Geological Survey of Denmark and Greenland Bulletin
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Nr. 4, Review of Survey activities 2003, pp. 69-72 |
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69
Southern West Greenland hosts a province of ultramafic alka-
line rocks, including swarms of dykes traditionally described
as kimberlites and lamproites (Larsen 1991; Jensen et al.
2002). Since the mid-1990s, commercial diamond explo-
ration has been focused on the Sarfartoq region and the
region south-east of Maniitsoq (Fig. 1), and has resulted in
numerous reports of diamond-favourable indicator minerals
from till sampling, finds of kimberlitic dykes, and recovery of
diamonds from kimberlitic rocks.
line rocks, including swarms of dykes traditionally described
as kimberlites and lamproites (Larsen 1991; Jensen et al.
2002). Since the mid-1990s, commercial diamond explo-
ration has been focused on the Sarfartoq region and the
region south-east of Maniitsoq (Fig. 1), and has resulted in
numerous reports of diamond-favourable indicator minerals
from till sampling, finds of kimberlitic dykes, and recovery of
diamonds from kimberlitic rocks.
A new digital compilation of company data released from
confidential status (Jensen et al. 2003a) presents a compre-
hensive overview of exploration activities and results that
have emerged since the Survey's first compilation of occur-
rences of kimberlitic and related rocks (Larsen 1991). The
new compilation in a GIS (geographic information system)
environment allows for refined assessment of the distribu-
tion, structural control and possible spatial and petrogenetic
relationships that characterise the kimberlitic occurrences.
hensive overview of exploration activities and results that
have emerged since the Survey's first compilation of occur-
rences of kimberlitic and related rocks (Larsen 1991). The
new compilation in a GIS (geographic information system)
environment allows for refined assessment of the distribu-
tion, structural control and possible spatial and petrogenetic
relationships that characterise the kimberlitic occurrences.
In 2003, the Geological Survey of Denmark and
Greenland (GEUS) and the Government of Greenland's
Bureau of Minerals and Petroleum (BMP) went further than
has been customary in investigating the economic potential
of specific sites. Four areas were temporarily closed to appli-
cation for exploration licences, pending sampling and testing
for diamond content of large samples of more than one tonne
each from significant kimberlitic occurrences. Additional
characterisation and research initiated on these and other
occurrences include magnetic mapping, detailed petrography
and studies of mantle xenoliths, as well as indicator mineral
chemistry. An extensive programme to determine the ages of
kimberlitic and related rocks was also initiated in 2003.
Bureau of Minerals and Petroleum (BMP) went further than
has been customary in investigating the economic potential
of specific sites. Four areas were temporarily closed to appli-
cation for exploration licences, pending sampling and testing
for diamond content of large samples of more than one tonne
each from significant kimberlitic occurrences. Additional
characterisation and research initiated on these and other
occurrences include magnetic mapping, detailed petrography
and studies of mantle xenoliths, as well as indicator mineral
chemistry. An extensive programme to determine the ages of
kimberlitic and related rocks was also initiated in 2003.
Distribution of kimberlitic occurrences
Alkaline ultramafic dykes in the SisimiutKangerlussuaq and
Sarfartoq regions intrude the border zone between the
Archaean craton and the Palaeoproterozoic Nagssugtoqidian
orogen (Fig. 1; Secher & Larsen 1980). The occurrences
south-east of Maniitsoq mark the southernmost extent of the
alkaline province.
Sarfartoq regions intrude the border zone between the
Archaean craton and the Palaeoproterozoic Nagssugtoqidian
orogen (Fig. 1; Secher & Larsen 1980). The occurrences
south-east of Maniitsoq mark the southernmost extent of the
alkaline province.
The alkaline rocks of interest here have previously been
described as kimberlites and lamproites (Larsen 1991; Jensen
et al. 2002). This classification was questioned by Mitchell et
et al. 2002). This classification was questioned by Mitchell et
al. (1999), who consider that they are not typical kimberlites,
but unusual ultramafic lamprophyres in that they are some-
times diamondiferous. In the absence of petrographic data
for many of the occurrences the broader terms `kimberlitic'
and `lamproitic' are applied here.
but unusual ultramafic lamprophyres in that they are some-
times diamondiferous. In the absence of petrographic data
for many of the occurrences the broader terms `kimberlitic'
and `lamproitic' are applied here.
Three clusters of dykes have been recognised within the
province during the last 2030 years (Larsen 1980, 1991;
Scott 1981). The `Sisimiut cluster', consisting mainly of
12141240 Ma lamproitic and c. 590 Ma kimberlitic dykes
(Larsen & Rex 1992), is characterised by vertical EW to
SENW trends. The `Sarfartoq cluster' has been described as
a cone-sheet structure centred on the 600 Ma Sarfartoq car-
Scott 1981). The `Sisimiut cluster', consisting mainly of
12141240 Ma lamproitic and c. 590 Ma kimberlitic dykes
(Larsen & Rex 1992), is characterised by vertical EW to
SENW trends. The `Sarfartoq cluster' has been described as
a cone-sheet structure centred on the 600 Ma Sarfartoq car-
Geological Survey of Denmark and Greenland Bulletin 4, 6972 (2004) © GEUS, 2004
Investigating the diamond potential of southern
West Greenland
West Greenland
Sven Monrad Jensen and Karsten Secher
Fig. 1. Map of kimberlitic and diamond occurrences of the West
Greenland alkaline province. Framed areas enclose the three kimberlitic
occurrences sampled for determination of diamond content in 2003.
bonatite complex (Larsen 1980). The 600 Ma kimberlitic
dykes of the `Maniitsoq cluster' have more variable orienta-
tions.
dykes of the `Maniitsoq cluster' have more variable orienta-
tions.
The cone-sheet model for kimberlitic dykes around the
Sarfartoq carbonatite complex was largely based on dyke ori-
entations in a broad EW-trending valley transecting the
core of the complex. With new knowledge of hundreds of
additional kimberlitic occurrences in the region presented in
Jensen et al. (2003c), it now appears that other structural ele-
ments have controlled the emplacement of dykes. For exam-
ple, some kimberlitic dykes follow the trends of the
Palaeoproterozoic Kangâmiut dolerite dykes, as outlined by
Escher et al. (1970, 1976), in reworked as well as unreworked
parts of the Archaean basement. Another example is an
apparent predominance of NS-trending kimberlitic dykes
in a zone reaching far beyond the Sarfartoq carbonatite com-
plex. Information from magnetic field data lends support to
the hypothesis that kimberlitic dyke emplacement may be
controlled by structures of regional character (Jensen et al.
2003b, c).
entations in a broad EW-trending valley transecting the
core of the complex. With new knowledge of hundreds of
additional kimberlitic occurrences in the region presented in
Jensen et al. (2003c), it now appears that other structural ele-
ments have controlled the emplacement of dykes. For exam-
ple, some kimberlitic dykes follow the trends of the
Palaeoproterozoic Kangâmiut dolerite dykes, as outlined by
Escher et al. (1970, 1976), in reworked as well as unreworked
parts of the Archaean basement. Another example is an
apparent predominance of NS-trending kimberlitic dykes
in a zone reaching far beyond the Sarfartoq carbonatite com-
plex. Information from magnetic field data lends support to
the hypothesis that kimberlitic dyke emplacement may be
controlled by structures of regional character (Jensen et al.
2003b, c).
Digital compilation of data
Exploration companies have produced a large volume of data
relevant to diamond exploration, especially since 1994. The
data include analyses of heavy minerals from till and stream
sediment samples, dyke and boulder distribution maps, air-
borne and ground geophysical surveys, results of testing for
diamond content of mini-bulk sampled dykes, drill logs, etc.
relevant to diamond exploration, especially since 1994. The
data include analyses of heavy minerals from till and stream
sediment samples, dyke and boulder distribution maps, air-
borne and ground geophysical surveys, results of testing for
diamond content of mini-bulk sampled dykes, drill logs, etc.
A GIS compilation of company exploration data now in
the public domain (Jensen et al. 2003a) constitutes the most
extensive overview of kimberlitic rocks and diamond occur-
rences in Greenland to date. The compilation contains
scanned text and maps, and selected digital data from com-
pany assessment reports submitted to BMP in fulfilment of
the standard terms for mineral exploration licences. A total of
146 company reports of relevance to diamond exploration,
with a total of approximately 9250 pages of text, tables and
maps, are included as PDF files. More than 53 000 tabulated
analyses of indicator minerals from till and stream sediment
are available, as are the details and results of drilling cam-
paigns and tests for diamond content.
extensive overview of kimberlitic rocks and diamond occur-
rences in Greenland to date. The compilation contains
scanned text and maps, and selected digital data from com-
pany assessment reports submitted to BMP in fulfilment of
the standard terms for mineral exploration licences. A total of
146 company reports of relevance to diamond exploration,
with a total of approximately 9250 pages of text, tables and
maps, are included as PDF files. More than 53 000 tabulated
analyses of indicator minerals from till and stream sediment
are available, as are the details and results of drilling cam-
paigns and tests for diamond content.
70
Fig. 2. Outcrop of one of the three kimberlitic occurrences from which more than 1000 kg of rock was collected (Area 1 in Fig. 1). Width of dyke
approx. 1 m.
Diamond occurrences and indicator
mineral chemistry
mineral chemistry
Most of the approximately 600 diamonds reported to date in
Greenland are from just two areas, both located in the unre-
worked Archaean craton (Jensen et al. 2003a). Some 95% of
the stones are classified as microdiamonds, i.e. smaller than
the minimum size recoverable in operating mine plants typ-
ically those passing a 0.5 mm screen. Another 20 microdia-
monds from kimberlitic occurrences to the west of the
Sarfartoq carbonatite complex were reported in late 2003
(Tuer 2003). The largest diamond from an in situ dyke
reported to date is c. 1.7 mm in its longest dimension and has
a weight of around 0.001 carat (1 carat = 0.2 g). Although
microdiamonds do not constitute an economic resource, they
are important to the characterisation of kimberlites and eval-
uation of diamond deposits (Rombouts 1995).
Greenland are from just two areas, both located in the unre-
worked Archaean craton (Jensen et al. 2003a). Some 95% of
the stones are classified as microdiamonds, i.e. smaller than
the minimum size recoverable in operating mine plants typ-
ically those passing a 0.5 mm screen. Another 20 microdia-
monds from kimberlitic occurrences to the west of the
Sarfartoq carbonatite complex were reported in late 2003
(Tuer 2003). The largest diamond from an in situ dyke
reported to date is c. 1.7 mm in its longest dimension and has
a weight of around 0.001 carat (1 carat = 0.2 g). Although
microdiamonds do not constitute an economic resource, they
are important to the characterisation of kimberlites and eval-
uation of diamond deposits (Rombouts 1995).
Nearly all of the reported diamonds have been recovered
using caustic dissolution, a method that is usually adjusted to
recover stones of all sizes down to around 0.1 or 0.15 mm. In
addition, a few diamonds have been found in large stream
sediment samples. Some occurrences have also been sub-
jected to dense media separation tests for larger diamonds,
where up to 11 tonnes of kimberlitic rock have been
processed (Boucher 2000), but none of these tests have recov-
ered any diamonds.
recover stones of all sizes down to around 0.1 or 0.15 mm. In
addition, a few diamonds have been found in large stream
sediment samples. Some occurrences have also been sub-
jected to dense media separation tests for larger diamonds,
where up to 11 tonnes of kimberlitic rock have been
processed (Boucher 2000), but none of these tests have recov-
ered any diamonds.
All in situ diamond occurrences lie within areas outlined
by diamond-favourable indicator minerals from till and
stream sediment samples. On a local scale, however, kimber-
lite tracing using indicator minerals from till samples is not
straightforward, probably due to the influence of complex
glacial dynamics on the formation of the till deposits. The
most diamond-favourable indicator minerals are distributed
far beyond the areas with known diamonds. This observa-
tion, together with the postulated regional structural con-
stream sediment samples. On a local scale, however, kimber-
lite tracing using indicator minerals from till samples is not
straightforward, probably due to the influence of complex
glacial dynamics on the formation of the till deposits. The
most diamond-favourable indicator minerals are distributed
far beyond the areas with known diamonds. This observa-
tion, together with the postulated regional structural con-
trols, suggests that the potential for diamonds is not restricted
to the known occurrences. The potential appears to exist on
both sides of the boundary between reworked and unre-
worked Archaean basement.
to the known occurrences. The potential appears to exist on
both sides of the boundary between reworked and unre-
worked Archaean basement.
Testing of three kimberlitic occurrences
for diamond content
for diamond content
In 2003, GEUS and BMP undertook sampling of three large
occurrences of kimberlitic rocks for subsequent testing for
diamond content using caustic dissolution.
occurrences of kimberlitic rocks for subsequent testing for
diamond content using caustic dissolution.
Composite samples of approximately 1000 kg from each
occurrence have been processed and examined for diamonds
by a certified Canadian testing laboratory. Two of the occur-
rences are vertical dykes with a length of approximately 2500 m
and a width of up to 2 m (Figs 24). The third occurrence is
a shallow-dipping sill with a length of at least 500 m and a
thickness of 12 m. The sill and one of the long dykes lie well
within the unreworked Archaean craton, while the second
long dyke lies a few kilometres inside the Palaeoproterozoic
deformed region (Fig. 1).
by a certified Canadian testing laboratory. Two of the occur-
rences are vertical dykes with a length of approximately 2500 m
and a width of up to 2 m (Figs 24). The third occurrence is
a shallow-dipping sill with a length of at least 500 m and a
thickness of 12 m. The sill and one of the long dykes lie well
within the unreworked Archaean craton, while the second
long dyke lies a few kilometres inside the Palaeoproterozoic
deformed region (Fig. 1).
The test resulted in 125 diamonds recovered from the
sampled dyke in Area 1 (Fig. 1), two diamonds from the dyke
in Area 2 and one diamond from the dyke in Area 3 (Jensen
et al. 2004). The largest diamond recovered measures 0.74
in Area 2 and one diamond from the dyke in Area 3 (Jensen
et al. 2004). The largest diamond recovered measures 0.74
0.63
× 0.54 mm, and the total weight of the 128 stones is
0.016 carat.
71
Fig. 3. Eclogitic xenolith in one of the dykes sampled for testing for dia-
mond content (Area 1 in Fig. 1). Scale bar is 2 cm.
Fig. 4. Boulders of a 1.5 m wide and 2500 m long dyke sampled for test-
ing for diamond content (Area 3 in Fig. 1).
72
Indicator minerals from the same three kimberlitic occur-
rences have been separated, picked and analysed by electron
microprobe. The indicator minerals studied are sub-calcic
pyrope, eclogitic garnet, chrome-diopside, chromite, ilme-
nite and olivine. The indicator mineral chemistry will be used
in conjunction with the diamond determination results to
assess the diamond potential of the occurrences. The mineral
chemistry fingerprint of the in situ occurrences may have
important implications for the interpretation of existing indi-
cator mineral chemistry data from till and stream sediment
samples.
microprobe. The indicator minerals studied are sub-calcic
pyrope, eclogitic garnet, chrome-diopside, chromite, ilme-
nite and olivine. The indicator mineral chemistry will be used
in conjunction with the diamond determination results to
assess the diamond potential of the occurrences. The mineral
chemistry fingerprint of the in situ occurrences may have
important implications for the interpretation of existing indi-
cator mineral chemistry data from till and stream sediment
samples.
Research in progress
The field work in 2003 included detailed magnetic mapping
of the three occurrences using a proton magnetometer.
Interpretation of the geophysical field data is ongoing.
of the three occurrences using a proton magnetometer.
Interpretation of the geophysical field data is ongoing.
Petrographic and geochemical characterisation of the kim-
berlitic occurrences has been largely neglected in previous
investigations, and accordingly a comprehensive programme
to systematically study and classify the groundmass, mantle
xenoliths and indicator minerals of these rocks has been ini-
tiated.
investigations, and accordingly a comprehensive programme
to systematically study and classify the groundmass, mantle
xenoliths and indicator minerals of these rocks has been ini-
tiated.
An extensive programme to determine the age of these and
many additional kimberlitic occurrences using the very pre-
cise U-Pb in perovskite method (e.g. Heaman et al. 2003) has
been launched. Some phlogopite-rich rocks will be dated by
the Rb-Sr method. A total of around 35 age determinations
have been commissioned.
cise U-Pb in perovskite method (e.g. Heaman et al. 2003) has
been launched. Some phlogopite-rich rocks will be dated by
the Rb-Sr method. A total of around 35 age determinations
have been commissioned.
An updated version of the digital data compilation (Jensen
et al. 2003a) planned for 2004 will include the results of the
ongoing Survey testing and analytical work, as well as recently
released company data, amounting to 3300 pages of text,
tables and maps, 50 000 indicator mineral analyses, around
100 previously undescribed kimberlitic occurrences and a
large volume of airborne geophysical data.
ongoing Survey testing and analytical work, as well as recently
released company data, amounting to 3300 pages of text,
tables and maps, 50 000 indicator mineral analyses, around
100 previously undescribed kimberlitic occurrences and a
large volume of airborne geophysical data.
Acknowledgement
The work reported on here has been supported and financed in part by
the Bureau of Minerals and Petroleum in Nuuk.
the Bureau of Minerals and Petroleum in Nuuk.
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Authors' address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: smj@geus.dk
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: smj@geus.dk
