Part2, Workshop on Greenland's diamond potential

G E U S

Indicator mineral signatures in basal till surrounding
the Lahtojoki and Seitaperä kimberlites, eastern
Finland

Lehtonen M., Marmo, J. & Nissinen, A.

Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland

Abstract

Since the 1980's diamondiferous kimberlitic rocks have been discovered in eastern Finland.
In order tOFurther the ongoing diamond exploration in the country, the Geological Survey of
Finland carried out detailed heavy mineral surveys of basal till around two of the known
kimberlitic bodies. The Lahtojoki kimberlite in Kaavi and the Seitaperä kimberlite in Kuhmo
have different geological and mineralogical characteristics. The kimberlitic indicator grains
down-ice from the Lahtojoki kimberlite form a well-defined fan, extending for at least 2 km.
The fan has been formed as a net effect of two known ice-flow events in the region. The
indicator dispersal trail from the Seitaperä kimberlite is shorter (~1 km) and more vague
than that at Kaavi. This is mainly due to the low indicator content in the Seitaperä kimber-
lite, as well as a large population of background chromites in till. These chromites are
probably derived from the Archean Kuhmo greenstone belt. Results of this study contribute
to the overall understanding of the Quaternary history of the Kaavi and Kuhmo areas, and
provide tools to diamond exploration in glaciated terrains.

1 Introduction

A commonly used diamond exploration method in recently glaciated terrains is to track
kimberlitic indicator mineral grains dispersed in glacial sediments (e.g. Gurney, 1984; At-
kinson, 1989; McClenaghan & Kjarsgaard, 2001) . The classic indicator suite includes Cr-
pyrope, Ti-pyrope and eclogitic garnet, Cr-diopside, Mg-ilmenite (picroilmenite) and high-
Cr, high-Mg chromite.

Diamondiferous kimberlites occur in eastern Finland around the Kaavi-Kuopio and Kuhmo
areas (Tyni, 1997; O'Brien & Tyni, 1999) . Major part of the Karelian Craton is, in fact, pro-
spective for diamond based on the empirical evidence necessary for diamond preservation
(Fig. 1). Active diamond exploration in Finland has been ongoing for over two decades
(Tyni, 1997) but systematic glacial dispersal studies around the known kimberlites have not
been published to date.

In 2001-2003 the Geological Survey of Finland (GTK) carried out sampling of Quaternary
till around two kimberlitic bodies in Finland, the Lahtojoki pipe (Pipe 7) in Kaavi and the
Seitaperä dyke swarm (Dyke 16) in Kuhmo. The selection of targets was based on their
different geological and mineralogical characteristics, as well as differences in the sur-
rounding bedrock and Quaternary deposits. The objective was to study their indicator min-

G E U S

eral signatures in the surrounding basal till. The ultimate aim of the work was to gather in-
formation that can be applied to diamond exploration in similar areas. This study has been
described in greater detail in Lehtonen et al. (in press)

Figure 1. Map illustrating the diamond prospective area of Fennoscandia characterized by
a) low heat flow (Kukkonen & Jõeleht, 1996) and b) lithosphere thicker than 170 km based
on seismic data (Calcagnile, 1982). The Archean/Proterozoic boundary marks the subsur-
face extent of the Archean craton. The black diamonds represent diamond-bearing kimber-
lite/lamproite clusters. The localities of this study are underlined.

2 Targets of study

2.1 The Lahtojoki kimberlite

The Lahtojoki kimberlite in Kaavi belongs to the 600-Ma Kaavi-Kuopio Kimberlite Province
situated at the edge of the Karelian Craton, see Figure 1 (Tyni, 1997; O'Brien & Tyni,
1999; O'Brien et al., 2005) . At least 19 kimberlite pipes have been emplaced into Archean
(3.1-2.6 Ga) basement gneisses and Proterozoic (1.9-1.8 Ga) metasediment sequences
(Kontinen et al., 1992)

The bedrock in Kaavi is mainly composed of mica schists, gneisses and intercalated black
schists (Huhma, 1975) . The Quaternary deposits consist of basal till (e.g. drumlin fields),
ablation till, outwash and glaciolacustrine sediments. The ablation till unit commonly rests
directly upon the basal till, altogether forming a till cover that rarely exceeds 3 m in thick-
ness. There are two known ice flow directions in the region (Hirvas & Nenonen, 1987) , the
older from 280° to 100° and the younger (main) from 335° to 155°.

G E U S

Figure 2. Pie graph showing the relative abundances of indicator minerals in the 0.25-1.0
mm fraction of: a) the Lahtojoki kimberlite; b) the Seitaperä kimberlite; c) till samples from
Lahtojoki; and, d) till samples from Seitaperä. Visual identification of indicator minerals was
confirmed using SEM-EDS and/or electron microprobe.

The 2-ha Lahtojoki kimberlite is a classical Group I kimberlite. The kimberlite is mainly
composed of macrocrystal tuffisitic kimberlite and subordinate tuffisitic kimberlite breccia
with rare hybabyssal kimberlite. An average diamond grade of 26 ct/100 t has been re-
ported for the pipe (Tyni, 1997) . The indicator minerals are abundant and include Mg-
ilmenite, Cr-pyrope, Ti-pyrope, eclogitic garnet and Cr-diopside (Fig. 2). The body is cov-
ered by 13-20 m of glacial deposits and peat.

2.2 The Seitaperä kimberlite

The Seitaperä kimberlite is located in Kuhmo (Fig.1), 200 km NE from Kaavi, closer to the
center of the Karelian craton. The bedrock in the area is composed of Archean gneisses,
migmatites and granites (3.1-2.6 Ga) through which runs the N-S trending Kuhmo green-
stone belt (3.0-2.7 Ga) (e.g. Piirainen, 1988) . The district is characterized by similar type
and age of Quaternary deposits as Kaavi, although there are some regional distinctions.
The glacial stratigraphy in the area consists of basal till covered by ablation till and/or a
widespread unit of glaciolacustrine sediments (Saarnisto et al., 1980) . The main ice flow
direction in the region was from 300° to 120°. The till cover usually exceeds 3 m in thick-
ness.

The Seitaperä kimberlite has characteristics of both olivine lamproite and Group II kimber-
lite (O'Brien & Tyni, 1999). The rock is mainly (~70%) composed of phlogopite microphe-
nocrysts while macrocrysts other than olivine are virtually absent. The kimberlite is known
to contain diamonds at extremely low contents, 1 ct/100 t. The SW-NE kimberlite dyke

G E U S

swarm intrudes a 300 x 600 m area, with approximately 20% of the area being kimberlitic.
The thickness of glacial sediments varies between 3 and 12 m.

Ar-Ar measurements from phlogopite microphenocrysts have resulted in age of ca. 1200
Ma for the kimberlite (Hugh O'Brien, personal communication). The most abundant indica-
tor mineral in the relatively indicator-poor kimberlite is chromite with rare pyrope and Cr-
diopside (Fig. 2).

3 Methodology

3.1 Sampling areas

3.1.1 Lahtojoki, Kaavi

Till sampling was carried out in Lahtojoki in 2001-2002. The results of a GTK heavy mineral
survey conducted in 1994 in the area were also included in this study. Indicator mineral
data on previous claims reported by exploration companies to the Ministry of Trade and
Industry were used as background information (Fig. 3). The GTK sampling program con-
sisted of 46 excavator pits, resulting in 84 samples. The initial sample size was 20 kg. The
first sample was taken from the uppermost layer of basal till, i.e. the C-horizon usually at ~
1 m depth, and the following in 1-2 m intervals. 30 pits reached the bedrock with an aver-
age overburden thickness of 2.3 m. The older basal till bed was encountered in only one
sampling site.

The objective of the longest sampling profile (Line 1L) parallel to the main ice flow was to
measure the length of the dispersal fan (Fig. 4A). Perpendicular profiles (Lines 2L and 3L)
were made in order to determine the width of the fan. A few samples were taken from the
distal areas to estimate the regional background indicator concentration.

In the vicinity of the kimberlite a drill rig was used for sampling due to the thick glacial de-
posits. 25 holes were drilled tOForm two crossing lines (4L and 5L). Altogether 87 till sam-
ples were recovered from the drill cores representing 1 m layers of the till bed and weighed
up to 9 kg apiece.

G E U S

Figure 3. Indicator distribution in till in the Kaavi area. Data compiled from expired claims
reported to the Ministry of Trade andIndustry. The known kimberlite pipes in the area are
marked as stars. Indicator counts are normalized to 10 kg and consist ofgrains below 1.2
mm in diameter. The arrows point to the main (2) and the older ice flow (1) directions in the
region.

3.1.2 Seitaperä, Kuhm

Till sampling was carried out in Seitaperä in 2002-2003 using an excavator and the same
methodology as in Lahtojoki. The sample size was increased to 60 kg due to the lower indi-
cator content of the Seitaperä kimberlite compared to that of the Lahtojoki pipe. A 2-km
profile (Line 1S) was sampled down-ice from the kimberlite parallel to the main ice flow
(Fig. 4B). A shorter perpendicular profile (Line 2S) was placed at the greatest extent on
Line 1S where indicators were still found in concentrations above background. In total 42
pits were excavated, resulting in 98 till samples. 21 pits reached the bedrock with an aver-
age overburden thickness of 2.5 m. Occasionally thick ablation till unit prevented sampling
of basal till.

In order to study the influence of the Archean ultramafics of the Kuhmo greenstone belt on
the regional chromite content in till, two pits were excavated on both up- and down-ice of
the voluminous Näätäniemi serpentinite massif. The massif is located approximately 30 km
up-ice from Seitaperä.

G E U S

3.2 Sample processing and analysis

The till samples were initially screened down to <1.0 mm grain size to reduce their volume.
Selected coarser fractions were also studied for indicators and kimberlite fragments. Pre-
concentrates were made from screened 15 kg (Lahtojoki) and 45 kg (Seitaperä) excavator
and 4-6 kg drilled (Lahtojoki) till samples using a GTK modified 3''Knelson Concentrator
(Chernet et al., 1999)

. The preconcentration reduced the 15 kg and 45 kg excavator sam-

ples into 200-300 g and 700-900 g, respectively, and the drilled till samples into 100-200 g.

The preconcentrates were separated by heavy medium (d=3.2 gcm

-3

). A dry low intensity

drum magnetic separator was applied to remove magnetite. Finally the 0.25-1.0 mm size
indicator grains were hand picked under binocular microscope. In order to confirm their
identification, all indicator grains were analyzed using an EDS equipped Jeol JSM-5900LV
scanning electron microscope. Accurate mineral compositions from selected indicator
grains were determined by a Cameca Camebax SX50 electron microprobe.

4 Results

4.1 The Lahtojoki indicator fan

Down-ice from the Lahtojoki kimberlite a well-defined indicator fan exists, parallel to and
symmetrically distributed around the main ice flow direction (Fig. 4A). The highest concen-
tration of indicator grains is found nearly 1.2 km down-ice from the pipe. At this distance the
indicator fan is approximately 600 m wide and the highest concentration of indicators usu-
ally exists at the base of the till bed. Further away the indicators are slightly enriched in the
uppermost part of the basal till. The further-most sampling site, 2 km from the kimberlite, is
still enriched in indicators and suggests that the fan extends further.

While studying the pipe area by drilling, another small kimberlitic body of yet unknown size
was discovered 300 m down-ice from the main pipe. The satellite kimberlite intersection of
9 m from a 45° dipping drill hole is composed of tuffisitic kimberlite breccia and bounded on
both sides by country rock. The intrusion is clearly related to the main pipe both spatially
and mineralogically. The till indicator population corresponds very closely to that of the
main Lahtojoki kimberlite (Fig. 2), suggesting that it is the main contributor of the dispersal
fan. The small satellite kimberlite must have its impact on the shape and indicator content
of the fan as well but this effect cannot be readily identified.

4.2 The Seitaperä indicator fan

The indicator content in till down-ice from the Seitaperä kimberlite is much lower than that
in Lahtojoki, as expected based on the different indicator concentrations in the kimberlites
themselves. The indicator maximum exists in samples directly over the kimberlite and im-
mediately down-ice from it, after which the concentration drops rapidly (Fig. 4B). The indi-
cators also seem to be enriched in the upper part of the basal till very soon after the kim-
berlite contact.

Special attention was paid to distinguishing kimberlitic till chromites from non-kimberlitic
background chromites. A separation method was applied based on chromite major element
composition (Cr, Ti, Mg). Altogether 1581 till chromites were microprobed and 559 of them
were categorized as kimberlitic. Figure 5 illustrates how the till chromite population evolves

G E U S

as the transport distance from the kimberlite 16 increases. The strongest background
population can be identified as a group of chromites with ca. 35-53 wt% of Cr

, 0.3-1.4

wt% of TiO

and 6-12 wt% of MgO. It is present literally in all till samples but not in the kim-

berlite. Based on regional geology, the most probable sources for these chromites are the
Archean ultramafic rocks of the Kuhmo greenstone belt, such as the Näätäniemi serpenti-
nite massif and associated metavolcanics. As expected, the basal till samples taken down-
ice from the Näätäniemi massif contain chromite in relatively high concentrations,> 200
0.25-1.0 mm grains/45 kg, whereas up-ice of the massif, the chromite contents are very low
(5-10 grains/45 kg). Most importantly, the bulk of the down-ice till chromites agrees in terms
of composition with the Seitaperä background chromite population.

Figure 4A and 4B. Indicator dispersal fans from the Lahtojoki (Pipe 7) and the Seitaperä
(Dyke 16) kimberlites subdivided (1-3) according to the content of kimberlitic material in till.
The arrows point to the ice flow directions in the regions. The indicatormineral (0.25-1.0
mm) counts in 15 kg (Lahtojoki) and 45 kg (Seitaperä) till samples from excavated sites are
marked in double boxes. The lower box represents the base of and the upper the surface of
the basal till bed. The drilling sites in Lahtojoki are marked with dots, the shades of which
indicate the indicator abundance in the deepest 2-4 m layer of till.

5 Discussion

5.1 The Lahtojoki case study

The results of the previous sampling programs (Fig. 3) combined with the outcome of this
study (Fig. 4A) suggest that the Lahtojoki dispersal fan has been formed as a net effect of
the two known ice flow events in the region (older ~W-E and younger ~NW-SE). Out of the
two ice flow events, the younger appears to have had more prominent effect on the shape
of the fan. During the latest glaciation the older till was reworked, transported and rede-
posited along the younger ice flow. The older till bed was virtually destroyed in the process.

G E U S

The distance of the indicator maximum in till from the kimberlite is generally a function of
the kimberlite type and/or the availability of soft kimberlite regolith for the glacier to erode
(McClenaghan et al., 2004) . The Lahtojoki kimberlite is mainly composed of soft diatreme
facies rocks with minor, more resistant hybabyssal component. On top of the pipe there
occasionally exists a several meters thick soft weathered kimberlite horizon whereas in
other parts glacial erosion has exposed fresh kimberlite. This variance in kimberlite resis-
tance probably explains the relatively long distance of the indicator maximum down-ice
from the kimberlite. In general it is not exceptional that the maximum for sand-sized indi-
cators in till exists some distance away from the kimberlite (e.g. McClenaghan et al., 2002)

The Lahtojoki dispersal fan parallel to the main ice flow can be roughly divided into three
zones (Fig. 4A): (1) The proximal zone extending approximately 500 m down-ice, charac-
terized by abundant kimberlitic fragments, and a high concentration of kimberlitic material
at the base of the till bed. (2) The intermediate zone starts where clearly elevated numbers
of indicator grains have reached the till surface, i.e. at ca. 500 m distance from Pipe 7. This
is probably mainly due to the effect of grain liberation from coarser kimberlite fragments.
The indicator maximum at 1.2 km distance marks the end of zone 2. (3) The distal zone
extends from this point further down-ice until the lake in the south (ca. 3 km). Zone 3 shows
decreasing numbers of indicator grains, with the highest concentrations occurring in the
upper parts of the basal till. Zone 3 is expected to dilute down-ice to background levels of
kimberlitic indicators.

5.2 The Seitaperä case study

In contrast to the Lahtojoki case, the mineral grain liberation from coarser kimberlite frag-
ments in Seitaperä has taken place immediately at the kimberlite contact (Fig. 4B). Dilution
from country rock has rapidly replaced the kimberlitic material from the base of the till bed
and soon reached also its upper parts. At 800 m distance the dilution is already so strong
that it is difficult to estimate the width of the dispersal fan.

The silicate indicator population in till does not correspond to that of the kimberlite (Fig. 2);
Cr-diopsides and Ti-pyrope and eclogitic kimberlitic garnets are considerably underrepre-
sented in the till whereas purple Cr-pyropes are found in low concentrations ( <5 grains in
45 kg of till) over the entire sampling area. This evidence suggests that the silicate indica-
tors in till most likely represent a regional feature.

The chromite content and composition in till samples strongly indicate that the Archean
ultramafics of the Kuhmo greenstone belt are the source for the bulk of the background
chromites. The distance between the Näätäniemi serpentinite massif and the Seitaperä
study area is approximately 30 km. The bulk of the till (i.e. the coarser material) has trav-
eled generally less than 10 km from its source based on various studies in Finland (e.g.
Hellaakoski, 1930; Virkkala, 1971; Perttunen, 1977; Saarnisto et al., 1980; Salminen, 1980;
Salonen 1986)

. However, studies on till size fractions indicate that finer material is trans-

ported far longer than coarser fractions (e.g. Peltoniemi, 1985) Saarnisto et al. (1980)
demonstrated that the proportion of 6-20 cm material originating from the Kuhmo green-
stone belt decreases almost to zero within a kilometer, but the 2-6 cm pebbles still contain
about 5% material from the greenstone belt at a distance of 15 km. Chromite grains in this
study are even considerably finer-grained than that, overwhelmingly 0.25-0.5 mm in size.

G E U S

Thus, it is reasonable to assume that they have endured in glacial transport all the way
from the greenstone belt.

Figure 5

Chromite analyses from the Seitaperä till samples plotted in a Cr

-TiO

diagram redrawn after Fipke et al. (1995). Analyses from the upper (top), middle and lower
(bottom) parts of the basal till bed are denoted: a) till sample 6538, 100 m southeast (down-
ice); b) 6535, 200 m southeast; c) 6530, 650 m southeast; and d) 6525, 1150 southeast
from the Seitaperä kimberlite. The field unique to lamproites and kimberlites is labelled as
L./K.F., diamond inclusion and intergrowth field asD.I.F., the nonlamproitic/kimberlitic field
as N.L./K.F., and the overlap field for kimberlitic and non-kimberlitic rocks as O.F.

6 Conclusions and implications for diamond exploration

At least in theory, the implications from this work seem to be easily applicable for diamond
exploration in Fennoscandia as well as in any other recently glaciated terrain. Most impor-
tantly, the methodology described here can be used in all till-covered regions. The ultimate
aim of commercial till heavy mineral surveys is, of course, to identify kimberlitic dispersal
fans. This study illustrates that defining a solitary fan requires a detailed indicator mineral

study with dense sample spacing, ~1 sample / 0.25 km

. The discovery of the Lahtojoki

satellite kimberlite emphasizes the significance of a dense sampling grid. The case studies
of Lahtojoki and Seitaperä show that the fans can, for a number of reasons, be different in
morphology, size, indicator content and internal structure.

G E U S

Based on this study, a sample size of at least 60 kg of basal till is recommended for indi-
cator mineral work in diamond exploration at reconnaissance and regional scale. It is also
important to take samples from different horizons of the basal till bed, because the vertical
distribution of indicators within the till may vary considerably as seen in both study areas.
Ideally this variance can be used to estimate the transport distance.

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G E U S

Kimberlites and ultramafic lamprophyres in West
Greenland: regional constraints

Nielsen, T.F.D. & Jebens, M.

Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copen-
hagen K, Denmark

Mitchell et al. (1999) noted regional differences in the mineral chemistry of ultramafic lam-
prophyres from the Sisimiut-Maniitsoq region in West Greenland. The Majuagaa dyke
(Nielsen et al., this volume) has now been shown to be a kimberlite (s.s.) and the distribu-
tion of kimberlites and ultramafic lamprophyres could be of importance for the evaluation of
the diamond potential in the West Greenland province.

Thirty occurrences of ultramafic lamprophyres and kimberlites throughout the province
have been selected for detailed studies. The compositions of groundmass phlogopite,
spinel and ilmenite and the presence or absence of cognate clinopyroxene can be used for
the identification of kimberlites (s.s.) (Mitchell, 1995 and Tappe et al, 2005). The ultramafic
lamprophyres from a single occurrence may, however, show several mineralogical charac-
teristics corroborating with a kimberlitic affinity, but one or more of the critical constraints do
not comply with the strict definition of kimberlite (see e.g. Hutchison, this volume). Whether
this reflects a gradation between kimberlite and ultramafic lamprophyre or mixing of lam-
prophyre, ultramafic lamprophyre and kimberlitic melts is not clear.

A first step in the investigation of this problem is the establishment of a large database for
the compositions of the groundmass minerals that forms the backbone of the classification
of kimberlites and ultramafic lamprophyres. This investigation has been initiated and some
preliminary results are presented. All occurrences investigated to date, except for the
Maniitsoq swarm (see below), are according to the current classifications ultramafic lam-
prophyres, but many may have characteristics of kimberlites. It is hoped that ultramafic and
mafic "endmember" types of dyke rocks can be identified and investigated in great detail.

A subsequent step would be a systematic investigation of the correlations between the
groundmass mineral chemistry and the occurrence of diamonds in the West Greenland
province and diamond-producing kimberlites and ultramafic lamprophyres world-wide. It is
a preliminary suggestion, but the data at hand do suggest that the prospectivity of the West
Greenland dykes is not dependent on the dykes being kimberlite (s.s.) or ultramafic lam-
prophyre.

Kimberlites in the Maniitsoq region
The Majuagaa dyke of the Maniitsoq region is concluded to be a genuine kimberlite (see
Nielsen et al., this volume). This is based on the compositions of groundmass phlogopite,
spinel and ilmenite and the absence of clinopyroxene. The same characteristics are seen in

G E U S

new data from the so-called MK02 occurrence mentioned in Mitchell et al. (1999) from
close to Maniitsoq. The characteristic of the kimberlites from these localities is that the dyke
rocks are phlogopite-poor (no brown phlogopite to be seen in hand specimens and thin
section) and carbonate-rich. Dykes collected from further in land in the E-W Maniitsoq
swarm of nodule-bearing dykes appear to be of the same type and it is suggested that the
Maniitsoq dyke swarm is dominated by kimberlites (s.s.). Lamprophyric dykes occur in the
same swarm, but are easily identified in hand specimens. They have grains of brown
phlogopite. One lamprophyre dike has been investigated in more detail and the phlogopites
are zoned towards tetraferriannite (Fe-rich biotite). Such dykes also occur in the Sarfartoq
region.

Ultramafic lamprophyres with kimberlitic affinities in the Sarfartoq region
Hutchison (this volume) clearly demonstrates the problems of classification of the ultrama-
fics from the Garnet Lake diamond prospect. Individual samples from the same occurrence
have few or more mineral kimberlitic charateristics. It becomes difficult to classify the occur-
rence. Is this a kimberlite occurrence diluted to variable extends with ultramafic lampro-
phyre and/or lamprophyre components? A similar complication is observed in the Sarfar-
tuup nuna (S) occurrence SE of Garnet Lake. Phlogopite trends in the "Jesper Blow" can
show one ore more compositional trends, even within the same thin section. One option is
to see the occurrence as the result of the mixing of magmas. A second possibility is that the
compositional trends of groundmass phlogopite reflect very local geochemical environ-
ments, such as volatile content, and composition (CO

O ratio) and oxygen fugacity. If

so, the fundation for the classifications of kimberlites may be questioned. It seems impera-
tive for an understanding of the diamond-related ultramafic magmatism and the diamond
prospectivity to understand the details of variability in the composition of groundmass
phases and their margins.

A call for co-operation
The observations in the West Greenland occurrences raise questions regarding the classi-
fication and the genesis of the ultramafic rocks. The aim of this presentation is to ask for
support for three investigations.

1: Is it important for diamond prospectivity if the host rocks are kimberlites, or ultramafic
lamprophyres including orangeites and such rock types? The investigation should study
samples from a larger suite of occurrences (50?) that have or do produce diamonds. The
groundmass minerals should all be analysed and the occurrence classified according to
Mitchell (1995) and Tappe et al. (2005). It is tentatively believed that the investigation would
show that petrographic type is irrelevant, as long as the "carrier" liquid originates from a
depth below the level at which diamond can form. All samples should be investigated by
the same standart methods and by the same group of researchers to ensure that the data
is comparable.

2: What are the compositions of the "carrier" melts of the ultramafic lamprophyres and kim-
berlites? It is argued that most of the matrix and megacrystic olivine in the Majuagaa dyke
is xenocrystic. This opens for the possibility for an evaluation of the composition of the "car-
rier" melt. Does this apply to other kimberlite occurrences? The olivines of the Wesselton
kimberlite (Mitchell, 1973) show a compositional variation similar to that of the Majuagaa

G E U S

dyke. It is suggested to study a suite of classic kimberlites and ultramafic lamprophyres
along the lines described in Nielsen and Jensen (2005). The investigation should comprise
morphological characterisation of the olivine grains and analysis of euhedral and anhedral
matrix grains, megacrysts and nodules. How much of the olivine in the kimberlites can be
interpreted as cognate? The amount of xenocrystic olivine should be evaluated and the
bulk recalculated to give an estimate of the "carrier" melt. What is the composition of the
carrier melt? A continuum of melts from magnesium-rich carbonatite to CO

-bearing high

Mg# silicate melt along the lines suggested by Dalton and Presnall (1998)? The investiga-
tion requires the study of hypabysal and very well preserved, unaltered, kimberlites in
which the morphologies of olivine can be interpreted.

3: Are the evolutionary trends in groundmass minerals directly related to the magma type?
Can marginal differences in CO

O and oxygen fugacity change the evolutionary trends

in, e.g., phlogopite? Detailed investigations of single samples from well-described occur-
rences should be used, e.g. Garnet Lake. Are groundmass mineral trends the results of
marginal differences and/or local geochemical environments during the final crystallisation?
If so, what would that suggest for the classification of kimberlites?

References

Dalton, J.A. & Presnall, D.C. 1998: The continuum of primary carbonatitic-kimberlitic.

Melt compositions in equilibrium with lherzolite: Data from the system CaO-MgO-Al2O3-

SiO2-CO

at 6 Gpa. Journal of Petrology 39 , 1953-1964.

Hutchison, M.T. 2005. Diamondiferous kimberlites from the Garnet Lake area, West

Greenland: Exploration methologies and petrochemistry. Danmarks og Grønlands Ge-
ologiske Undersøgelse Rapport, this volume.

Mitchell, R.H. 1973. Composition of olivine, silica activity and oxygen fugacity in kimberlite.

Lithos 6 , 65-81.

Mitchell, R.H. 1995. Kimberlites, orangeites and related rocks. New York, Plenum Press,

410 pp.

Mitchell, R.H., Scott-Smith, B.H. & Larsen, L.M. 1999. Mineralogy of ultramafic dikes from

the Sarfartoq, Sisimiut and Maniitsoq areas, West Greenland. In: Gurney, J.J., Gur-
ney, J.L., Pascoe, M.D. & Richardson, S.H. (es): Proceedings of the VIIth International
Kimberlite conference 2 , 575-583. Cape Town: Red Roof Design cc.

Nielsen, T.F.D., Jensen, S.M & Secher, K. 2005. The Majuagaa calcite-kimberlite dyke.

Danmarks og Grønlands Geologiske Undersøgelse Rapport, this volume.

Tappe, S., Foley, S.F., Jenner, G.A. & Kjarsgaard, B.A. 2005. Intergrating ultramafic lam-

prophyres into the IUGS classification for igneous rocks: Rationale and implications.
Journal of Petrology 46 , 1893-1900.

G E U S

The Majuagaa calcite-kimberlite dyke

Nielsen, T.F.D., Jensen, S.M. & Secher, K.

Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen
K, Denmark

Introduction

The Greenland diamond exploration is focused on swarms of ultramafic dykes in the
Sisimuit Maniitsoq region in West Greenland. The dykes have traditionally been referred
to as lamprophyres and "kimberlitic" dykes (e.g. Larsen, 1980). More detailed investiga-
tions of the mineral chemistry, however, let Mitchell et al. (1999) to conclude that the dykes
were best referred to suites of ultramafic lamprophyre grading into carbonatite. The dykes
are not referred to kimberlites, because the compositions of groundmass phlogopite, spinel
and ilmenite do not conform with kimberlites as defined in Mitchell (1995). The rationale
behind this conclusion is that the type of magma that transported nodules, megacrysts and
diamonds to upper crustal levels is revealed by the compositions of the minerals that crys-
tallised in the groundmass of the dykes.

The ultramafic dykes vary significantly in petrography and mineral chemistry from
phlogopite-rich and clinopyroxene-bearing in the Sisimiut region to phlogopite-rich or
phlogopite-poor in the Sarfartoq region to phlogopite-poor in the Maniitsoq region. Individ-
ual dykes may be composed of several segments, which may comprise several pulses of
contrasting types of ultramafic magma. The observations suggest that the magmas that
carried nodules and megacrysts to the surface varied and that individual dyke systems can
represent melts from several source regions, and may even be mixed melts with both aste-
nospheric and lithospheric origins.

A consistent description of the ultramafic magmatism in West Greenland apparently re-
quires identification and detailed descriptions of the different petrographic types. The Ma-
juagaa dyke from the Maniitsoq region (Jensen & Secher, 2004; Nielsen & Jensen, 2005) is
a diamond-bearing and phlogopite-poor ultramafic dyke, which in all field criteria should be
referred to as kimberlite.

The Majuagaa dyke

The Majuagaa dyke is 2.5 km long and up to 2 m wide. It consists of a number of "en
echelon" segments. The proportions of megacrysts and nodules vary, but the groundmass
appears quite uniform and composed of small, euhedral to anhedral grains of olivine in a
groundmass of calcite, serpentine, minor colourless or weakly greenish phlogopite, small
grains of spinel and ilmenite. Perovskite and apatite are rare and no clinopyroxene or mon-
ticellite has been encountered. The proportion of carbonate varies and late carbonatite
veins are common.

G E U S

The dyke rocks are a mixture of xenolithic and xenocrystic material and a "carrier" melt. It is
the composition and mineralogy of the "carrier" melt that allows the classification of a given
sample. This investigation of the petrography and the mineral chemistry of the Majuagaa
dyke focuses on: (1) the compositional variation in olivine and (2) the compositions of
groundmass phlogopite, spinel and ilmenite. The aim of the olivine investigation is an
evaluation of the proportion of liquidus olivine (as opposed to the xenocrystic olivine). If the
proportion and composition of xenocrystic material can be determined it is possible to cor-
rect the bulk compositions to give an estimate of the composition of the "carrier" melt. The
aim of the investigation of the groundmass minerals is to determine the affinity of the "car-
rier" magma.

Olivine
Small grains of olivine dominate the groundmass of the samples and can be divided into
euhedral crystals and anhedral grains. Euhedral crystals are rare to very rare. Systematic
analyses of all morphologies of olivine reveal that euhedral grains have Fo

90-89

cores and

89-88

margins, whereas anhedral grains have Fo

94-85

cores, in some cases with margins

similar to those of the euhedral olivines. It is suggested that the equilibrium olivine has a
starting high-T composition of Fo

90-89

and a margin composition of Fo

89-88

. Similar core and

margin compositions are described from Canadian kimberlites (Fedortchouk and Canil,
2004). Liquidus compositions between Fo

and Fo

conforms with liquidus olivine of kim-

berlite (s.s.). Note should be taken that the compositions of cores and margins of Majuagaa
olivine (euhedral as well as anhedral) are very similar to compositions of olivines from the
classic Wesselton kimberlite (Mitchell, 1973).

Compositions of olivine megacrysts and nodules overlap with the cores of the anhedral
groundmass olivine. Only a single nodule has olivine with compositions similar to those of
the cores of euhedral olivine grains. This appears to demonstrate that the anhedral
groundmass grains are xenocrystic and that a dominant proportion of groundmass olivine is
xenocrystic.

Ilmenite
Large rounded ilmenite megacrysts are common in the Majuagaa dyke. They show a small
compositional range and have on average 56% ilmenite and 44% geikielite. The ground-
mass ilmenite has a much higher geikielite component (up to 70%). The ilmenite megac-
rysts are xenocrystic and contributed little to the groundmass of the dyke. The groundmass
ilmenite conforms with ilmenite of kimberlite (s.s.).

Spinels
The groundmass of the Majuagaa dyke does not contain chromite. All spinels are poor in
Cr, but rich in Mg. They belong to the magmatic trend 1 and are Mg-rich Ti-magnetites.
Some margins are quite rich in MgO. Such margins are known from spinels of calcite-
kimberlites (Mitchell et al, 1999). The spinels of the Majuagaa dyke conform with composi-
tions from kimberlites (s.s).

Phlogopite
Phlogopite is rare and is often found in small areas of late residual calcite-rich material. The
phlogopite is pale or weakly greenish. The centres and the margins of the small grains are

G E U S

very similar in composition, except for BaO. The phlogopite is TiO2-, FeO- and Fe2O3-
poor, but rich in Al2O3. No tetraferriphlogopite is observed. Margins of the groundmass
grains show a strong increases in BaO, up to wt. 7.5%. Cores and margins all plot in the
fields of groundmass phlogopites of kimberlites (s.s.) and the enrichment in Ba in the mar-
gins conforms with observations from archetypal kimberlites (Mitchell, 1995).

Bulk composition
Nineteen samples with small proportions of megacrysts and nodules have an average
composition in the field of kimberlites. The average composition is relatively low in SiO2,
but compares to, e.g., the Benfontein kimberlite. The trace element compositions conform
with kimberlite. The REEs compare with classic kimberlites such as Wesselton and the
spidergram shows a composition very similar to that of Namibian kimberlite. The bulk com-
position conforms with compositions of kimberlite (s.s.).

The bulk composition is a mixture of xenocrystic olivine and ilmenite and a melt. An accu-
rate proportion of xenocrstic olivine is not at hand and the proportion of xenocrystic olivine
has been estimate by assuming that the Ni content of the quite carbonatitic "carrier" melt
was comparatively low ( <200 ppm). likewise the tio

content is assumed to be low

<0.5%). subtraction of 33% olivine (average of megacrystic olivine) and subtraction of a
further 10% of megacrystic ilmenite results in a composition in agreement with the experi-
mental melt compositions of Dalton and Presnall (1998). The estimated melt lies on the
borderline between kimberlite and silico-carbonatite. Addition of xenocrystic olivine makes
the bulk composition kimberlitic.

Discussion

The compositions of groundmass minerals and the absence of clinopyroxene conforms with
the definition of kimberlite (s.s.). Based on the criteria in Mitchell (1995) and the classifica-
tion scheme suggested by Tappe et al (2005) the Majuagaa dyke classifies as a kimberlite.
Some may hesitate to accept this conclusion, as the groundmass is calcite-rich and poor in
serpentine. The groundmass points towards carbonatitic rather that a CO

-rich silicate melt.

It is clear that all types of olivine grains, irrespectively of their origin, can have margins in
equilibrium with the melt. They are not surrounded by serpentine or altered to serpentine. It
is suggested that the volatile component of the Majuagaa dyke had a high CO

O ratio

and that the low proportion of silicate (serpentine) in the groundmass is due to the growth
of olivine margins rather than primary groundmass serpentine.

It is with reference to the spinel compositions and the calcite-rich nature of the groundmass
concluded that the diamond-bearing Majuagaa dyke is a calcite-kimberlite, with character-
istics referring to classic kimberlites such as Wesselton and Benfontein.

G E U S

References

Dalton, J.A. & Presnall, D.C. 1998: The continuum of primary carbonatitic-kimberlitic.

Melt compositions in equilibrium with lherzolite: Data from the system CaO-MgO-Al2O3-

SiO2-CO

at 6 Gpa. Journal of Petrology 39 , 1953-1964.

Fedortchouk, Y. & Canil, D. 2004: Variables in kimberlite magmas, Lac de Gras, Canada

and implications for diamond survival. Journal of Petrology 45 , 1725-1745.

Jensen, S.M. & Secher, K. 2004. Investigating the diamond potential of southern West

Greenland, GEUS Bulletin 4 , 69-72.

Larsen, M.L. 1980. Lamprophyric and kimberlitic dykes associated with the Sarfartoq car-

bonatite complex, southern West Greenland. Rapport Grønlands Geologiske Under-
søgelse 100 , 65-69.

Mitchell, R.H. 1995. Kimberlites, orangeites and related rocks. New York, Plenum Press,

410 pp.

Mitchell, R.H., Scott-Smith, B.H. & Larsen, M.L. 1999. Mineralogy of ultramafic dikes from

the Sarfartoq, Sisimiut and Maniitsoq areas, West Greenland. In: Gurney, J.J., Gur-
ney, J.L., Pascoe, M.D. & Richardson, S.H. (es): Proceedings of the VIIth Interna-
tional Kimberlite conference 2 , 575-583. Cap Town: Red Roof Design cc.

Nielsen, T.F.D. & Jensen, S.M. 2005. The Majuagaa calcite-kimberlite dyke, Maniitsoq,

souther West Greenland. Rappot Danmarks og Grønlands Geologiske Undersøgelse
2005/43 , 59 pp.

Tappe, S., Foley, S.F., Jenner, G.A. & Kjarsgaard, B.A. 2005. Intergrating ultramafic lam-

prophyres into the IUGS classification for igneous rocks: Rationale and implications.
Journal of Petrology 46 , 1893-1900.

G E U S

A Greenland petrographic atlas of kimberlites and
ultramafic lamprophyres

Nielsen, T.F.D., Secher, K. & Jensen, S.M.

Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copen-
hagen K, Denmark

An introduction to the great diversity of kimberlites, ultramafic lamprophyres and lamproites
in Greenland is proposed in the form of a petrographic atlas. The Atlas contains schematic
information and thin sections of a range of alkaline dyke rocks from the collections of the
GEUS archives.

An example of a page display from the proposed Atlas is introduced.

Locality number 2004KSE076

Locality name Majuagaa

Latitude 64,500

Longitude -52,00

Sample no. 48642

Groundmass

CAR

PHL

GAR

ILM

SPI

CPX

OPX

PER

LEU

MEL

SER

Megacrysts

PHL

GAR

ILM

SPI

CPX

OPX

Xenoliths

Harzburgite

Lherzolite

Wehrlite

Dunite

Eclgite

Basement

Diamonds

Tested +

Tested

Not tested

Size (mm)

Carats

0,56

0,0001

Remarks

Exceedingly rich in mantle xenoliths

G E U S

Assessment of diamond potential using kimberlitic
indicator minerals: key principles and applications

Nowicki, T.E. & Gurney, J.J.

Mineral Services Canada Inc., 205-930 Harbourside Drive, North Vancouver, B.C., Canada
V7P 3S7

Introduction

Most of the common indicator minerals (i.e. garnet, Cr-spinel, Cr-diopside, picro-ilmenite,
forsterite and enstatite) represent fragments of mantle material sampled and brought to
surface by volcanic intrusions such as kimberlite. The type, abundance and composition of
these minerals, recovered from surface sediment and kimberlite rock samples, provide
critical information at several stages in diamond exploration programs. In particular, indi-
cator mineral compositions provide a means of: 1) assessing regional diamond prospectiv-
ity; 2) identifying and locating primary diamond source rocks; and 3) evaluating the dia-
mond potential of source rocks. This contribution focuses on the use of indicator mineral
composition data to assess the diamond potential of kimberlites. The principles discussed
are applicable to other deep-seated intrusives that have potential to sample the diamond
stability field (e.g. olivine lamproites and group 2 kimberlites / orangeites) and may be used
to assess the regional prospectivity of terrains in which kimberlites or related rocks occur.

The basic premise

Most of our understanding of the link between indicator minerals and diamonds stems from
studies of inclusions in diamonds as well as of diamond-bearing upper-mantle xenoliths
from kimberlites. Such studies have defined the two well known diamond "parageneses",
i.e. sub-calcic garnet- and/or chromite-bearing harzburgite and high-pressure eclogite. In
the mantle sample within kimberlite, these are primarily represented by specific garnet
types G10 and group 1 eclogitic garnets.

The key diagnostic compositional characteristics of diamond-inclusion (DI) garnets are il-
lustrated in Figure 1. Approximately 85 % of all diamond inclusions of peridotitic garnet (> 2
wt% Cr

) are sub-calcic, plotting to the left of the G10-G9 divide, as defined by Gurney

(1984). These compositions correspond with those of garnets from mantle harzburgite (i.e.
clinopyroxene-free peridotite) or dunite. If one considers that the proportion of harzburgite
and dunite in average cratonic lithosphere, as represented by xenoliths and xenocrysts in
kimberlites, only ranges up to approximately 15 %, the high proportion of peridotitic DI gar-
nets with such compositions demonstrates a striking preferential association of diamond
with this material. Peridotitic chromite inclusions in diamond define a similarly unique com-
positional range (at least for mantle-derived chromite) and are characterized by very high
Cr and moderately high Mg contents. As in the case of G10 garnets, these chromite com-
positions are indicative of derivation from a peridotite with a specific bulk composition, i.e.

G E U S

harzburgite or dunite. In contrast to the harzburgite / dunite association, diamonds occur in
eclogite with a wide range of bulk compositions (Grütter and Quadling, 1999). However,
eclogitic garnets associated with diamond (i.e. as diamond inclusions or in diamond-
bearing eclogite xenoliths) are characterized by relatively high Na contents (McCandless
and Gurney, 1989; Figure 1b), indicative of high pressures of equilibration. This also ap-
plies to relatively rare websteritic garnets included in diamond.

These observed direct relationships between certain indicator minerals and diamond are
borne out in studies of xenocryst suites in kimberlites. In particular, as far as the authors
are aware, there are no significantly diamondiferous kimberlites worldwide that do not con-
tain one or more of these diamond-associated mineral types. This provides diamond ex-
plorers with a very powerful tool, i.e. it implies that the presence and amount of diamond-
indicator minerals are directly related to the diamond content of their source rock. However,
there are complexities and additional factors that need to be considered in order to ensure
reliable predictions of diamond potential.

Figure 1. Diagnostic composition plots of diamond inclusion garnets (from internal Mineral
Services database of published data). a) Cr

vs CaO plot of all garnets (n = 566) 

dashed lines show the Gurney (1984) G10-G9 divide (1), the 2 wt% Cr2O3 cut-off (2) and

the Cr-Ca graphite diamond constraint (Grütter et al,, 2004); b) TiO

vs Na

O plot of low-Cr

<2 wt% cr

), eclogitic garnet (n = 238) - dashed lines represent the 0.07 wt% Na

O cut-

off (4) and the approximate lower limit of the TiO

-Na

O compositional field defined by gar-

net megacrysts (5).

When is a G10 not a diamond indicator?

The compositional criteria used to identify diamond-associated G10 garnets (or chromites)
primarily reflect the bulk composition of the source rocks and a very strong association with
carbon in the mantle, but do not do not indicate whether the garnets are derived from the
diamond stability field or not. Recent studies have demonstrated that G10 garnets can be
derived from shallow depths in the graphite stability field (e.g. Griffin et al ., 1999). In addi-
tion, where hot geothermal gradients prevail, elevated temperatures can place deep, high-
pressure mantle peridotite outside of the diamond stability field. In these instances, the

0.4

0.8

1.2

0.08

0.16

0.24

0.32

0.4

O (wt%)

CaO (wt%)

G E U S

presence of G10 garnets (or associated DI chromites) may not have any relationship to the
diamond content of their kimberlite hosts. It is therefore critical to know the pressure-
temperature conditions at which these minerals equilibrated.

Equilibration temperatures for peridotitic garnet can be determined directly by Ni ther-
mometry (e.g. Ryan et al. , 1996) which requires measurement of Ni concentrations at trace
levels. A less precise, but useful alternative approach has been developed based on Mn
content (Grütter et al ., 1999; Grütter et al., 2004), which can be determined by high-quality
electron microprobe analysis. Unfortunately, equilibration pressure cannot be determined
reliably for single garnet grains. High-pressure G10 garnet can be broadly identified by
relatively high Cr content and this has been used by Grütter et al. (2004) to provide a con-
straint on likely diamond-associated (G10D) grains (Figure 1a). However, the Cr content of
garnet only provides an indication of the minimum pressure under which it is likely to have
equilibrated. Thus, while high Cr contents in G10 garnets indicate a high-pressure origin,
the reverse is not necessarily true (i.e. low-Cr garnets may be derived from high pressure if
they were not in equilibrium with chromite). An alternative approach is therefore commonly
required to confirm the likely proportion of G10 garnets derived from the diamond stability
field.

Equilibration pressures for garnets can be determined indirectly by projection of the garnet
temperature onto a known geothermal gradient (geotherm). The geotherm is most reliably
determined using well established thermobarometers that are based on co-existing mineral
pairs in xenoliths. However, these are commonly not available in the early stages of explo-
ration or kimberlite evaluation and typically it is necessary to rely on single-grain thermo-
barometry techniques. The most reliable published method is the clinopyroxene thermo-
barometer of Nimis and Taylor (2000). An alternative approach, proposed by Ryan et al
(1996), permits estimation of the geotherm based only on peridotitic garnet compositions.
This is a statistical approach (i.e. cannot determine pressures for individual grains and re-
lies on garnet population characteristics and assumptions about co-existence of garnet with
chromite) and, under typical cratonic conditions, significantly underestimates the geother-
mal gradient (Grütter and Sweeney, 2000). Nonetheless, in the absence of suitable pyrox-
ene minerals, it can provide a useful indication of the likely geotherm. Where present, or-
thopyroxene provides a very useful qualitative indicator of the geothermal gradient.

The eclogite problem

It is not possible to calculate equilibration temperatures or pressures for single grains of
eclogitic garnet. Inferences can be made on the likely P-T range for the eclogitic compo-
nent of kimberlite xenocryst suites based on associated peridotitic minerals but this is an
indirect and potentially unreliable approach. Thus the Na content of eglogitic garnet is the
primary indicator of equilibration depths and likely association with diamond. However,
high-Na garnet is also observed in graphite-bearing eclogite xenoliths (Grütter and Quad-
ling, 1999) indicating that the relationship between Na content and diamond in eclogite is
not straightforward. In addition, diamond-bearing eclogites with relatively low Na contents
(i.e. below the 0.07 wt% threshold typical of diamond inclusion compositions) have been
documented (Cookenboo et al. , 1997). These discrepancies do not negate the importance
of eclogitic garnet in assessing diamond potential, but demonstrate that care needs to be
taken in interpreting their compositions. Assessments of eclogitic diamond potential based

G E U S

on Na-Ti relationships need to be refined by consideration of bulk compositional factors that
influence the relationship between Na and equilibration pressure.

Diamond inclusion chromites some imposters

Diamond inclusion chromites are derived from highly depleted, high-pressure mantle peri-
dotite. However, chromites of equivalent compositions can occur in other, non-diamond
associated parageneses. In particular, chromite phenocrysts/microphenocrysts crystallizing
from kimberlite, and certain non-kimberlitic chromites from mafic to ultra-mafic crustal rocks
may have compositions that overlap with the DI chromite field. These "imposters" generally
occur in association with other non-mantle chromites that display specific compositional
trends. Thus they can generally be accounted for when evaluating large populations of
chromites. However, it is important to be cognizant of these alternative possible sources of
"DI type" chromites when interpreting data for individual grains or grains from small popula-
tions where such trends may not be evident.

The relationship between indicator mineral abundance and diamond content

In general, it can be assumed that the diamond content of a kimberlite is related to the con-
centration of diamond indicator minerals. This is broadly true and it is therefore critical to
not only measure the composition of indicator minerals recovered from kimberlite, but also
their abundance. In order to reliably interpret results, it is essential that procedures used to
extract and analyse indicator minerals provide quantitative and representative abundance
and composition data.

Reliable interpretation of indicator mineral abundance data in terms of diamond potential
requires cognizance of two key factors:
Only the specific mineral types indicative of the presence of diamond count (i.e. G10 gar-
nets and DI chromites from the diamond stability field, group 1 eclogitic garnets or Na-rich
websteritic garnets). The abundance of other mantle mineral types is not relevant.
The relationship between the abundance of specific diamond indicator minerals and that of
diamond is not constant and needs to be calibrated for different kimberlite fields and types.

Applications

Application of the above described principles will be illustrated with examples of indicator
mineral datasets from Greenland and elsewhere.

References

Cookenboo, H.O., Kopylova, M.G. and Daoud, D.K., 1997. A chemically and texturally dis-

tinct layer of diamondiferous eclogite beneath the central Slave craton, northern Can-
ada. Ext. Abstr. 7

Int. Kimb. Conf. , Cape Town, 164-165.

Griffin, W.L., Doyle, B.J., Ryan, C.G., Pearson, N.J., O'Reilly, S.Y., Davies, R., Kivi, K., van

Achterbergh, E. and Natapov, L.M., 1999. Layered mantle lithosphere in the Lac de
Gras area, Slave craton: composition, structure and origin. J. Petrol 40 , 705-727.

Grütter, H.S. and Quadling, K.E., 1999. Can sodium in garnet be used to monitor eclogitic

diamond potential? In: Gurney, J.J., Gurney, J.L.G., Pascoe, M.D. and Richardson,
S.H. (Eds), J.B. Dawson Volume, Prc. 7

Int. Kimb. Conf. , Red Roof Design, Cape

Town, pp. 314-320.

G E U S

Grütter, H.S and Sweeney, R.J., 2000. Tests and constraints on single-grain Cr-pyrope

barometer models: some initial results. Ext. Abstr. GAC/MAC Annual Joint Meeting
Calgary (CD-ROM, GeoCanada 2000).

Grütter, H.S., Apter, D.B and Kong, J., 1999. Crust-mantle coupling: evidence from mantle-

derived xenocrystic garnets. In: Gurney, J.J., Gurney, J.L.G., Pascoe, M.D. and
Richardson, S.H. (Eds), J.B. Dawson Volume, Prc. 7

Int. Kimb. Conf. , Red Roof

Design, Cape Town, pp. 307-313.

Grütter, H.S., Gurney, J.J., Menzies, A.H. and Winter, F., 2004. An updated classification

scheme for mantle-derived garnet, for use by diamond explorers. Lithos 77 , 841-857.

Gurney, J.J., 1984. A correlation between garnets and diamonds. In: Glover, J.E. and Har-

ris, P.G. (Eds), Kimberlite Occurrence and Origins: A Basis for Conceptual Models in
Exploration , Geology Department and University Extension, University of Western
Australia, Publication 8 , 143-166.

McCandless, T.E. and Gurney, J.J., 1989. Sodium in garnet and potassium in clinopyrox-

ene: criteria for classifying mantle eclogites. In: Ross, J. (Eds), Kimberlites and Re-
lated Rocks , Geol. Soc. Austr. Spec. Publ. 14 , pp. 827-832.

Nimis, P. and Taylor, W.R., 2000. Single clinopyroxene thermobarometer for garnet peri-

dotites: Part 1. Calibration and testing of a Cr-in-Cpx barometer and an enstatite-in-
Cpx thermometer. Contrib. Mineral. Petrol. 139 , 541-554.

Ryan, C.G., Griffin, W.L. and Pearson, N.J., 1996. Garnet geotherms: pressure-

temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. J. Geophys.
Res. 101 , 5611-5625.

G E U S

The North Atlantic alkaline rocks probes for test-
ing continuity of subcontinental lithospheric mantle

O'Brien, H., Peltonen P. & Lehtonen, M.

Geological Survey of Finland, P.O.Box 96, FIN-02151 Espoo, Finland

It has been 35 years since Doig (1970) published his paper titled "An alkaline rock province
linking Europe and North America", wherein he presented the North Atlantic Alkaline Igne-
ous Province (NAAIP) extending over an area from eastern Canada to European Russia.
Since that time a large body of age, petrological, geochemical and isotopic information has
been acquired on NAAIP rocks allowing us to "image" compositional and mineralogical res-
ervoirs in the subcontinental lithospheric mantle of the area. The main limitation of this
method is that information is only available for those specific time slices when there was
significant province-wide alkaline magma activity.

Three Mesoproterozoic alkaline intrusive fields, ca. 1200 Ma in age, are discussed here:
Western Greenland, Kalix in N. Sweden and the Kuhmo (Finland) and Kostomuksha (Rus-
sia) area. Even though the rock type names used for these localities range from lamproite
(Greenland) to ultramafic lamprophyre (Kalix) to K2L [Group II kimberlite-Lamproite hy-
brid](Kuhmo, Kostomuksha), there are, nevertheless, a number of similarities between
these occurrences. These include: 1. Mineralogy All of these rocks are Ti-phlogopite rich.
2. Major and trace element chemistry these show clear similarities 3. Intrusive habit
nearly all occur as relatively narrow dikes with a dominant N-S orientation.

The Neoproterozoic alkaline intrusive fields, ca. 600 Ma in age, within the NAAIP are more
numerous and include (from East to West): Kaavi-Kuopio Group I kimberlites in Eastern
Finland, alnöites from the type locality of Alnö in Eastern Sweden, damtjernites of the Fen
complex in Southern Norway, the West Greenland UMLs and kimberlites, the Aillikites from
Aillik in Labrador and the Torngat UML in Labrador and Quebec. This group is more di-
verse, mineralogically and geochemically but still shows a number of similarities, particu-
larly in terms of isotopic composition.

The use of alkaline magmas as mantle probes allows similarities and differences of the
lithospheric mantle to be discerned, as well as secular variations. This information, coupled
with the paleomagnetic plate reconstructions presented in Pesonen et al (this volume), pro-
vide significant tests for plausible craton correlations in the North Atlantic.

Doig, R. 1970. Can. J. Earth Sci. 7 , 22-28.

G E U S

Age, depth and composition of the W. Greenland
lithospheric mantle root and implications for dia-
mond prospecting

Pearson, D.G.

, Webb, M.

, Nowell, G.M.

, Sand, K.K.

2,3

, Luguet,

& Jensen, S.M.

Arthur Holmes Isotope Geology Laboratory, Department of Earth Sciences, Durham Uni-

versity, South Rd, Durham, DH1 3LE, United Kingdom

Geologisk Institut, University of Copenhagen, Øster Voldgade 10, DK-1350, Copenhagen

K, Denmark

Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copen-

hagen K, Denmark

Introduction

Alkaline magmatic activity across W. Greenland provides a record of lithosphere evolution
over the last 500 Ma. Kimberlite magmatism in particular has erupted an exceptional in-
ventory of mantle xenoliths allowing a detailed look at the lithospheric root beneath both the
exposed craton and the re-worked Archean terrane of the Nagssugtoqidian, 500 Ma ago.
This is complimented by less numerous xenoliths erupted during Tertiary times further
North and East, such that we can begin to understand the possible impact of a major plume
on the ancient lithospheric root. We are undertaking a major study aimed at constraining
the age, depth and composition of the lithospheric mantle root at the time of kimberlite
eruption, for the cratonic and circum-cratonic regions. This should enable us to provide
primary information on the likely diamond potential of the W. Greenland lithospheric mantle.

Petrography

Peridotites are the dominant lithology in the xenolith suite studied by us. Despite detailed
field investigations no eclogites were encountered in xenoliths from the reworked Archean
terrane or from the Archean terrance immediately South of the craton margin. Eclogite has
previously been reported from a dyke S. of the Sukkertoppen ice cap (Jensen et al., 2004).
Detailed petrography has yet to be completed but field and petrographical observations
indicate that the following generalisation can be made from initial data. Samples from within
the re-worked Archean terrane more commonly have diopside plus garnet, i.e. are more
lherzolitic whereas highly depleted harzburgites and dunitic lithologies are prevalent
amongst xenoliths erupted through the undisturbed Archean craton. We note that an abun-
dance of dunite is a feature of the peridotite xenolith suite from Weidemann Fjord studied
by Bernstein et al (1998). Modal orthopyroxene is mostly in the range 0 to 20%, i.e., signifi-
cantly less than values observed for Kaapvaal craton peridotites.

G E U S

Bulk compositions

20 peridotites have sOFar been analysed for bulk composition. Samples selected are as far
as possible taken from> 500 g coarse crush to ensure representative sampling an accurate
calculation of modes. The range in depletion indicated from modal mineralogy is reflected
in the major element compositions of the W. Greenland peridotites. Some dunitic samples
clearly have very low Al and Ca, as expected. Harzburgites are also depleted in CaO (most
<1 wt %) and al

(most <1wt%) and on this basis are as depleted as the those from

Wiedemann Fjord and cratonic peridotites from the Kaapvaal craton.

Mg/Si values are high and indicate, along with modal analysis that the W. Greenland man-
tle has not experienced significant SiO

enrichment, a feature also observed by Bernstein

et al. The low bulk SiO

contents suggest particularly orthopyroxene poor protoliths. This

contributes to growing evidence that the Kaapvaal cratonic lithosphere is the exception
rather than the rule in terms of craton lithosphere bulk compositions and evolution. FeO
contents of some W. Greenland peridotites are in the range shown by cratonic peridotites
but some extend to considerably higher values (> 9wt%) and, combined with elevated TiO

are indicative of metasomatic enrichment. This complicates the interpretation of major ele-
ments in terms of likely depths and environment of melting. Compatible trace element
analyses will be undertaken to try to resolve this issue.

Re-Os isotopes

A selection of dunitic, harzburgitic and lherzolitic lithologies have been analysed for Re-Os
isotopes. Some dunites have extremely low osmium contents, possibly due to reaction with
melt dissolving osmium-rich phases. Harzburgites and lherzolites have Os contents more
typical of melt residues and have unradiogenic Os isotope compositions. Os isotope ratios
of peridotites from dykes erupted through the Archean basement give Archean model melt
depletion ages (oldest T

age currently 3 Gyr). No clear age variation with depth is evident

sOFar. Analysis of peridotites from the re-worked Archean terrane will be reported.

References

S. Bernstein, P.B. Kelemen & C.K. Brooks (1998) Depleted spinel harzburgite xenoliths in

Tertiary dykes from East Greenland: Restites from high degree melting. Earth Planet.
Sci. Lett. 154 , 221-235.

S.M. Jensen, K. Secher & T.M. Rasmussen (2004) Diamond content of three kimberlitic

occurrences in southern West Greenland. Danmarks og Grønlands Geologiske Un-
dersøgelse Rapport 2004/14

Acknowledgements

We thank Thermo Electron for sponsorship of MW's MSc. Thesis.

G E U S

Mantle stratigraphy of the Karelian craton implica-
tions for diamond prospecting

Peltonen, P.

, O'Brien, H.

, Lehtonen, M.

and Brügmann, G.

Geological Survey of Finland, P.O.Box 96, FIN-02151 Espoo, Finland

Max-Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany

Mantle-derived xenolith and xenocryst studies indicate that the subcontinental lithospheric
mantle of the Karelian craton shows considerable variation from margin to core. At the
margin, the mantle is stratified into at least three distinct layers labeled A, B, and C. Shal-
low layer A (at ~60110 km depth) has a knife-sharp lower contact against layer B indica-
tive of a shear zone implying episodic construction of the SCLM. Layer A peridotites have
"ultradepleted" arc mantle -type compositions, and have been metasomatised by radio-
genic

187

Os/

188

Os, presumably from slab-derived fluids.

Xenoliths derived from the middle layer B (at ~110180 km depth), which is the main
source of harzburgitic garnets (G10) in Finnish kimberlites, are characterised by an unra-
diogenic Os isotopic composition.

187

Os/

188

Os shows a good correlation with indices of par-

tial melting implying an age of ~3.3. Ga for melt extraction. This age corresponds with the
oldest formation ages of the overlying crust, suggesting that layer B represents the un-
modified SCLM stabilised during the Paleoarchean.

The underlying layer C (at 180250 km depth) is the main source of Ti-rich pyropes of
megacrystic composition, which however, lacks G10 pyropes. The osmium isotopic compo-
sition of the layer C xenoliths is more radiogenic compared to layer B, yielding only Pro-
terozoic T

ages. Layer C is interpreted to represent a melt metasomatised equivalent to

layer B. This metasomatism most likely occurred at c. 2.0 Ga when the present craton mar-
gin formed following break-up of the proto-craton.

The mantle stratigraphy of the craton core, in the area of Kuhmo 150 km to the NE, shows
less variation. Layer A is absent. Layer B begins with the lowest temperature pyropes at
an inferred depth of 70 km and continues to a depth of about 250 km showing a relatively
homogenous distribution of harzburgite and lherzolite pyropes throughout. Differences in
craton core layer B compared to craton edge layer B include: 1. Wehrlite garnet is very
rare, as is chrome diopside. 2. The G10 to G9 ratio in both exploration samples and har-
drock sources is considerably higher implying core layer B is relatively harzburgite-rich.
Thus far there is only a weak indication of a G10 pyrope-free Layer C. However, Ti-rich
megacryst composition pyropes are very common, so evidence for a deep 250-300 km
Layer C may become available with further sampling.

G E U S

Kimberlites and lamproites in continental recon-
structions implications for diamond prospecting

Pesonen, L.J.

, O'Brien, H.

, Piispa, E.

, Mertanen, S.

, Pel-

tonen

Laboratory for Solid Earth Geophysics, Univ. of Helsinki, P.O. Box 64, FIN-00014 Helsinki,

Finland

Geological Survey of Finland, P.O. Box 96, FIN-02151 Espoo, Finland

Kimberlitic and lamproitic rocks are well known in every Precambrian Shield. The ages of
kimberlite/lamproite dykes vary from Mesoproterozoic to Cretaceous. Since all hardrock
economic diamond deposits occur in these rock types, it is worthwhile, from the prospecting
point of view, to determine whether kimberlite/lamproite occurrences form either continuous
belts or distinct provinces at various times in supercontinent assemblages. For this purpose
we initiated a new project where kimberlites and lamproites will be compiled into a data-
base, which allows the continuities of kimberlite/lamproite belts to be studied in former su-
percontinents. In the testing stage of the project we focus on two Precambrian age intervals
of kimberlite/lamproite magmatism: the 580-600 Ma interval and the 1200 Ma activity, re-
spectively. In the database we compile the ages of kimberlite/lamproite dyke swarms (and
pipes), their widths and orientations, their distances to known orogenic belts as well as their
geochemical (particularly trace element) and isotopic signatures. Using paleomagnetic data
of coeval rocks (such as mafic dyke swarms and other intrusions and occasionally the kim-
berlite dykes themselves) we first make the continental reconstructions using the tech-
niques outlined in Pesonen et al. (2003). We will then plot the kimberlite/lamproite dykes
and pipes onto these supercontinent reconstructions to delineate possible continuous belts
or provinces. The paleomagnetically made global reconstructions of continents with kim-
berlite/lamproite occurrences at 600 Ma and 1200 Ma will be presented and compared with
those made on traditional geological grounds. The implications for diamond prospecting will
be discussed.

References

Pesonen, L.J., Elming, S.-Å ., Mertanen, S., Pisarevsky, S., D´ Agrella-Filho, M.S., Meert,

J.,G., Schmidt, P.W., Abrahamsen, N., Bylund, G., 2003. Palaeomagnetic configura-
tion of continents during the Proterozoic. Tectonophysics 375 , 289-324.

G E U S

Magnetic signatures of circular geological features
with special emphasis on kimberlite prospecting

Plado

1,2

& Pesonen, L.J.

Institute of Geology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia

Department of Geology, Estonian Land Board, Taara pst 2, 51005 Tartu, Estonia

Solid Earth Geophysics Laboratory, Division of Geophysics, University of Helsinki, PO

Box 64, FIN-00014 Helsinki, Finland

1. Introduction

The magnetic anomaly patterns of idealistic 3-dimensional geological structures are inves-
tigated in an aim tOFind observable diagnostic features and to identify the type of structure
from the magnetic data. The analyzed structures - kimberlite pipe, kaolin deposit, gabbroic
intrusion, and meteorite impact structure - may all have circular or nearly circular plan view.
Because their shapes in vertical plans and their physical properties are different, they pro-
duce specific residual geophysical anomalies. The diagnostic features, as shape of the
anomaly, magnitude, amplitude, and derivatives, allow making first approaches in determi-
nation of the source body based on the magnetic map.

2. Methods

Four hypothetical models, one for every particular geological case, were created. All the
models consist of several vertical and symmetrical prism-like bodies with 16 corner points
on a plan view. To compare the modeling results, each structure has a diameter of 1 km,
i.e., the uppermost prism is always 1 km in diameter. The thickness, horizontal extent, and
physical properties of the prisms are different for every model. For comparison purposes,
the natural remanent magnetization (NRM) always has the direction that corresponds to an
age of 500 Ma at the Fennoscandian Shield. The magnetic anomalies for each case are
calculated for both polarity options of this 500 Ma remanence vector, hereafter called nor-
mal and reversed polarity.

The background rock has a magnetic susceptibility of 3300 10

-6

SI, typical to the granite-

gneiss basement in Finland (Puranen, 1989). The induced magnetic field corresponds to
that of a central location of Fennoscandia and is taken from Pesonen et al. (1994). The

magnetic anomalies of these four bodies were calculated over a 2 2 km area using the

ModelVision Software Package by Encom Technology Pty Limited, Australia (1995).

We defined kimberlite as a deep undeformed carrot-shaped body (Fig. 1), which has ob-
tained a strong and homogeneous thermal magnetization during the fast cooling at 500 Ma.
We didn't consider post-intrusion changes of the magnetic properties (oxidation, weather-
ing, metamorphism etc.). The model, for calculation purposes, contains 8 vertical prisms,
which have different thickness and diameter. The lowermost prism extends to the depth of

G E U S

5 km. All prisms possess similar magnetic properties, derived from Korhonen and Kivekäs
(1997), as mean values of kimberlites in the central Baltic Shield.

In the model of kaolin , the low magnetizations are assigned to a lens-like body that ex-
tends to the depth of 180 m (Fig. 2). We assumed that the remanent magnetization is of
chemical origin, produced by the weathering of the underlying basement. The gabbro
model has lens-like shape with a maximum diameter of 1250 m in a depth of 250 - 300 m
(Fig. 3). The vertical extent is 700 m. All the prisms possess similar magnetic properties
(Table 1), derived from Korhonen et al. (1993), whereas the NRM is of thermal origin. To
describe the magnetic field above the simple impact structure two units, which are differ-
ent in their magnetic properties, were created. The uppermost 140 m thick unit simulates
crater filling post-impact sediments and possesses low magnetization (Fig. 4). Another unit
is the 300 m thick impact breccia lens, located around and below the sediments. Due to the
shock effects, the magnetization of the breccias is usually higher than in sediments, and
the NRM is dominating (e.g. Iso-Naakkima, Finland; Järvelä et al., 1995) over induced
magnetization (Q> 1).

3. Results

Magnetic sources in the upper part of the crust produce anomalies (A), which depend on
depth (h) of the source structure, its orientation (O), shape (S) and volume (V):

A = (1/h)

M F(O, S, V),

where M is magnetization contrast of the source body and its host rock and is calculated

as a vector sum of the induced and remanent magnetization. Thus, the amplitude of a
magnetic anomaly decreases rapidly in 3

power with depth thus reducing the depth and

height sensitivity of magnetic methods. The detectable magnetic sources are confined to
the uppermost few kilometers of the crust (Henkel and Reimold, 1997). Moreover, vector
nature of the magnetization will produce complex magnetic anomalies, which have both

positive and negative parts. Due to the very steep (72 - 77 ) inclination and moderate decli-

nation (2-12 ; Pesonen et al., 1994) of the Earth's magnetic field, highly magnetic bodies at

the Fennoscandian Shield produce intensive positive anomalies followed by a negative side
on the northern edge. The anomalous picture is overturned if K is negative, i.e. the mag-
netization of background is higher that those of the anomalous body, or the highly magnetic
source body possesses the reversed polarity.

The circular positive magnetic anomaly, due to a normally polarized kimberlite pipe with
dimensions and physical properties given in Fig. 1, reaches the maximum value of 580 nT.
Due to the eastwards directed NRM, acquired at 500 Ma, the central maximum is shifted
slightly to south-east. The narrow negative ring, most intensive in the northern side of the
structure surrounds the positive part of the anomaly. The vertical and horizontal gradients
(Fig. 1c) show great variations at the edges of the model, and reach values of several thou-
sands nTkm

-1

. In the case of reversed polarity of the NRM, the magnetic anomaly of kim-

berlite shows minimum of -260 nT in its central part, shifted sligthly to north-west from the
center. The gradients are up to 2300 nTkm

-1

, which are more than 2 times less than in the

case of normal polarity.
Compared to other modeled structures, the residual magnetic anomalies over the kimberlite
bodies are most similar to that of the gabbroic intrusions showing the most intensive

G E U S

anomalies up to thousands of nT (Fig. 3). Due to high magnetization, their anomalies are
positive in the case of normally polarized NRM, and vice versa in the case of reversed
magnetization. The amplitudes of the anomalies are much higher in the normal polarity
case. In a case the magnitude of the NRM is approximately equal and oppositely directed
to the induced magnetization, the resulting magnetization is zero and the source is invisible
in the magnetic data.

The magnetic anomalies of the meteorite impact structure (Fig. 4) and kaolin deposit (Fig.
2) may be similar to each other in amplitude, but most likely different from those of kimber-
lite and gabbroic intrusion. They both exhibit weak magnetic amplitudes. Kaolin deposit
produces negative anomalies in both normal and reversed cases, whereas impact structure
has a negative magnetic anomaly in the case of normal, and positive in the case of re-
versed polarity of the NRM.

4. Acknowledgements

We thank Olli Sarapää, Petri Peltonen, Matti Tyni and Seppo Elo (all at Geological Survey
of Finland) for helpful discussions. The research was supported by VMY, Partek Oy, and
the Estonian Science Foundation (grant #5500).

References

Encom Technology Pty Limited, 1995. ModelVision, Geophysical data display, analysis and

modelling, Version 1.20, 212 p.

Henkel, H. and Reimold, W.U., 1997. Integrated gravity and magnetic modelling of the Vre-

defort impact structure - reinterpretation of the Witwatersrand basin as the erosional
remnant of an impact basin. Royal Institute of Technology, Department of Geodesy
and Photogrammetry Stockholm , 90 p.

Järvelä, J., Pesonen, L.J. and Pietarinen, H., 1995. On palaeomagnetism and petrophysics

of the Iso-Naakkima impact structure, southeastern Finland. Geological Survey of
Finland, Internal report Q19/29.1/3232/95/1, 53 pp.

Korhonen, J.V., Säävuori H., Wennerström, M., Kivekäs, L., Hongisto, H. and Lähde, S.,

1993. One hundred seventy eight thousand petrophysical parameter determinations
from the regional petrophysical programme. Geological Survey of Finland, Special Pa-
per, 18 , 137-141.

Korhonen, J.V. and Kivekäs, L., 1997. Petrophysical properties of kimberlites and rocks of

Archaean basement of central Fennoscandian shield. In: H. Papunen (ed.), Mineral
Deposits . Balkema, Rotterdam, 771-774.

Pesonen, L.J., Nevanlinna, H., Leino M.A.H. and Rynö, J., 1994. The Earth's magnetic field

maps of 1990.0. Geophysica 30 , 57-78.

Puranen, R., 1989. Susceptibilities, iron and magnetite content of Precambrian rocks in

Finland. Geological Survey of Finland, Report of Investigation 90 , 45 p.

Figure 1 4:

G E U S

-1000

-500

500

1000

-1000

-500

500

1000

200

400

5000

Dept

h (m)

MAGNETIC FIELD

H=52µT

D="8"

I="75"

KIMBERLITE

=0.0122 SI

Q="2"

D="109"

I="75"

BACKGROUND

=0.0033 SI

KIMBERLITE: MAGNETIC ANOMALY

Prisms 1...8

-550

-450

-350

-250

-150

-50

150

250

350

450

Figure 1. (a) plan view of the magnetic anomaly (total intensity, nT) of kimberlite pipe at
the normal magnetization (N). (b) S-N profile of the magnetic anomaly (nT) across the
model. (c) South-north profile of the first vertical (V) and horizontal (H) gradient (nT/km) of
the magnetic anomaly of kimberlite at the normal magnetization. (d) The geophysical model
of kimberlite producing the calculated map (a) and profiles (b and c). Model consists of 8

polygonal vertical prisms with different thickness but homogeneous magnetic properties (

= magnetic susceptibility; Q = Koenigsberger ratio; D = declination; I = inclination; H =
magnetic field).

G E U S

-1000

-500

500

1000

-1000

-500

500

1000

200

h (

KAOLIN: MAGNETIC ANOMALY

KAOLIN; =0.0001 SI; Q=0.3;

D=109; I="75"

MAGNETIC FIELD

H=52µT

D="8"

I="75"

BACKGROUND

=0.0033 SI

Prisms 1...11

-60

-40

-20

Figure 2. (a) plan view of the magnetic anomaly (total intensity, nT) of kaolin deposit at
the normal magnetization (N). (b) S-N profile of the magnetic anomaly (nT) across the
model. (c) South-north profile of the first vertical (V) and horizontal (H) gradient (nT/km) of
the magnetic anomaly of kaolin at the normal magnetization. (d) The geophysical model of
kaolin producing the calculated map (a) and profiles (b and c). Model consists of 8 polygo-

nal vertical prisms with different thickness but homogeneous magnetic properties ( =

magnetic susceptibility; Q = Koenigsberger ratio; D = declination; I = inclination; H = mag-
netic field).

G E U S

-1000

-500

500

1000

-1000

-500

500

1000

200

400

600

Depth

(m)

GABBRO

=0.0111SI

Q=1.3

D="109"

I="75"

MAGNETIC FIELD

H=52µT

D="8"

I="75"

BACKGROUND

=0.0033 SI

GABBRO: MAGNETIC ANOMALY

-100

100

200

300

400

500

600

Prisms 1...14

Figure 3. (a) plan view of the magnetic anomaly (total intensity, nT) of gabbro at the nor-
mal magnetization (N). (b) S-N profile of the magnetic anomaly (nT) across the model. (c)
South-north profile of the first vertical (V) and horizontal (H) gradient (nT/km) of the mag-
netic anomaly of gabbro at the normal magnetization. (d) The geophysical model of gabbro
producing the calculated map (a) and profiles (b and c). Model consists of 14 polygonal

vertical prisms with thickness of 50 m each but homogeneous magnetic properties ( =

magnetic susceptibility; Q = Koenigsberger ratio; D = declination; I = inclination; H = mag-
netic field).

G E U S

-1000

-500

500

1000

-1000

-500

500

1000

200

Depth

(m)

IMPACT: MAGNETIC ANOMALY

MAGNETIC FIELD

H=52µT

D="8"

I="75"

BACKGROUND

=0.0033 SI

=0.0001 SI

Q="10"

D="109"

I="75"

400

SEDIMENTS; =0.00005 SI;

Q=0.1; D=109; I="75"

-50

-40

-30

-20

-10

Prisms 1...16

IMPACT BRECCIA

Figure 4. (a) plan view of the magnetic anomaly (total intensity, nT) of impact structure at
the normal magnetization (N). (b) S-N profile of the magnetic anomaly (nT) across the
model. (c) South-north profile of the first vertical (V) and horizontal (H) gradient (nT/km) of
the magnetic anomaly of the impact structure at the normal magnetization. (d) The geo-
physical model of the structure producing the calculated map (a) and profiles (b and c).
Model consists of 7 polygonal vertical prisms of sediments and 16 prisms for breccias with

thickness of 20 m each ( = magnetic susceptibility; Q = Koenigsberger ratio; D = declina-

tion; I = inclination; H = magnetic field).

G E U S

101

Application of geophysical methods to diamond ex-
ploration in Greenland experiences and data re-
view

Rasmussen, T.M.

Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copen-
hagen K, Denmark

Introduction

Geophysics has been an integral part of the methods used in the search for kimberlites in
Greenland. The geophysical data are valuable both with respect to the general under-
standing of gross structures of the lithosphere and with respect to the direct search for and
mapping of kimberlites.

In summary, magnetic data are found to be most valuable, whereas detailed electromag-
netic data have sOFar not proved very useful in the search for kimberlites in Greenland.
This presentation focus on the magnetic data.

Available data

Regional airborne magnetic surveys
Regional magnetic survey data covers the entire ice-free part of South and Central West
Greenland. The regional magnetic data set (Figure 1) is produced by merging the data from
different regional aeromagnetic surveys collected in the Airborne Geophysical Survey pro-
gramme during the years 19922001 financed jointly by the Geological Survey of Denmark
and Greenland (GEUS) and the Bureau of Minerals and Petroleum the (BMP). The line
spacing for these surveys is 500 m and the nominal sensor altitude above ground is 300 m.

Detailed airborne magnetic and electromagnetic surveys
Most of the detailed airborne surveys include combined electromagnetic and magnetic
measurements. The electromagnetic measurements have mainly been carried out with the
frequency domain Dighem V system, but one of the larger surveys was done with the tran-
sient electromagnetic GEOTEM system.

The coverage with detailed aeromagnetic survey data is shown by the black polygons in
Figure 1. The survey data originate from the AEM Greenland 1995 & 1996 projects fi-
nanced by the Bureau of Minerals and Petroleum (BMP) and from projects carried out by
prospecting companies. The data from the surveys carried out by the prospecting compa-
nies have been delivered to the BMP as part of the mandatory reporting by licensees. The
data are made publicly available after termination of the license agreement within which the
data were collected. Note that the surveys displayed in Figure 1 were only partly carried out

G E U S

103

caesium-vapour magnetic gradiometer manufactured by Geometrics Inc. that measures the
magnetic field intensity with two sensors. A GSM856 proton magnetometer was used for
recordings of diurnal magnetic variations at a base station placed within distances of a few
km from the profiles. The vertical gradient option for the GSM858 instrument with a top and
bottom sensor mounted on a vertical rod was used during the measurements. Recordings
of the magnetic field are made with a sampling frequency of 10 Hz while walking along the
profiles. A 10 Hz sampling corresponds approximately to 10 cm sampling distance. The
bottom and top sensors were approximately 0.5 m and 1.5 above the ground respectively.

Prospecting companies have also carried out ground profiling or made grid surveys with
measurements of the total field, although with a lower resolution than the GSM858 surveys.

Results from data processing and interpretations

Crustal domains and large-scale lineaments from regional and detailed aeromagnetic data
Large-scale lineaments have been proposed as a guide to kimberlite emplacement. The
magnetic field data from Greenland contain numerous large-scale features of which some
can be traced for several hundreds of kilometres. Some of the larger linear features are
shown in Figure 2. Most of the linear features defined by magnetic lows are most likely as-
sociated with faults or shear zones, where magnetite has been destroyed by oxidation.
Valleys associated with fracture zones often contribute significantly to the anomalous field
and the topographic effects are in some cases very significant. Dykes, mainly dolerites,
often cause the linear features defined by positive anomalies. The lineaments can be
viewed as elongated anomalies that are superimposed on the background of regional
anomalies. In most cases, it is evident from the shape of the background magnetic anoma-
lies that only minor lateral displacements are associated with the supposed fault and shear
zones.

Some coincidence between major linear features in the magnetic field and kimberlite occur-
rences is seen, but no clear correlation is evident. The kimberlite dykes located south of
Maniitsoq Ice Cap are in most cases oriented N80°E, but this direction is only observed for
two of the larger lineaments (marked a-A and b-B in Figure 2), and for numerous dolerite
dykes that crosses the boundary of the Inland Ice between latitudes 65°15' and 65°45'. It is
noticeable that the kimberlite dykes south of Maniitsoq Ice Cap are located in an area with
closely spaced lineaments. A clear correlation between lineaments and kimberlite occur-
rences is seen in the area around Sarfartoq. Figure 3 shows an example of linear magnetic
anomalies from the area around WE oriented Sarfartoq valley. The topographic low is
associated with a low in the magnetic field, but superimposed on that is a positive anomaly
that extends minimum 16 km in the valley. The positive anomaly is caused by a dyke of
ultramafic composition.

G E U S

105

Figure 3. Magnetic total field anomaly in the area around the Sarfartoq Valley. Black dots
marks kimberlite occurrences. The magnetic data are from regional and detailed airborne
surveys. The rectangle in black colour outlines the area shown in Figures 5 and 6.

Small-scale lineaments detailed magnetic data
In addition to the large-scale structures discussed above, the detailed magnetic survey data
contain significant amounts of structural information on a smaller scale. An automatic pro-
cedure has been developed that extracts linear features based on flight line data. The pro-
cedure identifies local minima and maxima in the flight line data, and evaluates the similar-
ity of the field variation across flight lines along line segments defined by locations of ex-
tremum values on adjacent flight lines. Thus, the term lines segment describes here a con-
nection between points on two adjacent flight lines . A standard root mean square (RMS)
function is used as a measure for the deviation between the field along the line segments.
The line segments are concatenated into lineaments by mean of RMS values in conjunction
with criteria for maximising the number of points on a lineament and minimising the angle
difference between connecting line segments. The procedure provides an objective deter-
mination of lineaments and is not subjected to the artefacts that often are produced from
levelling, interpolation and gridding. However, the result depends on the flight line direc-
tions, because magnetic anomalies elongated along a flight line direction will not have well-
defined extremum values. Threshold values for both anomaly amplitude and width are used
in the selection of extremum points.

Figure 4 shows an example for an area south of Maniitsoq Ice Cap (the rectangle shown in
Figure 2 outlines the area) Locations of minima and maxima are shown by dot in light blue
and red colour respectively. Corresponding lineaments that fulfil the selection criteria are
shown as lines in blue and cyan colour. Kimberlite occurrences are shown by green dots. A
system of kimberlite dykes with mean orientation N°80E is located at latitude 65°18'. The
thickness of the kimberlite dykes is of the order of 1 m, which is insufficient to produce a
detectable anomaly in the airborne data measured at an altitude of 45 m above ground.
The same orientation as for the kimberlite dykes can be seen in the preferred orientations
of the magnetic anomalies. Another prominent orientation of the magnetic anomalies are

106

G E U S

along N°45E and some lineaments are oriented approximately N-S. The results indicate
that the intrusion of the kimberlite dykes was controlled by a system of N°80E structures
that are seen in the entire area.

Figure 4. Minima (red dots) and maxima (light blue dots) of magnetic field data from an
area south of Maniitsoq Ice Cap outlined by the black rectangle in Figure 2. The lineaments
obtained from the minima are shown by lines in blue colour and lineaments obtained from
the maxima are shown by lines in cyan colour. Kimberlite occurrences are shown by green
dots. The magnetic total field is shown as background colour. The extremum points for the
analysis are limited by applying a selection criteria based on anomaly width (25
m <500m) and field strength.

G E U S

107

Ground magnetic surveys
The high spatial resolution provided by the GSM858 magnetometer results in data that are
very useful in an attempt to understand magnetic response from kimberlites and the host
rocks. The small width of most known kimberlite dykes requires a very dense sampling in
order to map the occurrences. Figures 5 and 6 illustrates typical responses from kimberlites
but they also illustrate some of the difficulties in distinguishing between responses caused
by variations in magnetisation of the host rocks. Figure 5 shows results of 20 magnetic pro-
files marked 1-20 that crosses mapped kimberlite dykes (black lines). Magnetic field data
from airborne data are also shown. Figure 6 shows the responses measured along profile
3. Along profile 3, two large magnetic peaks are observed. The southernmost anomaly is
caused by a narrow zone of high magnetic susceptibility of the host rock (see alsOFigure
3), whereas the northernmost anomaly is caused by a kimberlite dyke. Adjacent to the large
anomaly caused by a kimberlite dyke is a peak caused by a thin kimberlite. There is no
possibility to discriminate between the causes for the observed anomalies solely from the
magnetic data. Profile 7-12 cross an area covered by boulders where visual mapping of
kimberlites where difficult. The profile data from these profiles were useful by excluding the
existence of significant kimberlite occurrences in the surveyed area.

Figure 5. Magnetic total field anomaly measured at the top-sensor (large coloured dots)
along the ground profiles numbered 120 and magnetic total field anomaly from the air-
borne survey (small coloured dots). The class limit annotations above the legend bar are
for the ground survey data and the annotations below are for the airborne survey data.
Black lines mark the trace of the kimberlitic dykes.

108

G E U S

Figure 6. Magnetic total field anomaly at the top sensor (upper panel), vertical gradient
(middle panel) and total field anomaly at bottom sensor (lower panel) along profile 3. The
horizontal axis is the north co-ordinate in zone UTM22. The kimberlitic dykes are at co-
ordinates 7365508 m and 7365513 m. The peaks around co-ordinate 7365397 m are
caused by gneiss with high magnetic susceptibilities

Conclusion

The experience obtained from both airborne and ground surveying in Greenland is that
magnetic data can be used in a direct search for kimberlites provided the resolution is high
and that the data contains very valuable information about structures in general. The
structural information may be useful for interpreting kimberlite emplacement patterns.

G E U S

109

Distribution of kimberlite indicator minerals in till
within the Neoproterozoic
Sarfartoq-Maniitsoq province of kimberlite and ul-
tramafic lamprophyres, southern
West Greenland

Steenfelt, A., Jensen, S.M. & Sand, K.K.

Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen
K, Denmark

Introduction
The recovery of kimberlite indicator minerals (KIM) from overburden material such as till,
soil or stream sediment has been used widely as prospecting method for kimberlites, par-
ticularly in glaciated terrain. In Greenland, thousands of samples of till and stream sediment
have been collected and processed by exploration companies, and abundant indicator
minerals including garnet, chromite, ilmenite and clinopyroxene have been identified, par-
ticularly in the Sarfartoq-Maniitsoq (S-M) province of abyssal kimberlite and ultramafic lam-
prophyre. The compilation of all released company data by Jensen et al. (2004) enabled an
overview of the KIM distribution in West Greenland. In the Sarfartoq-Maniitsoq province,
the KIM distribution patterns have south-westerly trends and may be seen as trails subpar-
allel to the general direction of ice movement. This leaves the possibility that the KIM or
at least a large proportion of the grains are derived from unknown kimberlite bodies up-
ice from the present location.

2004-05 investigation by GEUS-BMP
An investigation was undertaken in 2004 to 2005 by GEUS and BMP with the aim of finding
the likely source for the KIM. Data have been acquired to compare the abundance and
composition of KIM within known dykes and sills with those in the surrounding till and gla-
ciofluvial deposits. In addition, samples have been collected with the aim of verifying previ-
ously obtained company data, because individual companies have used distinct methods
and laboratories to process and analyse their samples.

Geology and topography of study areas
The Neoproterozoic (0.560.6 Ga) Sarfartoq and Maniitsoq kimberlite fields comprise nu-
merous dykes and sills intruded into the Archaean craton of West Greenland. The craton is
a part of the North Atlantic Craton that also comprises parts of Labrador to the west as well
as parts of East Greenland and Scotland to the East. The southern Maniitsoq field lies en-
tirely within the craton, whereas the Sarfartoq field straddles the craton's northern boundary
towards the Palaeoproterozoic Nagssugtoqidian Orogen. The country rocks for both kim-
berlite fields are dominated by tonalitic to granodioritic orthogneisses, but also comprises

110

G E U S

enclaves of supracrustal rocks of predominantly mafic volcanic origin, and a Palaeopro-
terozoic dolerite dyke swarm affects most of the Sarfartoq field.
Topographically, the study areas display large variations. In the Sarfartoq area gently roll-
ing lowlands gradually rise in altitude from north (average about 500 m) to south (average
about 1000 m). The Maniitsoq area presents a landscape dominated by steep SSW-
trending ridges with peaks in the range of 500 to1200 m and a coastline characterised by
deep fjords.

Quaternary geology
With the exception of a few very high mountain peaks, the entire West Greenland has been
glaciated. The deposits of till resulting from the retrieval of the ice are irregularly distributed
and generally thin except where the ice margin has been stationary over a period of time,
and undulating ridges of terminal moraines have formed. Also the main valleys have large
amounts of glacial deposits, both terminal moraines from valley glaciers and abundant gla-
cio-fluvial material. Although ice and melt water movements have been generally towards
the west, the late valley glaciers have caused transport in more varied directions from NW
to SW, so that directions of transported overburden are not easily deduced at a local scale.

Sampling and sample treatment
Sampling was performed from 8 field camps supplied by helicopter-supported visits to se-
lected localities. The distribution of sample sites was intended to cover some known kim-
berlite occurrences, previously sampled areas and background areas outside and up-ice
from known kimberlite occurrences.

Rock samples were collected by hammer as representatively as possible to obtain c. 5 kg
of material. They were treated at Overburden Drilling Ltd. where they were milled in stages
to maximize 0.25 to 2 mm grain size fraction. KIM were picked from three fractions (0.25-
0.5, 0.5 1, and 1-2 mm) of non-ferromagnetic, heavy (d> 3.2 g/cm

) minerals.

Till samples (overburden samples)
A pit was dug with a common field spade at each sample site in the local overburden, most
commonly till, but locally glaciofluvial material were present at the sampling site. Material
from below the vegetation was loaded onto and passed through a 4-mesh (6.35 mm)
screen fitted on top of a 20-l bucket. Both screen and bucket had been rinsed carefully with
a brush, and further rinsed by the discarded first load of material from each new site. Then
till material was passed through the screen and collected in the bucket until an amount of c.
20 kg was achieved. The resulting pit was typically 40 to 50 cm deep with a diameter of 30
to 40 cm. The surroundings of the site and the character of the sampled till (proportion of
gravel, sand and silt, humidity) was noted. The samples were treated at Overburden Drilling
Ltd. to produce three fractions (0.25-0.5, 0.5-1, 1-2 mm) of non-ferromagnetic heavy miner-
als (d> 3.2 g/cm

) from which kimberlite indicator minerals were picked.

A small sample c. 400 g was collected representatively (though avoiding stones larger than
1 cm) from the sides of each pit using a small plastic shovel and a paper sample bag. The
sides of the pit were first scraped with the plastic shovel to avoid contamination from the
metal of the spade. The samples were further dried at GEUS, then screened into a number
of fractions to characterise the grain size composition of the till at the sampling sites. The

G E U S

111

0.1 mm fractions have been analysed chemically for major and trace elements (ICP-ES and
ICP-MS upon dissolution of fused samples) at Activation Laboratories Ltd.

Mineral chemistry
About 7000 mineral grains have been analysed by microprobe at Geocenter Copenhagen.
Preliminary results are presented by Jensen et al. (this volume).

Results
KIM were picked from all but one of the 27 rock samples submitted, and from 80 of the 131
till samples submitted.

Garnet

Rock

Peridotic

Eclogitic

Clinopyroxene

Ilmenite

Chromite

Max

21650

6125

47250

151700

5312

Min

Median

536

12272

Till

Max

3205

31600

746

Min

Median

Table 1. Total number of picked kimberlite indicator mineral grains in the 0.25 to 2 mm
grain size fraction.

Local dispersion
The abundance of KIM is high close to kimberlite dykes (sills) or among kimberlite boul-
ders, but the number decreases significantly within a short range down-ice from the occur-
rence. Many new occurrences and boulders were found at the sampling sites, so that
where KIM have been recorded previously in till samples, a local source were found pres-
ent. In most of the sites where our samples were collected close to previous company
samples, the order of magnitude of KIM was the same (Figure 1). KIM were not recorded in
till samples from areas without kimberlite occurrences, including those up-ice from known
occurrences. The chemistry of the fine fraction of till sustains that the till surrounding kim-
berlite occurrences contains kimberlite-derived material.

Regional variation
The till results mimics regional variations observed in rock samples, namely that peridotitic
and eclogitic garnets are more abundant in the Maniitsoq than in the Sarfartoq region (Fig-
ure 2), and that chromite and clinopyroxene relative to peridotitic garnets are higher in the
Sarfartoq region. Rock samples from a locality north of the boundary towards the
Nagssugtoqidian orogen also contain KIM, like the till samples.

G E U S

113

Figure 2. Regional distribution of pyrope (peridotitic garnet) from rock samples (left) and
till samples (right).

Conclusion
The similarity in abundance and proportion between KIM from kimberlite rocks and the sur-
rounding till, both locally and regionally, together with the absence of KIM in till in kimberlite
barren areas are taken to indicate that the KIM in overburden have a local origin. The KIM
in till samples north of the Nagssugtoqidian front are likewise probably sourced by local
rocks. Jensen et al. (this volume) provides further evidence for our conclusion.

Testing the implication
The compilation of company KIM in till data (Jensen et al. 2004) shows a small cluster of
picked garnet south-east of Nuuk. Neither lamprophyres nor kimberlites were known from
that area until several ultramafic lamprophyre dykes and a carbonatite complex were dis-
covered by GEUS during this summer's fieldwork (2005).

Jensen, S.M., Secher, K., Rasmussen, T.M. & Schjøth, F. 2004: Diamond Exploration data

from West Greenland: 2004 update and revision. Danmarks og Grønlands Geologiske
Undersøgelse Rapport 2004/117 , 90 pp., 1 DVD-ROM.

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115

Ultramafic Lamprophyres and carbonatites of Lab-
rador and New Quebec: towards a genetic model for
Neoproterozoic rift-related alkaline magmatism in
the North Atlantic region

Tappe, S.

, Foley, S.F.

, Kjarsgaard, B.A.

, Heaman, L.M.

Jenner, G.A.

, Stracke, A.

& Romer, R.L.

Institut für Geowissenschaften, Universität Mainz, Becherweg 21, D-55099 Mainz,

Germany

Geological Survey of Canada, Ottawa, Ontario, Canada K1A 0E8

Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta,

Canada T6G 2E3

Department of Earth Sciences, Memorial University, St. John's, Newfoundland, Canada

A1B 3X5

Max-Planck-Institut für Chemie, Postfach 3060, D-55020 Mainz, Germany

GeoForschungszentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany

Abstract

Late Neoproterozoic potassic to carbonatitic dyke rocks of deep origin occur throughout
northern Labrador and New Quebec forming part of an alkaline province which encom-
passes the former Laurentian margin, West Greenland and the Scandinavian Peninsula.
Characteristic rock types of this North Atlantic Alkaline Province (Doig, 1970; Tappe et al.
2004) are ultramafic lamprophyres (UML) and associated carbonatites, but not kimberlites
(Tappe et al. , 2005).

Our U-Pb perovskite results for the UML dykes from northern Labrador/New Quebec and
from central Labrador (Aillik Bay) indicate similar emplacement ages ranging between 606-
568 and 590-555 Ma, respectively. Reported ages for carbonate-rich ultramafic dykes from
Sisimiut, Sarfartoq and Maniitsoq in West Greenland (~ 607-583 Ma; see Larsen & Rex,
1992) fall within the same time frame. This implies coeval and comparatively long lasting
UML/carbonatite igneous activity at the borders of the present-day Labrador Sea during the
Late Neoproterozoic.

The northern Labrador/New Quebec UML (aillikites and mela-aillikites) define an array in
Sr-Nd and Nd-Hf isotope space that extents from fairly depleted compositions with positive

(up to +1.8), positive

(up to +3.5) and unradiogenic

Sr/

(582)

<0.7037) towards

long-term enriched isotope compositions with negative

(down to -3.3), negative

(as

low as 6.6) and moderately radiogenic

Sr/

(582)

(0.7039-0.7046). The carbonate-rich

aillikites predominately fall on the depleted end of this "Torngat array", whereas the car-
bonate-poorer mela-aillikites tend to be isotopically more enriched; however, there are gra-
dations between rock types. The generally carbonate-richer aillikites (> 10 wt.% CO

) from

116

G E U S

Aillik Bay have more homogenous Sr-Nd-Hf isotope compositions than their northern Lab-

rador/New Quebec analogues and fall close to the depleted end of the "Torngat array" (

= 0.1-1.8;

= -0.9 to 2.6;

Sr/

(582)

typically <0.7040).

Any petrogenetic scenariOFor UML magma generation at the borders of the Labrador Sea
has to account for the presence of residual phlogopite in the melting assemblage, which
places the magma source region in comparatively cold cratonic lithospheric mantle (
1300°C). Furthermore, the unradiogenic Nd and Hf isotope compositions of some UML
forming the long-term enriched end of the "Torngat array" indicate a contribution from iso-
lated subcontinental lithospheric mantle (SCLM). However, the carbonate seems to be de-

rived from convective upper mantle given the positive

and

values of the carbonate-

richest UML. These observations are best interpreted by a large-scale upwelling of the as-
thenosphere beneath the present-day Labrador Sea area due to incipient continental rifting,
thereby heating and successively converting the stretched base of the SCLM. Potassic to
carbonatitic fluids/melts were injected into the cold base of the SCLM, where they solidified
as a phlogopite + carbonate-dominated vein network shortly prior to UML magmatism

(there was not enough time for Hf-Nd isotope decoupling; delta

= -1.7 to -4.6). Re-

melting of the veins plus melting of the lithospheric wall-rock peridotite occurred because
the asthenosphere-lithosphere boundary beneath the present-day Labrador Sea area
moved further upward and sideward during rift propagation (from ~ 180 to 120 km). The
fact that UML from the northernmost occurrences (i.e. Torngat UML) span a well developed
isotope mixing array points to a significant contribution from older SCLM to those magmas.
The carbonate-rich type aillikites, however, represent UML magmas with a much higher
vein/SCLM wall rock ratio in the melting assemblage than their Torngat counterparts, which
implies advanced lithospheric thinning in the southern part of the Labrador Sea rift zone at
~ 582 Ma. The presence of a long-term enriched SCLM reservoir deep underneath Aillik
Bay is evident from the Nd-Hf-Sr-Pb isotope signature of recently discovered Mesoprotero-

zoic lamproites (

= -5.3 to -8.4;

= -7.8 to -10;

Sr/

(1376)

typically> 0.7047;

206

Pb/

204

(1376)

= 14.2-14.8).

Taken together, enhanced UML and carbonatite igneous activity affected the cratonic as-
sembly of the North Atlantic region as a consequence of large-scale plate reorganization
(breakup of Rodinia) during the Late Neoproterozoic. Ascending potassic carbonate-rich
asthenosphere-derived fluids/melts had been concentrated underneath stretched lithos-
pheric blocks, where they solidified as veins. Subsequent high pressure re-melting (~ 4-6
GPa) of these carbonate-phlogopite dominated vein assemblages together with variable
amounts of their host peridotites during the incipient rifting stages produced carbonate-rich
UML magmas. These primary UML magmas intruded the rift margins as dyke swarms or
central complexes, but low-pressure processes such as liquid immiscibility and filter press-
ing may have produced a variety of associated rock types including carbonatites.

G E U S

117

References

Doig, R. (1970). An alkaline rock province linking Europe and North America. Canadian

Journal of Earth Sciences 7 , 22-28.

Larsen, L. M. & Rex, D. C. (1992). A review of the 2500 Ma span of alkaline-ultramafic,

potassic and carbonatitic magmatism in West Greenland. Lithos 28 , 367-402.

Tappe, S., Jenner, G. A., Foley, S. F., Heaman, L. M., Besserer, D., Kjarsgaard, B. A. &

Ryan, A. B. (2004). Torngat ultramafic lamprophyres and their relation to the North
Atlantic Alkaline Province. Lithos 76 , 491-518.

Tappe, S., Foley, S. F., Jenner, G. A. & Kjarsgaard, B. A. (2005). Integrating ultramafic

lamprophyres into the IUGS classification of igneous rocks: rational and implications.
Journal of Petrology 46, 1893-1900.

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119

Detection of kimberlitic rocks using airborne hyper-
spectral data from southern West Greenland

Tukiainen, T. & Thorning, L.

Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copen-
hagen K, Denmark

High-resolution hyperspectral (HS) remote sensing data have been successfully used for
the location of kimberlitic rocks, e.g. in Australia and Africa. However, its potential as a vi-
able method for the mapping of kimberlite occurrences in arctic glaciated terrain with high
relief was previously uncertain. In July - August 2002, GEUS conducted an airborne hyper-
spectral survey in central West Greenland using the commercially available HyMap hyper-
spectral scanner operated by HyVista Corporation, Australia (Fig.1). Data were processed
in 2003, and in 2004 follow-up field work was carried out in the Kangerlussuaq region to
test possible kimberlites indicated by the HS data. The project was financed by the Bureau
of Minerals and Petroleum, Government of Greenland.

Figure 1. Coverage of the HyperGreen2002 survey in West Greenland as indicated by a
shaded topographic map.

Kimberlites consist of predominantly ultramafic material that has crystallised in situ , and
commonly host megacrysts formed in the upper mantle from the kimberlite magma and
mantle derived xenoliths (dunite, lherzolite, wehrlite, harzburgite, eclogite and granulite)

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G E U S

incorporated during magma transport. Common matrix minerals are olivine, phlogopite,
perovskite, spinel, chromite, diopside, monticellite, apatite, calcite and Fe-rich serpentine.
The most interesting minerals with respect to hyperspectral mapping are phlogopite, Fe-
rich serpentine (antigorite) and calcite; these minerals have characteristic spectral re-

sponses in the Short Wave Infrared (SWIR) spectral region (2.0 2.5 m).

TOFully exploit the possibilities of hyperspectral image data delivered "at sensor radiance
data", they must be converted to surface reflectance data. The small size of potential tar-

gets and the relatively subtle spectral characteristics as
established by a ground truth survey included in the proj-
ect, demonstrated that the rugged terrain conditions of
West Greenland required the use of advanced atmos-
pheric correction methods.

The conversion of the data to surface reflectance was
done by producing a detailed digital elevation model and
using this with the hyperspectral data in a commercially
available software package, which take sensor viewing
geometry and terrain information into consideration.
Comparison of the HyMap spectrum of kimberlite to the
spectra measured with a field instrument at the same
locality shows a close match (Fig. 2) demonstrating the
quality of the conversion to surface reflectance.

Figure 2. Comparison of laboratory mineral spectra to
the kimberlite spectra measured by field instruments (FS)
and airborne HyMap hyperspectral scanner.

The field measurements have shown that the spectral response from kimberlitic rocks

within wavelengths of 2.02.5 m is remarkably uniform. Thus the simplest way to locate

the kimberlitic rocks is to use selected characteristic kimberlite field spectra as end mem-
bers for the spectral processing. The Spectral Angle Mapper (SAM) was used in this project
for comparing the HS image spectra to selected, characteristic kimberlite field spectra and
mineral spectra such as phlogopite and antigorite.

The results were encouraging. The known occurrences could be seen in the hyperspectral
data; new ones were also suggested and later proven during the fieldwork. Examples will
be shown and some cases of similar anomalies originating from other types of rocks will be
discussed.

G E U S

123

Papers published from GEUS 20002005 with rela-
tion to Greenland kimberlites

Jensen, S.M., Hansen, H., Secher, K., Steenfelt, A., Schjøth, F. & Rasmussen, T.M.

(2002): Studies of kimberlitic rocks in the Sisimiut-Kangerlussuaq region, southern
West Greenland, Abstract in: Nielsen, B.M. & Thrane, K. (eds.): Workshop on
Nagssgtoquidian and Rinkian geology, West Greenland, Danmarks og Grønlands
Geologiske Undersøgelse Rapport 2002/9 , 22-24.

Jensen, S.M., Hansen, H., Secher, K., Steenfelt, A., Schjøth, F. & Rasmussen, T.M.

(2002): Kimberlites and other ultramafic alkaline rocks in the Sisimiut-Kangerlussuaq
region, southern West Greenland. Geology of Greenland Survey Bulletin 191 : 57-66.

Jensen, S.M., Lind, M., Rasmussen, T.M., Schjøth, F. & Secher, K (2003): Diamond explo-

ration data from West Greenland. Danmarks og Grønlands Geologiske Undersøgelse
Rapport 2003/21 : 50 pp +1 DVD.

Jensen, S.M., Secher, K., Rasmussen, T.M. & Schjøth, F. (2003): Distribution and mag-

netic signatures of kimberlitic rocks in the Sarfartoq region, southern West Greenland.
8th International Kimberlite Conference, Victoria, British Columbia, Canada.

Jensen, S.M., Secher, K., Rasmussen, T.M., Tukiainen, T., Krebs, J.D. & Schjøth, F.

(2003): Distribution and magnetic signatures of kimberlitic rocks in the Sarfartoq re-
gion, southern West Greenland. 8th International Kimberlite Conference, Victoria,
B.C., Canada. Extended abstracts CD-ROM, 5 pp. [Poster presentation in PDF format
available from GEUS on request].

Jensen, S.M., & Secher, K. (2004): Investigating the diamond potential of southern West

Greenland. Geological Survey of Denmark and Greenland Bulletin 4 , 6972.

Jensen, S.M & Secher, K. (2004): Diamond exploration in Greenland, Fact Sheet 7 , 2 pp.

Jensen, S.M., Secher, K. & Rasmussen, T.M. (2004): Diamond content of three kimberlitic

occurrences in southern West Greenland. Diamond identification results, field descrip-
tion and magnetic profiling. Danmarks og Grønlands Geologiske Undersøgelse Rap-
port 2004/19 : 41 pp.

Jensen, S.M., Secher, K., Rasmussen, T.M. & Schjøth, F. (2004): Diamond exploration

data from West Greenland: 2004 update and revision, Danmarks og Grønlands
Geologiske Undersøgelse Rapport 2004/117 : 90pp + 1 DVD.

Nielsen, T.F.D. & Jensen, S.M. (2005): The Majuagaa calcite-kimberlite dyke, Maniitsoq,

southern West Greenland, Danmarks og Grønlands Geologiske Undersøgelse Rap-
port 2005/43 , 59 pp.

Secher, K. & S.M., Jensen (2004). Diamond exploration in Greenland. Geology & Ore 4 : 12

pp.

Steenfelt, A. (2001): Geochemical atlas of Greenland - West and South Greenland. Dan-

marks og Grønlands Geologiske Undersøgelse Rapport 2001/46 , 39 pp + CD-ROM.

Tukiainen, T., Krebs, J.D., Kuosmanen, V., Laitinen, J. & Schäffer, U.

2003): Field and

laboratory reflectance spectra of kimberlitic rocks, 0.35 - 2.5

m , West Greenland.

Danmarks og Grønlands Geologiske Undersøgelse Rapport 2003/43 , 25 pp.

Tukiainen, T. & Krebs, J.D. (2004): Mineral resources of the Precambrian shield of central

West Greenland (66 to 70 15'N), Part 4. Mapping of kimberlitic rocks in West Green-
land using airborne hyperspectral data. Danmarks og Grønlands Geologiske Under-
søgelse Rapport 2004/45 , 40 pp + 1 DVD.

www.geus.dk > Main areas of work > Raw materials and mapping > Greenland > Report 2005/68 - overview > This page

Workshop on Greenland's diamond potential

Danmarks og Grønlands Geologiske Undersøgelse Rapport 2005/68