Geological Survey of Denmark and Greenland Bulletin
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Nr. 4, Review of Survey activities 2003, pp. 77-80 |
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The discovery in 2002 of a gold mineralised quartz-carbon-
ate vein at Ubekendt Ejland, central West Greenland, yield-
ing 0.6 ppm Au over 0.7 m, led to a reconnaissance sampling
project in summer 2003. Most of the accessible quartz-car-
bonate veins on the south-east coast of the island (Figs 1, 2)
were sampled during boat-supported field work. Massive sul-
phide mineral deposits (Fe-Zn-Pb) were located in the centre
of brecciated quartz-carbonate vein systems at several places
along the south and south-east coast of the island, and gold
anomalies mainly associated with the occurrence of the mas-
sive sulphides were identified. Pervasive hydrothermal altera-
tion of the volcanic wall rocks surrounds the quartz-carbonate
ate vein at Ubekendt Ejland, central West Greenland, yield-
ing 0.6 ppm Au over 0.7 m, led to a reconnaissance sampling
project in summer 2003. Most of the accessible quartz-car-
bonate veins on the south-east coast of the island (Figs 1, 2)
were sampled during boat-supported field work. Massive sul-
phide mineral deposits (Fe-Zn-Pb) were located in the centre
of brecciated quartz-carbonate vein systems at several places
along the south and south-east coast of the island, and gold
anomalies mainly associated with the occurrence of the mas-
sive sulphides were identified. Pervasive hydrothermal altera-
tion of the volcanic wall rocks surrounds the quartz-carbonate
vein systems, which comprise low-temperature mineral
assemblages dominated by dolomite and veined by chal-
cedony and fibrous silica. Evidence of oil migration into vol-
caniclastic rocks prior to the intense hydrothermal activity
was found in several places in the form of organic carbon,
interpreted to be pyrobitumen, that infills pores and cavities
in hyaloclastites.
assemblages dominated by dolomite and veined by chal-
cedony and fibrous silica. Evidence of oil migration into vol-
caniclastic rocks prior to the intense hydrothermal activity
was found in several places in the form of organic carbon,
interpreted to be pyrobitumen, that infills pores and cavities
in hyaloclastites.
Geological setting
Ubekendt Ejland comprises early Palaeogene volcanic and in-
trusive rocks (Fig. 1; Drever & Game 1948; Larsen 1977a, b).
trusive rocks (Fig. 1; Drever & Game 1948; Larsen 1977a, b).
77
Epithermal gold and massive sulphide mineralisation in
oil impregnated Palaeogene volcanic rocks of Ubekendt
Ejland,West Greenland
oil impregnated Palaeogene volcanic rocks of Ubekendt
Ejland,West Greenland
Stefan Bernstein and Christian Knudsen
Geological Survey of Denmark and Greenland Bulletin 4, 7780 (2004) © GEUS, 2004
Fig. 1. Geological map of Ubekendt Ejland.
Most quartz-carbonate veins (red lines)
visited in 2003 are located from 5 km east
of Nuunngutak to 7 km west of Qeqertalik.
Modified from Larsen (1977a, b). Spot heights
(e.g. 420) are in metres.
Most of the island consists of picritic and olivine-phyric lavas,
hyaloclastites and volcanic breccias, assigned to the Vaigat
Formation (Fig. 1). The Vaigat Formation succession dips
2030° to the west and is overlain by plagioclase-phyric lavas,
pyroclastic units and an upper lava succession including alka-
line basalts (Larsen 1977b). A maficfelsic intrusive complex,
that includes layered gabbros and fine-grained granite, is
found at Saqqaata Qaqqaa in the southern part of the island.
hyaloclastites and volcanic breccias, assigned to the Vaigat
Formation (Fig. 1). The Vaigat Formation succession dips
2030° to the west and is overlain by plagioclase-phyric lavas,
pyroclastic units and an upper lava succession including alka-
line basalts (Larsen 1977b). A maficfelsic intrusive complex,
that includes layered gabbros and fine-grained granite, is
found at Saqqaata Qaqqaa in the southern part of the island.
The quartz-carbonate veins are particularly abundant in
the south-eastern part of the island, but veins also occur close
to the south-west coast, and on the north-west coast at
Qarusuk (Fig. 1). Most veins on the south-eastern coast can
be traced for a few kilometres northwards until they are hid-
den beneath Quaternary glacial deposits. One vein system
has been traced for 6 km along strike. The highest density of
vein systems occurs along a 5 km long stretch of the coast east
of Nuunngutak.
to the south-west coast, and on the north-west coast at
Qarusuk (Fig. 1). Most veins on the south-eastern coast can
be traced for a few kilometres northwards until they are hid-
den beneath Quaternary glacial deposits. One vein system
has been traced for 6 km along strike. The highest density of
vein systems occurs along a 5 km long stretch of the coast east
of Nuunngutak.
Quartz-carbonate veins
Some vein systems appear to be developed as relatively well-
defined planar structures that extend for some distance along
strike, judging from the orange-buff outcrop coloration. In
other cases, the veins form complex anastomosing systems,
where sets of veins often four or more follow irregular trends
and join upwards (Fig. 2). In general, the vein systems are
subvertical. It is unclear whether the vein systems were for-
med during one or several events, although at several loca-
tions the veins exhibit evidence of distinct stages of brecciation
and mineralisation (see below). Samples were taken up to
about 5 km west of the intrusive complex at Saqqaata
defined planar structures that extend for some distance along
strike, judging from the orange-buff outcrop coloration. In
other cases, the veins form complex anastomosing systems,
where sets of veins often four or more follow irregular trends
and join upwards (Fig. 2). In general, the vein systems are
subvertical. It is unclear whether the vein systems were for-
med during one or several events, although at several loca-
tions the veins exhibit evidence of distinct stages of brecciation
and mineralisation (see below). Samples were taken up to
about 5 km west of the intrusive complex at Saqqaata
Qaqqaa, but no significant changes in mineralisation assem-
blages or structures were observed.
blages or structures were observed.
The quartz-carbonate veins on Ubekendt Ejland typically
consist of banded carbonates with centres filled by quartz or
carbonate. Thin ( <1 mm) cross-cutting veins filled with quartz
and/or fibrous silica also occur. The banded carbonates are
medium- to fine-grained and some veins also have coarse-
grained centres with carbonate crystals up to 20 mm across
(Fig. 3). The outer margins of the veins are often intensely
brecciated, showing a mixture of carbonate and quartz crys-
tals and fragments, including fragments of chalcedony, set in
a very fine-grained matrix of ground carbonate. The matrix
sometimes shows signs of recrystallisation. Carbonate is pre-
dominantly dolomite, and grades into ankerite. Some quartz-
carbonate veins exhibit later sulphide mineralisation (see
below) lining late fractures and veins (Fig. 3). The late frac-
turing can be extensive and has resulted in brecciation zones
within the quartz-carbonate veins. The latest veining and
mineralisation stage resulted in the formation of chalcedony
veins that are usually 12 mm thick, but chalcedony com-
monly occurs in cavities and vein centres within massive sul-
phides. In places, the chalcedony grades into coarser-grained
quartz (0.5 mm). Vugs filled with fibrous quartz, probably
replacing amorphous sillica, are also common.
carbonate. Thin ( <1 mm) cross-cutting veins filled with quartz
and/or fibrous silica also occur. The banded carbonates are
medium- to fine-grained and some veins also have coarse-
grained centres with carbonate crystals up to 20 mm across
(Fig. 3). The outer margins of the veins are often intensely
brecciated, showing a mixture of carbonate and quartz crys-
tals and fragments, including fragments of chalcedony, set in
a very fine-grained matrix of ground carbonate. The matrix
sometimes shows signs of recrystallisation. Carbonate is pre-
dominantly dolomite, and grades into ankerite. Some quartz-
carbonate veins exhibit later sulphide mineralisation (see
below) lining late fractures and veins (Fig. 3). The late frac-
turing can be extensive and has resulted in brecciation zones
within the quartz-carbonate veins. The latest veining and
mineralisation stage resulted in the formation of chalcedony
veins that are usually 12 mm thick, but chalcedony com-
monly occurs in cavities and vein centres within massive sul-
phides. In places, the chalcedony grades into coarser-grained
quartz (0.5 mm). Vugs filled with fibrous quartz, probably
replacing amorphous sillica, are also common.
Sulphide and gold mineralisation
In the larger quartz-carbonate veins, the sulphide mineralised
fractures and breccias may be up to 15 cm in width and 40
cm long and comprise semi-massive to massive sulphides. In
brecciated quartz-carbonate veins, pyrite and other sulphide
minerals are euhedral or subhedral and fine-grained, while in
fractures and breccias may be up to 15 cm in width and 40
cm long and comprise semi-massive to massive sulphides. In
brecciated quartz-carbonate veins, pyrite and other sulphide
minerals are euhedral or subhedral and fine-grained, while in
78
chalcedony
sulphides
Fig. 2. Quartz-carbonate vein systems cutting picritic lava flows of the
Vaigat Formation, about 3 km east of Nuunngutak (Fig. 1). Height of cliff
about 150 m. The orange-buff coloured patterns include both the quartz-
carbonate veins and their alteration halos, typically 210 m wide.
Fig. 3. A 20 cm thick quartz-carbonate vein in an altered dyke, 6 km west
of Qeqertalik (Fig. 1). The vein shows several stages of brecciation, with
fragments of banded dolomite overgrown with pyrite. Some cavities are
lined with chalcedony, while others are lined with coarse calcite crystals.
Pen is 15 cm long.
the massive sulphides, pyrite, pyrrhotite and sphalerite are
subhedral and coarse-grained, most often enclosing other sul-
phides such as galena, arsenopyrite, millerite, pentlandite and
chalcopyrite. In most vein systems, the sulphides occur as
veins and in the brecciated matrix within the quartz-carbon-
ate veins, but disseminated sulphides also occur in the altered
wall rocks. Fresh sulphides have only been found in coastal
outcrops, while elsewhere they are extensively altered to
limonite.
subhedral and coarse-grained, most often enclosing other sul-
phides such as galena, arsenopyrite, millerite, pentlandite and
chalcopyrite. In most vein systems, the sulphides occur as
veins and in the brecciated matrix within the quartz-carbon-
ate veins, but disseminated sulphides also occur in the altered
wall rocks. Fresh sulphides have only been found in coastal
outcrops, while elsewhere they are extensively altered to
limonite.
Massive sulphide bodies comprise mainly pyrrhotite or
pyrite, sphalerite, galena, arsenopyrite and native silver. Gold
has not been seen in thin section, but samples of the massive
sulphides contain up to 1300 ppb Au and 110 ppm Ag, with
3.8 wt% Pb and 2.2 wt% Zn. The gold appears to reside
within the sulphide-mineralised vein centres.
has not been seen in thin section, but samples of the massive
sulphides contain up to 1300 ppb Au and 110 ppm Ag, with
3.8 wt% Pb and 2.2 wt% Zn. The gold appears to reside
within the sulphide-mineralised vein centres.
Alteration of wall rocks
The igneous host rocks are lava flows, volcaniclastites, hyalo-
clastites and basaltic dykes, all of which have suffered exten-
sive hydrothermal alteration adjacent to the quartz-carbonate
veins. The alteration halos may extend up to tens of metres,
but more usually are in the range of 25 m on either side of
the vein system. Veins that follow older thick basaltic dykes
often only show hydrothermal alteration in a narrow (12 m)
zone adjacent to the vein, while the remaining dyke retains its
igneous mineral assemblage.
clastites and basaltic dykes, all of which have suffered exten-
sive hydrothermal alteration adjacent to the quartz-carbonate
veins. The alteration halos may extend up to tens of metres,
but more usually are in the range of 25 m on either side of
the vein system. Veins that follow older thick basaltic dykes
often only show hydrothermal alteration in a narrow (12 m)
zone adjacent to the vein, while the remaining dyke retains its
igneous mineral assemblage.
At one location, a hyaloclastite unit is cut by a vertical, 2 m
thick basaltic dyke. The eastern dyke contact is cut by a par-
allel quartz-carbonate vein system, and alteration extends to
the centre of the dyke. On the east side of the vein, alteration
allel quartz-carbonate vein system, and alteration extends to
the centre of the dyke. On the east side of the vein, alteration
within the hyaloclastite host rock extends for about 810 m.
The unaltered part of the basaltic dyke contains sparse, fresh
clinopyroxene phenocrysts set in a groundmass of fine-
grained plagioclase, clinopyroxene and Fe-Ti oxide grains.
Intersticies are filled with sericite, and vesicles with chal-
cedony, carbonate and illite/phengite. In the altered part of
the dyke, plagioclase laths in the groundmass are still visible,
but are invariably altered to sericite. Former clinopyroxene
phenocrysts are replaced by fine-grained yellow clay minerals,
and bands of fine-grained euhedral pyrite cross-cut the rock,
together with thin (50 micron) veins of carbonate or chal-
cedony. Chemical analyses demonstrate that the alteration
has resulted in the loss of SiO
The unaltered part of the basaltic dyke contains sparse, fresh
clinopyroxene phenocrysts set in a groundmass of fine-
grained plagioclase, clinopyroxene and Fe-Ti oxide grains.
Intersticies are filled with sericite, and vesicles with chal-
cedony, carbonate and illite/phengite. In the altered part of
the dyke, plagioclase laths in the groundmass are still visible,
but are invariably altered to sericite. Former clinopyroxene
phenocrysts are replaced by fine-grained yellow clay minerals,
and bands of fine-grained euhedral pyrite cross-cut the rock,
together with thin (50 micron) veins of carbonate or chal-
cedony. Chemical analyses demonstrate that the alteration
has resulted in the loss of SiO
2
, MgO, CaO, Na
2
O, Sr and
Ba, and in an increase in FeO
total
, K
2
O, Ni, Rb, Pb and
volatiles. Other elements appear unaffected by the alteration;
these include Ti, Al, P, the transition metals, Zr and the rare
earth elements. These changes reflect the dissolution of
igneous silicates and growth of hydrous phyllosilicates, car-
bonate and pyrite. We deduce that the added volatiles must
include H
these include Ti, Al, P, the transition metals, Zr and the rare
earth elements. These changes reflect the dissolution of
igneous silicates and growth of hydrous phyllosilicates, car-
bonate and pyrite. We deduce that the added volatiles must
include H
2
O, CO
2
and S.
Trace of oil on Ubekendt Ejland
Evidence of oil migration into the onshore Palaeogene vol-
canic rocks is common in central West Greenland (Bojesen-
Koefoed et al. 1999). On the east coast of Ubekendt Ejland
(Fig. 1), minor oil staining has earlier been reported at the
contact of a dyke (Christiansen et al. 1998). During field
work in 2003, several localities with quartz-carbonate veins
and alteration zones on the south-east coast of Ubekendt
Ejland were found to bear evidence of oil migration into the
canic rocks is common in central West Greenland (Bojesen-
Koefoed et al. 1999). On the east coast of Ubekendt Ejland
(Fig. 1), minor oil staining has earlier been reported at the
contact of a dyke (Christiansen et al. 1998). During field
work in 2003, several localities with quartz-carbonate veins
and alteration zones on the south-east coast of Ubekendt
Ejland were found to bear evidence of oil migration into the
79
Fig. 4. Upper portion of altered
hyaloclastite, some 3 m from a
quartz-carbonate vein. The black
matrix between the lithic fragments
is organic carbon, which also fills
fine cracks in the host rock. To the
right is seen a subset of thin
carbonate veins. Pencil is 15 cm
long. About 4 km east of
Nuunngutak.
80
volcanic rocks. At one location, this is manifested by the
patchy development of black staining in altered hyaloclastic
rocks (Fig. 4). In thin section, organic carbon occurs as
almost opaque material enveloping lithic fragments identical
to the surrounding altered hyaloclastite (Fig. 5). Analysis of
the rock material yielded 1.25 wt% organic carbon, 2.89
wt% carbon and 0.14 wt% sulphur. These patchy zones in
the hyaloclastites are interpreted to be remnants of oil (pyro-
bitumen) that migrated into the volcanic rocks and filled
pore space in the hyaloclastites. In this case the migration of
oil occurred some time before the intense hydrothermal alter-
ation associated with the emplacement of the quartz-carbo-
nate vein systems. Prolonged exposure to elevated temperatures
has caused thermal alteration of the oil to leave pyrobitumen
as the solid residue. Assuming a density of the pyrobitumen
of about 1 g/cm
patchy development of black staining in altered hyaloclastic
rocks (Fig. 4). In thin section, organic carbon occurs as
almost opaque material enveloping lithic fragments identical
to the surrounding altered hyaloclastite (Fig. 5). Analysis of
the rock material yielded 1.25 wt% organic carbon, 2.89
wt% carbon and 0.14 wt% sulphur. These patchy zones in
the hyaloclastites are interpreted to be remnants of oil (pyro-
bitumen) that migrated into the volcanic rocks and filled
pore space in the hyaloclastites. In this case the migration of
oil occurred some time before the intense hydrothermal alter-
ation associated with the emplacement of the quartz-carbo-
nate vein systems. Prolonged exposure to elevated temperatures
has caused thermal alteration of the oil to leave pyrobitumen
as the solid residue. Assuming a density of the pyrobitumen
of about 1 g/cm
3
, the 1.25 wt% carbon corresponds to about
3 vol.% organic carbon in the rock, which in turn would rep-
resent 10% of the original volume of oil. An inferred 30
vol.% porosity in the matrix of the hyaloclastites appears
likely, either reflecting primary porosity or attained by disso-
lution of primary pore fillings prior to the invasion of oil.
resent 10% of the original volume of oil. An inferred 30
vol.% porosity in the matrix of the hyaloclastites appears
likely, either reflecting primary porosity or attained by disso-
lution of primary pore fillings prior to the invasion of oil.
Conclusion
The presence of chalcedony and fibrous silica possibly replac-
ing amorphous silica suggest that the quartz-carbonate veins
formed at a shallow level in hydrothermal conduits. The
quartz-carbonate veins are similar to epithermal Au-rich vein
ing amorphous silica suggest that the quartz-carbonate veins
formed at a shallow level in hydrothermal conduits. The
quartz-carbonate veins are similar to epithermal Au-rich vein
mineralisations described from other volcanic terrains associ-
ated with organic-rich sediments, in terms of geological set-
ting, vein mineralogy, alteration of wall rocks and the
evidence of migration of hydrocarbons (see e.g. Sketchley &
Sinclair 1991; Sherlock & Lehrman 1995; Sillitoe et al.
2002). The vein mineralogy and alteration assemblage fur-
ther suggest the mineralisation is a low sulphidation type
(e.g. Bonham 1988). The presence of Fe-Pb-Zn sulphides in
several veins may indicate that the sampled level of the veins
is below that of precious metal deposition, that according to
the model of Buchanan (1981) would appear above that of
base metal sulphides. Our findings suggest there is a poten-
tial for economic precious metal mineralisation on Ubekendt
Ejland.
ated with organic-rich sediments, in terms of geological set-
ting, vein mineralogy, alteration of wall rocks and the
evidence of migration of hydrocarbons (see e.g. Sketchley &
Sinclair 1991; Sherlock & Lehrman 1995; Sillitoe et al.
2002). The vein mineralogy and alteration assemblage fur-
ther suggest the mineralisation is a low sulphidation type
(e.g. Bonham 1988). The presence of Fe-Pb-Zn sulphides in
several veins may indicate that the sampled level of the veins
is below that of precious metal deposition, that according to
the model of Buchanan (1981) would appear above that of
base metal sulphides. Our findings suggest there is a poten-
tial for economic precious metal mineralisation on Ubekendt
Ejland.
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Fig. 5. Thin section of sample from locality in Fig. 4 (plane-polarised
light). The dark matrix contains lithic fragments of varying size and of a
similar composition to the host rock. Note the fine veins (arrowed) of
dark carbon-rich material extending into the altered hyaloclastite.
Authors' address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: sb@geus.dk
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark. E-mail: sb@geus.dk
