mapping lithology of the sarfartoq carbonatite complex, southern west greenland, using hymap imaging...

Remote Sensing of Environment 113 (2009) 1208–1219

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Remote Sensing of Environment

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Mapping lithology of the Sarfartoq carbonatite complex, southern West Greenland,using HyMap imaging spectrometer data

Enton BediniGeological Survey of Denmark and Greenland (GEUS), Øster Voldgade 10, 1350 Copenhagen K, DenmarkInstitute of Geography and Geology (IGG), Øster Voldgade 10, 1350 Copenhagen K, Denmark

E-mail address: [email protected].

0034-4257/$ – see front matter © 2009 Elsevier Inc. Adoi:10.1016/j.rse.2009.02.007

a b s t r a c t
a r t i c l e i n f o
Article history:
The Sarfartoq carbonatite c Received 9 November 2008Received in revised form 10 February 2009Accepted 10 February 2009
Keywords:Lithologic mappingCarbonatitesImaging spectrometryGreenland

omplex occurs in the southern West Greenland in a transition zone betweenArchaean gneiss complex to the south and a Proterozoic mobile belt to the north. The Sarfartoq carbonatitecomplex consists of a core zone composed of dolomite carbonatite and minor søvite (calcite carbonatite)surrounded by a fenite zone and a marginal zone of gneisses frequently altered due to hydrothermal activity.High spatial and spectral resolution imaging spectrometer data recorded by the HyMap imaging systemwereused to map lithology of the Sarfartoq carbonatite complex. A careful analysis of the spectral reflectanceproperties of the carbonatite lithology preceded the HyMap data analysis stage. The spectral reflectancemeasurements showed that the various lithologic units including dolomite carbonatite, søvite, fenite and themarginal alteration zone have distinct spectral reflectance characteristics. The analysis of the HyMap datawas based on an unsupervised clustering algorithm, the Self Organizing Maps (SOM), for the mapping of themain lithology and a hierarchical tree for the mapping of sparsely occurring søvite rocks. Spectral mixtureanalysis was applied to map fractional abundances and compare with the SOM results. The resultinglithological map shows the spatial distribution of dolomite carbonatite, søvite, fenite with abundantcarbonatite dykes (representing the outer core of the carbonatite complex), fenite and hematized gneiss(marginal alteration zone). The results compare well with the field data collected for the assessment of themapping accuracy and due to the spatially contiguous nature of the hyperspectral data could be used tobetter map the outcropping carbonatite lithology. The spectral reflectance measurements and the mappingresults provide information of petrological importance for the carbonatite core zone.

© 2009 Elsevier Inc. All rights reserved.

1. Introduction

The reflectance spectrum in the 0.45- to 2.5 µmwavelength regionprovides mineralogical information based on analysis of electronicabsorption features in transitional metals, especially iron (Clark, 1999;Hunt, 1977), and of molecular absorption features in carbonate,hydrate and hydroxide minerals (Clark, 1999; Hunt, 1977). Imagingspectrometers or ‘hyperspectral’ sensors measuring hundreds ofspectral bands from aircraft and satellite platforms provide uniquespatial/spectral datasets for analysis of surface mineralogy (Goetzet al., 1985; Kruse et al., 2003). Imaging spectrometry or “hyperspec-tral remote sensing” is a promising technology for assisting thegeological mapping and mineral exploration studies of the remoteArctic terrains (e.g., Harris et al., 2005; Rivard & Arvidson, 1992).

An extensive and complex suite of alkaline igneous rocks ofcarbonatitic and kimberlitic affinity is known to occur in the basementrocks of southern West Greenland (Larsen et al., 1983). One of themost important and major carbonatite intrusions is the Sarfartoq

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carbonatite (Secher & Larsen, 1980). The Sarfartoq carbonatitecomplex consists of inner and outer carbonatite core zones, a fenitezone and a marginal alteration zone (Secher & Larsen, 1980). Theinteresting carbonatite lithology and its remote location in the Arcticlatitudes of southern West Greenland make the Sarfartoq carbonatitecomplex a very good target to evaluate the use of imaging spectro-metry for geological mapping of carbonatite complexes and in theArctic environment of West Greenland. The purpose of this paper is toreport on the application of high spatial and spectral resolutionairborne imaging spectrometer data recorded by the HyMap imagingsystem to map lithology of the Sarfartoq carbonatite complex insouthern West Greenland.

2. Geological setting

The Sarfartoq carbonatite complex was emplaced in Neoproter-ozoic time in the transition zone between Archaean gneiss complex tothe south and the Proterozoic Nagssugtoqidian mobile belt to thenorth, in southern West Greenland (Fig. 1A), (Larsen & Rex, 1992;Secher et al., in press; Secher, 1986; Secher & Larsen, 1980). TheSarfartoq carbonatite complex was discovered in 1975 as a result ofregional airborne gamma-ray spectrometry survey carried out by the

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Fig. 1. (A) Geographic and geologic position of the Sarfartoq carbonatite complex in southern West Greenland (modified from Allaart, 1982). (B) Geological map of the Sarfartoqcarbonatite complex (modified from Secher, 1986). Used with permission from the Geological Survey of Denmark and Greenland. The rectangle indicates the approximate spatialextent of the HyMap data analyzed in this study.

1209E. Bedini / Remote Sensing of Environment 113 (2009) 1208–1219

Geological Survey of Greenland (Secher, 1976). The Sarfartoqcarbonatite is an intrusive conical body with a core of carbonatitesheets surrounded by a marginal zone of hematized gneiss withcarbonatite dykes (Secher & Larsen, 1980), (Fig. 1B). The carbonatitecore zone consists of an inner core zone (N50% carbonatite), an outer

core zone (b50% carbonatite) and a narrow rim of fenite surroundingthis (Fig. 1B). The predominating carbonatite type in the core zone isdolomite carbonatite. Søvite rocks (calcite carbonatite) occur onlysporadically as discrete layers in the inner core zone and in schlieren inthe outer core zone (Secher & Larsen, 1980). The main minerals in the

Fig. 2. Photo (facing north) of the outcroppingpart of the Sarfartoq carbonatite complex, along the Paradise Valley (ArnangarnupQoorua), southernWest Greenland. Theheight of the slopeis ca. 400–500 m. Photo by the author.

1210 E. Bedini / Remote Sensing of Environment 113 (2009) 1208–1219

core zone are ferroan dolomite, ankerite, phlogopite, alkali amphibole,magnetite, and apatite. The most frequently found fenite is a light-greyaegirine bearing fenite. Themarginal alteration zone consists of gneissesfrequently altereddue to hydrothermal activity and cataclastic deforma-tion of varying intensity (Secher & Larsen, 1980). Locally largeradioactive shear zones 50–200 mwide are found oriented tangentiallyto the core. They consist of strongly limonitized, hematized, jointed andcrushed gneisses (Secher, 1986).

Most of the rocks of the carbonatite core zone are covered byglaciofluvial sediments in the floor of the Paradise Valley (the Green-landic name for this valley is ArnangarnupQoorua). The peripheral partsof the carbonatite core are exposed in the slopes of the valley (Fig. 2). Inthis study the focus is at the outcroppingpart of the carbonatite complexnorth of the Paradise Valley. All recognized zones of the Sarfartoqcarbonatite complex (except radioactive shear zones) are exposed forabout 5 km along the slopes of the valley within the study area.

3. HyMap data

The HyMap® is an airborne imaging system developed by IntegratedSpectronics, Sydney, Australia, and operated by HyVista Corporation. Itconsists of sensors located on a fixed wing aircraft typically flown at analtitude of 2.5 km. The sensors collect reflected solar radiation in 126bands covering the 0.45–2.5 µmwavelength range, including the visibleto near infrared (VNIR) and short-wave infrared (SWIR) regions of theelectromagnetic spectrum (Table 1), (Cocks et al., 1998).

Table 1HyMap instrument specifications (Cocks et al., 1998).

Spectral module Wavelength range (µm) Bandwidth (nm) Spectral sampling (nm)

VIS 0.45–0.89 15–16 15NIR 0.89–1.35 15–16 15SWIR1 1.40–1.80 15–16 13SWIR2 1.95–2.48 18–20 17

IFOV: 2.5 mrad along track; 2.0 mrad across trackFOV: 60° (512 pixels)Swath: 2.3 km at 5 m IFOV; 4.6 km at 10 m IFOV

IFOV, instantaneous-field-of-view; FOV, field-of-view; mrad, milliradian.

The Sarfartoq HyMap scenes are part of the HyperGreen-2002campaign of the Geological Survey of Denmark and Greenland(Tukiainen & Thorning, 2005). The Sarfartoq HyMap scenes wererecorded on 09.08.2002 in 126 narrow bands, from 0.44 to 2.48 µmwith 4 m nominal pixel size. This study is based on two HyMapsubscenes covering the outcropping part of the Sarfartoq carbonatitecomplex. The spatial extent of the HyMap subscenes is shownmosaicked in Fig. 3. The Sarfartoq HyMap data were geometricallyand atmospherically corrected using the software PARGE and ATCOR4.The parametric geocoding procedure (PARGE) reconstructs thescanning geometry for each image pixel using position, attitude andterrain elevation data (Schläpfer & Richter, 2002). The AirborneAtmospheric and Topographic Correction Model (ATCOR4) performsthe combined atmospheric/topographic correction accounting for theangular and elevation dependence of the atmospheric correctionfunctions and calculates surface reflectance (solar spectral wave-length region) based on the geocoded and orthorectified imagery(Richter & Schläpfer, 2002).

4. Lithologic composition and spectral reflectance properties

Spectral reflectance measurements were considered important tounderstand the spectral reflectance of the carbonatite rocks andanalyze the HyMap data. A collection of rock samples collected in thecourse of previous geological studies of the Sarfartoq carbonatitecomplex (Secher, 1986; Secher & Larsen, 1980; Nielsen, 1982) wasacquired from the rock samples' archive of the Geological Survey ofDenmark and Greenland. This made possible a detailed study of thespectral reflectance properties of the carbonatite rocks before the fieldactivities. Reflectance spectra of more than forty rock samples fromthe Sarfartoq carbonatite complex were recorded using an AnalyticalSpectral Device (ASD), which records 2151 channels within the 350-to 2500 nm wavelength region.

4.1. Carbonatites

Carbonatites by definition are igneous rocks that contain more than50% modal carbonate minerals. Dolomite-rich carbonatites are calleddolomite carbonatiteswhereas the term søvite is used for coarse grained

Fig. 3. The spatial extent of the HyMap data used in this study, covering the outcropping part of the Sarfartoq carbonatite complex north of the Paradise Valley in the southern WestGreenland. The boundaries of the geological map of the carbonatite complex are shown for reference with Fig. 1. White circles show the locations of the field stations used to assessthe accuracy of the mapping results. The rectangle (white dashed line) indicates the subset of the image analyzed using the spectral mixture analysis.


calcite carbonatites (Winter, 2001). Several studies have considered thespectral properties of carbonate minerals (Gaffey, 1985; Gaffey, 1986,1987;Hunt,1977;Hunt& Salisbury,1971) in thewavelengths coveredbymost remote sensing instruments. A study on the spectral reflectance ofthe carbonatite rocks has been reported by Rowan et al. (1986).

Spectra of powders of carbonate minerals containing no transi-tional metal cations are nearly straight lines near unity reflectance atwavelengths shorter than 1.6 µm (Gaffey, 1986). At wavelengthsN1.6 µm there is a series of absorptions due to vibrational processes ofthe carbonate anion (e.g., Gaffey, 1985). Gaffey (1986) observed thatin general absorption bands in dolomite spectra are centered atshorter wavelengths than the equivalent absorption bands in calcitespectra.

Reflectance spectra of dolomite carbonatites and søvites from theSarfartoq carbonatite core zone are shown in Fig. 4A. The dolomitecarbonatites (Fig. 4A, DLC1, DLC2, DLC3) display characteristicdolomite absorption features with themain CO3

−2 absorption centerednear 2.32 µm. The reflectance spectra DLC1 andDLC2 are characterizedby broad ferrous (Fe2+) absorption feature in the 1.0–1.3 µm region(Gaffey, 1986). Increase in intensity of this broad band is positivelycorrelated with increasing Fe2+ content in dolomites or calcites(Gaffey, 1986). However, in the carbonatite samples with limoniticcoating the intensity of the Fe2+ absorption feature diminishes (e.g.,spectrum DLC3). The søvite spectrum (Fig. 4A, SØV1) has the maincarbonate absorption feature at 2.333µm. In the visible to near infrared(VNIR) wavelength region, in addition to the broad ferrous-ironabsorption feature this spectrum exhibits characteristic Rare EarthElements (REE) absorptions at 0.58 µm, 0.74 µmand 0.80 µmwhich are

generally attributed to electronic transitions in Nd3+ contained in rareearth element-bearing minerals (Rowan et al., 1986). The cause of asimilar absorption feature in the same spectrum at 0.62 μm has notbeen determined.

The HyMap image spectra of the carbonatites (Fig. 5A) despite thelower spectral resolution are similar to the reflectance spectraacquired in laboratory, except for the atmospheric obscuration regionsaround 1.40 µm and 1.90 µm. The HyMap image spectrum DLC4(Fig. 5A) from an erosive area within the core zone of the carbonatiteis characteristic for the ferroan dolomite carbonatite, exhibiting thebroad Fe2+ absorption feature in the 1.0–1.3 µm wavelength region.The spectrum DLC5 (Fig. 5A) is typical for the inner core zone of thecarbonatite representing ferroan dolomite carbonatite with limoniticcoating. The spectral reflectance features of the limonite (e.g., Hunt,1977; Hunt and Ashley, 1979) are combined with the spectralreflectance features of the ferroan dolomite carbonatite in the VNIRwavelength region. It is to be noted that the limonitic coating (whichis ubiquitous at the Sarfartoq carbonatite core zone) does notinfluence the SWIR region of the spectrum and the main dolomiteabsorption at 2.32 µm. The HyMap image spectrum SØV2 (Fig. 5A)which represents a søvite zone, is similar to the spectrumDLC5, exceptfor the main carbonate absorption that is located at the HyMap bandrecording at 2.34 µm, indicative of the presence of calcite. Rare EarthElements (REE) absorptions have not been noted in the HyMap imagespectra of the carbonatites in the study area. This could be related tothe level of concentration of the REE, their narrow absorption featuresand spectral resolution of the hyperspectral data. Absorption featuresdue to very high concentrations of Light Rare Earth Elements in

Fig. 4. (A) Representative reflectance spectra of rock samples of dolomite carbonatite (DLC1; DLC2; DLC3) and søvite (SØV1). (B) Representative reflectance spectra of rock samplesfor fenites (FEN1; FEN2), carbonatite and fenite (SIL-CRB1) and hematized gneiss (HEM+SER). The vertical short arrows show important absorption features in the adjacent spectra.


carbonatites have been mapped in AVIRIS data of the Mountain Pass,California carbonatites (Rowan & Mars, 2003).

4.2. Fenites

An almost universal characteristic of carbonatite complexes is thepresence of a distinctive metasomatic aureole in which the wall rockshave been converted to aegirine-rich and alkali amphibole-rich rocksand in some cases to K-feldspar rich rocks (Winter, 2001). Thesemetasomatic rocks are commonly called fenites and the processfenitization (Winter, 2001). Rowan et al. (1995) showed reflectancespectra of fenites from the Iron Hill carbonatite complex, Colorado,displaying an Mg–OH doublet absorption feature due to amphiboleand weathered fenites displaying only weak Al–OH due to illite and/orMg–OH absorption features caused by biotite or chlorite. The feniterocks spectrally analyzed in this study, in the SWIR wavelength regiondisplay Mg–OH absorption features (Fig. 4B; FEN1 and FEN2) that arecentered near 2.32 µm and 2.38 µm. These absorption features areattributed to the alkali amphibole phase present in the fenites accordingto petrographic descriptions (Nielsen, 1982). In some cases (Fig. 4B;FEN2) the Mg–OH doublet absorption feature is associated with ashallowAl–OHabsorption featurenear2.20µmdue to sericite. The latteris attributed to the surface weathering of the feldspar. Both reflectancespectra (FEN1 and FEN2) display a rapid fall-off in intensity from 2.0 µmto the blue due to broad Fe2+ and Fe3+ absorptions (e.g., Hunt &Salisbury, 1970; Hunt et al., 1973).

The HyMap image spectra from zones where field mapping hasrecorded fenite in the boundary between the carbonatite core zone andthemarginal alteration zone show both anMg–OH doublet absorptionfeature and an Al–OH absorption feature in the SWIR region (Fig. 5B;

FEN3 and FEN4). The spectra exhibit a rapid fall-off of the spectrum inthe VNIR region due to combinations of broad Fe2+ and Fe3+

absorption bands (e.g., Hunt & Salisbury, 1970). However, fenitesoften show ferric-iron absorption features in the VNIR wavelengthregion.

4.3. Outer core zone (fenites with carbonatite dykes)

The outer core zone of the Sarfartoq carbonatite consists offenitized basement rocks intersected from abundant carbonatitedykes that make up between 10 and 50% of the rock (Secher & Larsen,1980). The spectral reflectance properties of these rocks represent amixture of the spectral reflectance of the carbonatite and fenite. In theSWIR wavelength region the spectral reflectance of the rocks of thiszone is characterized by CO3

−2 absorption features of the carbonateminerals combined with Mg–OH absorption features of the fenite orfenitized country rocks (Fig. 4B, SIL-CRB1; Fig. 5A, SIL-CRB2). As theabundance of the carbonate phase decreases the Mg–OH absorptionfeatures dominate the reflectance spectrum. The increase in the fenitecontent is also associatedwith the reduction of the characteristic broadferrous-iron absorption of the ferroan dolomite in the VNIR region ofthe reflectance spectrum.

4.4. Country rocks

The country rocks consist mainly of granitic gneisses. Spectral ref-lectance properties of these rocks vary from featureless to displayingAl–OH features due to muscovite and/or Fe,Mg–OH features due tobiotite and chlorite (Fig. 5B). Field observations also showed that thecountry rocks are extensively covered from lichen, usually of black

Fig. 5. Representative HyMap image spectra of the main lithologies of the Sarfartoq carbonatite complex. (A) Ferroan dolomite carbonatite displaying very intense ferrous-iron absorptionfeature (DLC4), ferroan dolomite with limonitic coating (DLC5), søvite (SØV2), carbonatite and fenite (SIL-CRB2). (B) Fenites (FEN3; FEN4), hematite and sericite (HEM+SER) from themarginal alterationzone, gneiss (GNEISS), carbonate andgreenvegetation (VEGETATION+CARBONATE). The vertical short arrows show important absorption features in the adjacent spectra.


colour. This is also evident in the image pixel spectra especially by thepresence of the characteristic broad cellulose absorption feature oflichens around 2.10 µm (e.g., Ager & Milton, 1987).

4.5. Marginal alteration zone

The marginal alteration zone is distinguished by the strong hema-tization/limonitization of the gneisses. In the VNIR region, thereflectance spectra of these rocks display intense ferric-iron absorp-tion features due to hematite or limonite (e.g., Fig. 4B, HEM+SER).This spectrum (Fig. 4B, HEM+SER) in the SWIR region exhibits Al–OHabsorption features at 2.20 µm, 2.35 µm and 2.45 µm due to sericite.Depending on the composition of the gneiss, in the SWIR regionspectra from these rocks often display both Al–OH absorption featuresand Fe,Mg–OH absorption features due to chlorite and biotite.

TheHyMap image spectra from themarginal alteration zone (Fig. 5B,HEM+SER) display ferric-iron spectral features in the VNIR region andAl–OH absorption feature at 2.20–2.22 µm. These are sometimesassociated with an absorption feature at 2.25 µm which could beattributed to the presence of an Fe,Mg–OH phase probably chlorite orbiotite, based on field observations.

4.6. Vegetation cover

In the plateau (see Fig. 2), tundra vegetation and the presence oflichens constitute the major challenges for the remote sensing appliedto lithologic mapping. On the other hand, the high levels of the slope,where is also the best exposure of the carbonatite complex (see Fig. 2),are practically devoid of vegetation cover. In general even in theplateau the lichen cover is minimal on the carbonatites probably dueto the chemical composition. Sparse vegetation cover associates debrisof carbonatite material and weathered surfaces in the lowest parts of

the carbonatite core zone outcrop. The HyMap reflectance spectrafrom these areas show often carbonate absorption features in theSWIR wavelength region and green vegetation spectral reflectancefeatures superimposed on the VNIR wavelength region of thespectrum (Fig. 5B) as indicated by the characteristic chlorophyllabsorption at about 0.68 µm (Knipling, 1970).

5. Methods of HyMap data analysis

5.1. Unsupervised classification of HyMap data to map lithology of theSarfartoq carbonatite complex

Due to the low sun angle in high geographic latitudes, areas inshadowandpresence ofwater bodies, theHyMapdata in several parts ofthe study areawere dominated from noise. To avoid artefacts from darkpixels in the data analysis stage and to focus at the rock outcrop, pixelswith low reflectance (less than 5%) and pixels with high NDVI-scorewere masked out. The HyMap data analysis was based in the 0.45–2.48μm spectral region. The HyMap bands which measure close or in theatmospheric water absorption zones around 1.40 µm and 1.90 µmwereexcluded from further analysis. As the study area is covered from twoHyMap flights, due to different image statistics the subscenes wereanalyzed separately. A spectral reduction and data compression wasperformed using the Minimum Noise Fraction (MNF) transformation(Green et al.,1988). TheMNF is a form of principal components analysisbut instead of ordering thedata in terms of variance the data are orderedbased on the signal to noise ratio (Green et al., 1988). The first 20 MNFbands were used as input to an unsupervised artificial neural network,the Kohonen self-organizing maps (SOM).

The SOM belongs to a class of unsupervised artificial neuralnetworks which are trained by competitive learning (Kohonen, 1982;Kohonen, 2001). A brief description of the SOM given below is based

Fig. 6. HyMap image spectra (mean normalized) used as reference spectra in the spectralmixture analysis. Image spectraMAZ-1 andMAZ-2 represent themarginal alteration zone.Gapsnear1.40 and1.90 µmdue todeletion of channels affectedbyatmospheric absorption.Note that the MNF transformed spectra were used in the spectral mixture analysis.


on a summary by Canty (2006). The network consists of an input layerand an output layer of neurons. The input signal (in our case a pixelspectrum in MNF space) is represented by the vector x=(x1, x2… xn)T

where “T” denotes transpose and “n” is the number of bandsconsidered. The vectors {x(i) |i=1… p} (where “p” is the number ofall pixel spectra used in the training process) are used as training datafor the neural network. Each neuron is associated with a synapticweight. The components of the vectorwk=(wk1, wk2… wkn)T are thusthe synaptic weights of the kth neuron. The initial synaptic weightsfor each neuron are usually randomly chosen from the data. When atraining vector x is presented to the network, the neuron whoseweight vector is most similar to x (usually the Euclidian Distance isused as a measure of similarity) is called the “winner”. Then theweight vector of the winner neuron (k⁎) is shifted in the direction ofthe training vector:

wk⁎ i + 1ð Þ = wk⁎ ið Þ + η x ið Þ− wk⁎ ið Þ½ � ð1Þ

wherewk⁎(i+1) is the adjustedweight vector of thewinner neuron. Theparameter η is called the learning rate of the network. In order for thismethod to function it is necessary to allow the learning rate to decreasegradually during the training process. A convenient function for this is:

η ið Þ = ηmaxηmin

ηmax

� �i=p

: ð2Þ

During the learning phase not only the winner neuron but also theneurons in its “neighbourhood” are moved in the direction of thetraining vectors:

wk i + 1ð Þ = wk ið Þ + η ið Þλ k⁎; kð Þ x ið Þ− wk ið Þ½ �; k = 1 N M ð3Þ

where λ(k⁎, k) defines a neighbourhood function for thewinner neuronon the network of neurons usually a Gauss function of the form:

λ k⁎; kð Þ = exp −d2 k⁎; kð Þ= 2σ2h i

: ð4Þ

The d2(k⁎,k) is the square of the distance between neurons k⁎ andk. The extent of the neighbourhood is allowed to shrink steadily:

σ = σmaxσmin

σmax

� �i=p

: ð5Þ

Typically the neighbourhood is initially the entire network andtoward the end of the training is very localized.

Despite the design of the SOM as an algorithm for the visualizationof similarities in a multi-dimensional space, successful applications ofthe SOM as a classification tool of remote sensing imagery have beenshown (Gonçalves et al., 2008; Ji, 2000; Penn & Livo 2001; Villmanet al., 2003; among others). In this study the SOM was based on twodimensional eight-by-eight hexagonally oriented units, implementedusing the kohonen package (Wehrens & Buydens, 2007) in the Renvironment for statistical computing (R Development Core Team,2007). A key point in the application of the SOM was its implementa-tion in theMNF space (e.g., Penn& Livo, 2001). All the imagepixels thatwere notmasked in the pre-processing stepwere used as training data(input layer) for the SOM. The start value for the learning rate wasηmax=0.05 and the value to stop ηmin=0.01. The number of iterations(i.e. the number of times the data set is presented to the SOM)was 100.In this implementation (kohonen package in the R) the size of theneighbourhood decreases linearly during training such that after one-third of the iterations only thewinning unit is being adapted (Wehrens& Buydens, 2007). The resulting clusters were labelled (using ENVI™)based on their mean spectrum (in the original hyperspectral space)interpreted in accordance with the spectroscopic knowledge gainedfrom the reflectance spectroscopy stage. Spectrally similar classesweremerged together.

5.2. Methods to map the søvite zones of the carbonatite

An important objective of geological mapping and economicgeology studies of carbonatite complexes is the mapping of dolomitecarbonatite and søvite (calcite carbonatite). The Sarfartoq carbonatitecomplex is predominantly composed of dolomite carbonatite withminor søvite (Secher and Larsen, 1980). As discussed in Section 4.1positions of carbonate absorption bands in the SWIR region ofreflectance spectra can be used to distinguish between dolomiteand calcite (e.g., Gaffey, 1985). Reflectance spectra obtained fromhyperspectral imaging sensors allow mapping of the surface spatialdistribution of these minerals over large areas, providing informationwhich could not be easily obtained from field observations. Thehierarchical tree approach has been shown successful for thedifferentiation between dolomite and calcite (e.g., Kruse, 1990;Kruse et al., 1990). The hierarchical tree used here consists of a two-step procedure:

(1) mapping of the carbonatite class (obtained from the SOMclassification);

(2) in the region defined from step one, in the HyMap reflectancedata for every pixel spectrum the following rule is applied: if thestrongest carbonate absorption feature is located at the HyMapband recording at 2.32 µm then it is dolomite carbonatite; else ifthe strongest carbonate absorption feature is located at theHyMap band recording at 2.34 µm then it is søvite.

The second step is similar to one of the nodes of the hierarchicaltree used by (Kruse et al., 1990) for the identification of calcite anddolomite.


5.3. Spectral mixture analysis (SMA)

In order to obtain fraction abundances of the classes of interest andcompare the SOM thematic mapping results with the spectral unmixingapproach, the spectral mixture analysis (Settle & Drake, 1993) wasapplied to analyze a part of the study area (see Fig. 3) noted for thelithologic transition carbonatite inner core zone→carbonatite outer corezone→ fenite→marginal alteration zone. The spectral mixture analysis(SMA) was applied as follows. Pixels with low reflectance (less than 5%)and pixels with high NDVI-score were masked out. The HyMapreflectance data in the 0.45–2.48 µm region were mean normalized(i.e. each pixel spectrumwas divided by its mean). This form of norma-lization eliminates the effects of different albedos in the spectralunmixing results (e.g., Berman et al., 2004). Minimum Noise Fraction(MNF) transformation (Green et al., 1988) was applied to the meannormalized data. Image derived reference spectra (Fig. 6) representingcarbonatite, fenite and the marginal alteration zone (two referencespectra were selected for the latter) were used as input to the spectralmixture analysis. The SMA was applied (using ENVI™ linear spectralunmixing) in the20firstMNF bands space. The application of the SMA ina subset of MNF bands is of advantage, as noise isolated in the excludedMNF bands does not influence the spectral unmixing (e.g., Nielsen,2001). The sum of the fractions was not constrained. The fractions

Fig. 7. Classification results of the HyMap data for the main lithologic units of the Sarfartoq cashown for reference with Fig. 1. White circles show the locations of the field stations used tothe spatial extent of Fig. 9.

produced from the spectral mixture analysis were filtered using a 3×3median filter.

6. Results and discussion

6.1. Mapping lithology of the Sarfartoq carbonatite complex

The result of the SOM-based unsupervised classification of theHyMap data was a classified image in seven classes which representthe following lithologic units and rock–vegetation mixed spectralclasses: carbonatite, carbonatite outer core zone (fenite withcarbonatite dykes), fenite, marginal alteration zone (hematizedgneiss), gneiss, CLASS-1, and CLASS-2 (Fig. 7). The SOM clusters thatmapped pixels dominated by the spectral response of lichen or by thecombined spectral response of lichen and green vegetation (that hadnot been masked in the pre-processing step) are not shown in Fig. 7.

The carbonatite class mapped areas with a clear carbonate (over-whelmingly dolomite) spectral response in the SWIRwavelengths. In theVNIR region the image spectra from these areas display ferric-ironabsorption features of the limonitic coating combined with the broadferroan-iron absorption at 1.0–1.3 µm, similar to the spectra shown in thesection of reflectance spectroscopy. A class that mapped erosive areaswithout limonitic coating in the carbonatite inner core zone (mainly

rbonatite complex. The boundaries of the geological map of the carbonatite complex areassess the accuracy of the mapping results. The rectangle (white dashed line) indicates


along the gullies) exhibiting typical ferroan dolomite absorption features(similar to spectrum DLC4 in Fig. 5A) is merged in this class as well. Theresults show that carbonatite rocks dominate the central part of thecarbonatite core zone outcrop but extend somewhat more (up to the topof the slope) thanpreviouslymapped (the inner coreboundary inFig.1B).The results for the fenite class (distinguished by the Mg–OH doubletabsorption feature often combined with Al–OH absorption at around2.20 µm) are in agreement with the map of the carbonatite complex(Fig. 1B) in the south-western part of the image where there is also thebest outcrop of the fenite zone. The rocks of the outer core zone have thespectral characteristics of amixture between carbonatite and theMg–OHsilicate phase in fenites. Classification results show that these rocksoutcrop in the western and eastern part of the carbonatite core zone aswell as in several areas in thenorthwithin the carbonatite outer core zoneof (Fig.1B). Field observations, and by interactively checking pixel spectrafrom the HyMap image cube showed a very high accuracy of theclassification for the delineation of the spatial extent of the outcroppingcarbonatite inner core and outer core zones. Such a detailed mapping ofthe carbonatite rocks outcrop is a direct contribution of the remotesensing approach to the study of the carbonatite. Themarginal alterationzone is spectrally distinguished by the ferric-iron spectral features in theVNIR region and Al–OH absorptions at 2.20–2.20 µm in the SWIR region,due to sericite. The marginal alteration zone is well mapped at theoutcropping part along the valley slope (Fig. 2) and numerous alteredspots have been mapped in the plateau. Gneiss often mixed with lichenwas mapped in other parts of the study area. In the lowest parts of thecarbonatite core zone vegetation is more abundant as well as looseweatheredmaterial. These areas aremapped in CLASS-1. Amiscellaneousspectral class (CLASS-2) is recognized at the top of the outcroppingcarbonatite. In general it has strong ferric oxide/hydroxide spectralreflectance features and exhibits often Mg–OH absorption features. Few

Fig. 8. Thematic map showing the distribution of dolomite carbonatite and søvite at the outanalysis of the hyperspectral data.

pixels within this area were mapped in the carbonatite outer core zoneclass. Field observations showed that this area represents a weatheredcrust over the carbonatite outcrop combined with not in-situ materialand it is not relevant for the study of the carbonatite lithology.

6.2. Mapping the søvite zones

The map of søvite and dolomite carbonatite is shown in Fig. 8. Thethematic mapping results indicate limited presence of søvite in theareamapped as the carbonatite inner core and outer core zones. This isin agreement with previous studies (Secher & Larsen, 1980; Secher,1986) that report dolomite carbonatite predominance at the out-cropping part of the Sarfartoq carbonatite core zone. However, there isa tendency for more abundant presence of søvite in the lowest parts ofthe carbonatite inner core zone. Especially the carbonatite outcrops atlocations ‘A’ and ‘B’ (Fig. 8) are rich in søvite.

6.3. Results of spectral mixture analysis

The results of spectral mixture analysis (SMA) for carbonatite,fenite, marginal alteration zone (hematized gneiss) as well as a colourcomposite of these are shown in Fig. 9. The fraction image of themarginal alteration zone (Fig. 9) is the sum of the fraction abundancesproduced from the SMA for the reference spectra MAZ-1 and MAZ-2(Fig. 6). The SMA produced very good results for the carbonatite class.The fenite zone and the marginal alteration zone are well mappedwithin the exposed part of the carbonatite complex along the valley.Themost important result of the SMA is the mapping of the outer corezone of the carbonatite consisting of fenitized country rock andcarbonatite dykes. This lithology is distinguished by the imageanalysis as a mixture of fenite and carbonatite (yellow in the colour

cropping inner and outer core zones of the Sarfartoq carbonatite complex, based on the

Fig. 9. Fraction abundances produced from the SMA for (A) carbonatite, (B) fenite, (C) marginal alteration zone and (D) a colour composite (carbonatite = red, fenite = green,marginal alteration zone = blue). Note the outer core zone of the carbonatite complex in yellow as a mixture of fenite and carbonatite. The boundaries of the geological map aresuperimposed on the colour composite. The image covers an area of 2.56×2.56 km. (For interpretation of the references to colour in this figure legend, the reader is referred to theweb version of this article.)


composite). It is to be noted the high similarity of the results ofSpectral Mixture Analysis and the SOM classification.

6.4. Accuracy assessment

Field work at the Sarfartoq carbonatite complex to evaluate theaccuracy of the lithological mapping results was carried out in August,2008. Eighty-one reference sites representative of carbonatite (dolo-mite carbonatite and søvite), outer core zone (fenite with abundantcarbonatite dykes), fenite, marginal alteration zone (hematizedgneiss) and gneiss were field checked (Fig. 7). The location of eachof these reference sites was determined using GPS measurements.Rock samples were collected from almost one-third of the referencesites, especially from the sites mapped as søvite from the remotesensing image analysis.

The relationship between ground reference information and thethematic classification map (Fig. 7) of the hyperspectral data usingthe SOM algorithm is summarized in the confusion matrix (Table 2).The confusion matrix (sometimes referred to as the error matrix)provides the basis on which to both describe classification accuracyand characterize the errors (Congalton & Green, 1999). The overall

accuracy is 87.6%. The confusion matrix shows that carbonatite(undifferentiated between dolomite carbonatite and søvite) is thebest mapped class with some confusion with the outer core zoneclass. The fenite class has error of commission with the outer corezone and gneiss classes (i.e. outer core zone and non-fenitized gneissrocks wrongly have been mapped as fenite). The marginal alterationzone has no error of commission but the error of omission in whichthree sites that belong to the marginal alteration zones have beenincluded in the gneiss class indicates that this zone is morewidespread than mapped from the hyperspectral data.

Twelve sites mapped from the analysis of the HyMap data as søvite(calcite carbonatite) were field checked and samples were collectedfor further analysis. One of the concerns before the field work stagewas that the spectral response of calcite could be due to mono-hydrocalcite (a weathering mineral in this Arctic environment); inthis way the mapping of the calcite mineral would have nopetrological significance. However, in all the field-checked sites,which were mapped as søvite from the hyperspectral data analysis,was found a white variety of carbonatite (Fig. 10). Rock samplescollected from these twelve sites showed strong effervescence withdilute hydrochloric acid (HCl), a characteristic of the calcite mineral as

Table 2Confusion matrix for the field observations versus image classification results.

Reference data

Carbonatite Outer corezone (fenite+carbonatitedykes)

Fenite Marginalalterationzone

Gneiss Rowtotal

MAP Carbonatite 28 2 0 0 0 30Outer corezone (fenite+carbonatitedykes)

1 18 0 0 0 19

Fenite 0 2 8 0 1 11Marginalalterationzone

0 0 0 10 0 10

Gneiss 0 0 1 3 7 11Column total 29 22 9 13 8 81

Overall accuracy=71/81=87.6%Khat coefficient of agreement=83.6%Users' accuracy Producer's accuracyCarbonatite=28/30=93.3% Carbonatite=28/29=96.5%Outer core zone=18/19=94.7% Outer core zone=18/22=81.8%Fenite=8/11=72.7% Fenite=8/9=88.9Marginal alteration zone=10/10=100%

Marginal alteration zone=10/13=76.9%

Gneiss=7/11=63.6% Gneiss=7/8=87.5%


opposed to dolomite. Spectral reflectance measurements on thesesamples showed that the carbonate main absorption feature variesbetween 2.33 and 2.34 µm. X-ray diffraction (XRD) analyses carriedout on five of these samples indicated that the calcite is thepredominant mineral in four of them, although dolomite is alsopresent in all of these samples. The XRD analysis of one sampleindicated an almost equal abundance of calcite and dolomite. As themajority of the field-checked sites are located at the area “A” (Fig. 8)more field checking is needed for the zones mapped as søvite in otherparts of the carbonatite.

Fig. 10. Field photograph of søvite rocks mapped from

7. Conclusions

The results of this study show that analysis of high spatial andspectral resolution HyMap imaging spectrometer data can providedetailed maps of surface lithology/mineralogy for outcroppingcarbonatites and their associated rocks. Through an image processingstrategy based on a careful analysis of rock spectral reflectancemeasurements, the inner carbonatite core zone, the outer carbonatitecore zone, the fenite zone and the marginal alteration zone of theSarfartoq carbonatite complex in southern West Greenland weremapped. The mapping of the spatial distribution of the søvite rockswithin a predominantly dolomite carbonatite core zone has igneouspetrology significance, indicating søvite-rich zones in the lower levelsof the inner core zone of the Sarfartoq carbonatite complex. Theresults obtained from the remote sensing approach especially for thecarbonatite inner and outer core zones have the potential to improvethe geological mapping of the carbonatite complex, by providingspatially contiguous mineralogic and lithologic information, which forinaccessible areas cannot be effectively obtained in any other way.

Acknowledgments

This study was funded by the Geological Survey of Denmark andGreenland (GEUS). I thank Leif Thorning, Karsten Secher, TapaniTukiainen, Thorkild M. Rasmussen (all with GEUS) and Birger UlfHansen (Institute of Geography and Geology, University of Copenha-gen) for helpful discussions and their continuous support for thisstudy. Hudson Resources Inc. is thanked for the hospitality at their“Garnet Lake” field camp in southern West Greenland and for thelogistic support of the field work. I thank Karsten Secher (GEUS) forthe geological field guide to the Sarfartoq carbonatite complex. I thankLlewellyn Pilbeam (GEUS) for his assistance during the field work atthe Sarfartoq carbonatite complex. The HyMap data were geome-trically corrected by Tapani Tukiainen (GEUS). The spectral reflectancemeasurements of rock samples were carried out at the spectrallaboratory of the Geological Survey of Finland (GTK). I thank Leah

the hyperspectral data at the area “A” (see Fig. 8).


Roach (with Planetary Geosciences Group of Brown University, USA)and three anonymous reviewers for their helpful comments andsuggestions on the manuscript.

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