data source. The Landsat data series has been used widely in mapping lithological and altered rocks and in geology in general. The study area contains three gold mines part of Ariab mining district in Red Sea Hills, Northeastern Sudan. There are three types of gold deposits in the study area (Supergene deposits, polymetallic massive sulphide deposits and The Ganaet deposits) are being mined in Hadal Auatib mine, Hassai mine and Kamoeb mine. The objective of this study was to find new high potential areas for gold mineralization in the area. Conventional image processing methods such as (color composite, principle component analysis and band ratio) and minimum noise fraction have been used in this study for the purpose of lithological and alteration zones mapping. The visible and short infrared region was useful for mapping the iron oxides and the clay minerals, in which the thermal bands were used for silicate mapping. The results of this study showed the distribution of the lithological units and the hydrothermal alteration zones along with new high potential areas for gold mineralization which can be used in the future and proved the ability of Landsat data in mapping these feature.
Trachyandesite rocks, occurring over an area of about 1 km2 in the southwest part of Limnos Island, Greece, are altered mainly to halloysite. The samples were collected and analysed by polarizing microscopy, powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and chemical analysis. The alteration of plagioclase to halloysite follows seven discrete stages that are described in detail. The geochemical evaluation of the data shows enrichment of the light REE (LREE) over heavy REE (HREE) as expressed by the (La/Yb)n ratio. The ΣLREE range from 206.44 to 272.30, while the sum of HREE varies from 11.01 to 26.26. The (La/Yb)n ratio ranges from 9.72 to 27.64. Fractionation among LREE expressed as (La/Sm)n and between middle REE (MREE) and HREE is shown as (Tb/Yb)n ratios. The most altered rocks close to the fault zone have high (Tb/Yb)n ratios and low (La/Sm)n and Eu/Eu* ratios. Although mineralogy and clay mineral textures indicate hydrothermal genesis of halloysite, the geochemical data are not conclusive due to a secondary weathering effect.
Remote sensing is the science of acquiring, processing, and interpreting images and related data, acquired from aircraft and satellites, that record the interaction between matter and electromagnetic energy. Remote sensing images are used for mineral exploration in two applications: (1) map geology and the faults and fractures that localize ore deposits; (2) recognize hydrothermally altered rocks by their spectral signatures. Landsat thematic mapper (TM) satellite images are widely used to interpret both structure and hydrothermal alteration. Digitally processed TM ratio images can identify two assemblages of hydrothermal alteration minerals; iron minerals, and clays plus alunite. In northern Chile, TM ratio images defined the prospects that are now major copper deposits at Collahuasi and Ujina. Hyperspectral imaging systems can identify individual species of iron and clay minerals, which can provide details of hydrothermal zoning. Silicification, which is an important indicator of hydrothermal alteration, is not recognizable on TM and hyperspectral images. Quartz has no diagnostic spectral features in the visible and reflected IR wavelengths recorded by these systems. Variations in silica content are recognizable in multispectral thermal IR images, which is a promising topic for research.
Reduced intrusion-related gold systems are generally characterized by a Au-Bi-Te-W metal assemblage genetically linked to the emplacement of granitoids. The Palea Kavala ore system, Greece, consists of ~150 minor Fe-Mn (PbyZnyAg), Fe-Mn-Au, Fe-As-Au, Fe-Cu-Au, and Bi-Te-Au ore occurrences that occur primarily in quartz-calcite-sulfide veins (hypogene mineralization), or as supergene bodies, in overlapping zones centered on the ~21-22 Ma granodioritic Kavala pluton, which intrudes metamorphic rocks of the Paleozoic Rhodope metamorphic core complex. The pluton consists mostly of granodiorite with lesser amounts of diorite, tonalite and monzodiorite, which was emplaced along the regional E-W trending Kavala-Komotini fault. The recently discovered, ~4 km long, E-W trending so-called Kavala vein is a sheeted quartz vein system of Bi-Te-Pb-SbyAu mineralization that crosscuts the Kavala pluton and the schists and gneisses of the Rhodope Massif. The Kavala vein system is comprised of quartz with lesser amounts of K-feldspar, plagioclase, and muscovite. Quartz-sericite-pyrite alteration is pervasive but minor kaolinite is also present. Pyrite (~5% of vein volume) contains inclusions of tetradymite (some gold-bearing), bismuthinite, and cosalite. Sulfur isotope values (n = 27) of pyrite from the Kavala and Chalkero veins, as well as pyrite and galena from Garizo Hill Fe-Mn-Pb vein range from -1.9 to 1.0 per mil (with one outlier of -4.6 per mil) and suggest a magmatic sulfur source. Homogenization temperatures (Th) of type I (two-phase aqueous liquid-vapor) and type II (three-phase, H2O-CO2-rich) fluid inclusions that homogenize into the liquid phase in quartz from the Kavala and Chalkero veins range from 216.0y to 420.0yC (n = 216) and 255.7y to 414.0yC (n = 112), respectively. The Th of type III (two-phase aqueous liquid-vapor), which homogenize into the vapor phase, ranges from 210.4o to 323.4yC (n = 28). The salinities of type I and type II inclusions range from 15.9 to 22.6 wt. % NaCl equiv. and 5.5 to 11.2 wt. % NaCl equiv., respectively. Eutectic temperatures of -58.5o to -44.3yC for type I inclusions suggest the presence of appreciable CaCl2 in addition to NaCl. Clathrate melting temperatures for type II inclusions of ~-56.7yC indicate that CO2 is the major component of the gaseous phase. The presence of a zoned metallogenetic district centered on Bi-Te-Pb-SbyAu mineralization within the Kavala pluton and the two high-temperature, high-salinity, immiscible carbonic and aqueous fluids associated with the Kavala and Chalkero veins are consistent with them being part of a reduced intrusion-related gold system. The Fakos porphyry Cu-(Mo) and epithermal-style Au-Te deposit, Limnos Island, Greece, is hosted in ~20 Ma quartz monzonite and shoshonitic subvolcanic rocks that intruded Paleogene sedimentary basement rocks. Metallic mineralization formed in three stages in quartz and quartz-calcite veins. Early porphyry-style (Stage 1) metallic minerals consist of pyrite, chalcopyrite, galena, bornite, sphalerite, molybdenite and iron oxides, which are surrounded by halos of potassic and propylitic alteration. Stage 2 mineralization is composed mostly of quartz-tourmaline veins associated with sericitic alteration, whereas stage 3, epithermal-style mineralization is characterized by polymetallic veins containing pyrite, chalcopyrite, sphalerite, galena, enargite, bournonite, tetrahedrite-tennantite, hessite, petzite, altaite, an unknown cervelleite-like Ag-telluride, Au-Ag alloy, and native Au. Stage 3 veins are spatially associated with argillic, silicic, and alunitic alteration. Fluid inclusions in quartz from stage 1 (porphyry-style) mineralization contain five types of inclusions. Type I, liquid-vapor inclusions, which homogenize at temperatures ranging from 189.5 to 403.3yC have salinities of 14.8 to 19.9 wt. % NaCl equiv. Type II, liquid-vapor-NaCl, type III liquid-vapor-NaCl-CaCl2, and type IV, liquid-vapor-hematiteyNaCl homogenize to the liquid phase at temperatures of 209.3y to >410.0y C, 267.6y to >410.0y C, and 357.9y to >410.0y C, respectively. The porphyry-style inclusions are associated with type V, vapor-rich inclusions. Stage 3 quartz contains two types of fluid inclusions, type I, liquid-vapor inclusions that homogenize to the liquid phase (191.6y to 310.0y C) with salinities of 1.40 to 9.73 wt. % NaCl equiv., and type II, vapor-rich inclusions. Mixing of magmatic fluids with meteoric water in the epithermal environment is responsible for the dilution of the ore fluids in stage 3 veins. Eutectic melting temperatures of -35.4y to -24.3yC for type I inclusions in both porphyry and epithermal veins suggest the presence of CaCl2, MgCl2, and/or FeCl2 in the magmatic- hydrothermal fluids. Sulfur isotope data from sulfides show a range in δ34S of -6.82 to -0.82 per mil and values overlap for porphyry and epithermal sulfides, which suggest a common sulfur source for the two styles of mineralization. The source of sulfur in the system was likely the Fakos quartz monzonite for which the isotopically light sulfur isotope values arose from changes in oxidation state during sulfide deposition (i.e. boiling) and/or from disproportionation of sulfur-rich magmatic volatiles upon cooling. It is less likely that sulfur was derived from the reduction of seawater sulfate or leaching of sulfides from sedimentary rocks given the absence of primary sulfides in sedimentary rocks in the vicinity of the deposit. Petrological, mineralogical, fluid inclusion, and sulfur isotope data indicate that the mineralization at Fakos Peninsula represents an early porphyry system that is transitional to a later intermediate- to high-sulfidation epithermal gold system. This style of mineralization is similar to porphyry-epithermal mineralization found elsewhere in northeastern Greece (e.g. Pagoni Rachi, St. Demetrios, St. Barbara, Perama Hill, Mavrokoryfi, and Pefka).