Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 37-40 |
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37
Recently discovered volcaniclastic rocks of andesitic compo-
sition form major parts of the mid-Archaean, amphibolite
facies supracrustal belts at Qussuk, on Bjørneøen and on part
of Storø in western Godthåbsfjord (Fig. 1). These rocks are
interpreted as an island arc that represents the onset of the
magmatic accretion of the Akia terrane 3070 Ma ago; this
terrane is the north-westernmost of several Archaean tec -
tono-stratigraphic terranes in the Nuuk region, which were
all amalgamated by 2720 Ma (cf. Hollis et al . 2006). The
presence of the arc in the Akia terrane points to similarities
between high-grade orthogneiss-amphibolite associations in
West Greenland and lower-grade granite-greenstone terrains
of other Archaean cratons e.g. in Canada and Western
Australia. Volcaniclastic rocks belonging to the ancient arc
have been subject to intense synvolcanic, hydrothermal alter-
ation associated with gold-copper mineralisation especially in
parts of the Qussuk area. Another important gold prospect
occurs on central Storø, which is currently being explored by
NunaMinerals A/S (Knudsen et al . 2007 - this volume). This
contribution presents new field observations from some of
the best preserved parts of the ancient arc at Qussuk and on
Bjørneøen, while it remains unclear if the volcano-sedimen-
tary associations and their gold mineralisation at Qussuk,
Bjørneøen and the nearby Storø share a common mid-
Archaean geological history.
sition form major parts of the mid-Archaean, amphibolite
facies supracrustal belts at Qussuk, on Bjørneøen and on part
of Storø in western Godthåbsfjord (Fig. 1). These rocks are
interpreted as an island arc that represents the onset of the
magmatic accretion of the Akia terrane 3070 Ma ago; this
terrane is the north-westernmost of several Archaean tec -
tono-stratigraphic terranes in the Nuuk region, which were
all amalgamated by 2720 Ma (cf. Hollis et al . 2006). The
presence of the arc in the Akia terrane points to similarities
between high-grade orthogneiss-amphibolite associations in
West Greenland and lower-grade granite-greenstone terrains
of other Archaean cratons e.g. in Canada and Western
Australia. Volcaniclastic rocks belonging to the ancient arc
have been subject to intense synvolcanic, hydrothermal alter-
ation associated with gold-copper mineralisation especially in
parts of the Qussuk area. Another important gold prospect
occurs on central Storø, which is currently being explored by
NunaMinerals A/S (Knudsen et al . 2007 - this volume). This
contribution presents new field observations from some of
the best preserved parts of the ancient arc at Qussuk and on
Bjørneøen, while it remains unclear if the volcano-sedimen-
tary associations and their gold mineralisation at Qussuk,
Bjørneøen and the nearby Storø share a common mid-
Archaean geological history.
The age and setting of the ancient arc
The eastern Akia terrane comprises
c
. 3060-3000 Ma
tonalitic to trondhjemitic orthogneisses, isoclinally folded
panels derived from the older andesitic arc, and granites
mobilised from the orthogneisses during a late-kinematic
thermal event at c . 2980 Ma which also led to granulite facies
P-T conditions west of Qussuk and Bjørneøen (Garde 1997,
2007; Garde et al . 2000). Andesitic metavolcanic and vol-
cano-sedimentary rocks were first reported from the eastern
Akia terrane by Garde (1997) and Smith (1998). However,
volcanic textures are mostly very poorly preserved, and it was
not appreciated then, how widespread the andesitic arc is,
and that it forms large parts of the supracrustal belts exposed
at Qussuk and on Bjørneøen.
tonalitic to trondhjemitic orthogneisses, isoclinally folded
panels derived from the older andesitic arc, and granites
mobilised from the orthogneisses during a late-kinematic
thermal event at c . 2980 Ma which also led to granulite facies
P-T conditions west of Qussuk and Bjørneøen (Garde 1997,
2007; Garde et al . 2000). Andesitic metavolcanic and vol-
cano-sedimentary rocks were first reported from the eastern
Akia terrane by Garde (1997) and Smith (1998). However,
volcanic textures are mostly very poorly preserved, and it was
not appreciated then, how widespread the andesitic arc is,
and that it forms large parts of the supracrustal belts exposed
at Qussuk and on Bjørneøen.
Recent age determinations of volcanic zircon grains from
central Bjørneøen show that the age of the andesitic arc is
3071 ± 1 Ma (sample 479827, Garde 2007). The arc is thus
3071 ± 1 Ma (sample 479827, Garde 2007). The arc is thus
marginally older than its orthogneiss host, in agreement with
recently observed intrusive contacts into the supracrustal belt
(Fig. 2). Similar field relationships in the adjacent Qussuk
area further document that the orthogneiss precursors
intruded into the arc. However, zircon from volcaniclastic
rocks here, on strike with those on Bjørneøen, yields meta-
morphic ages of 2990-2970 Ma which coincide with the
thermal maximum and associated fluid movement; only a
few zircon cores approach the true volcanic age of c . 3070 Ma
recently observed intrusive contacts into the supracrustal belt
(Fig. 2). Similar field relationships in the adjacent Qussuk
area further document that the orthogneiss precursors
intruded into the arc. However, zircon from volcaniclastic
rocks here, on strike with those on Bjørneøen, yields meta-
morphic ages of 2990-2970 Ma which coincide with the
thermal maximum and associated fluid movement; only a
few zircon cores approach the true volcanic age of c . 3070 Ma
Pre-metamorphic hydrothermal alteration with gold in a
mid-Archaean island arc, Godthåbsfjord, West Greenland
mid-Archaean island arc, Godthåbsfjord, West Greenland
Adam A. Garde, Henrik Stendal and Bo Møller Stensgaard
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 37-40. Available at:
www.geus.dk/publications/bull
Fig. 1. Simplified map of north-western Godthåbsfjord with supracrustal
belts (green), gold anomalies, and locations of Figs 2-8. Modified from
Hollis (2005).
(Garde 2007). Zircon grains from yet another volcano-sedi-
mentary rock on central Bjørneøen (Fig. 3A) yielded a range
of 2908-2742 Ma ages with a cluster around 2825 Ma (sam-
ple 479745, Hollis 2005 p. 55). The 2825 Ma cluster was
interpreted as the depositional age, and a complex tectonic
model for central Bjørneøen was proposed with thrust-stack-
ing of supracrustal rocks and orthogneisses of different ages
and origins at the eastern margin of the Akia terrane.
However, further field observations in 2006 uncovered that
this sample locality lies within the same volcaniclastic
sequence as the sample dated at 3071 Ma. With the estab-
lished intrusive contacts of orthogneisses dated at c . 3065-
3050 Ma (Fig. 2; Hollis 2005 pp. 30-39), this tectonic
model is no longer tenable, as the supracrustal belt is older
than the orthogneisses and its contacts are not tectonic.
Furthermore, a duplicate sample collected at the locality of
Fig. 3A by the first author in 2006 yielded a metamorphic U-
Pb zircon age of 2986 ± 3.6 Ma (13 stubby, very indistinctly
zoned zircon grains with Th/U <0.01; unpublished ion probe
data, May 2007). Hence, the zircon grains allegedly extracted
from sample 479745 seem to have been incorrectly labelled
during the sample preparation.
mentary rock on central Bjørneøen (Fig. 3A) yielded a range
of 2908-2742 Ma ages with a cluster around 2825 Ma (sam-
ple 479745, Hollis 2005 p. 55). The 2825 Ma cluster was
interpreted as the depositional age, and a complex tectonic
model for central Bjørneøen was proposed with thrust-stack-
ing of supracrustal rocks and orthogneisses of different ages
and origins at the eastern margin of the Akia terrane.
However, further field observations in 2006 uncovered that
this sample locality lies within the same volcaniclastic
sequence as the sample dated at 3071 Ma. With the estab-
lished intrusive contacts of orthogneisses dated at c . 3065-
3050 Ma (Fig. 2; Hollis 2005 pp. 30-39), this tectonic
model is no longer tenable, as the supracrustal belt is older
than the orthogneisses and its contacts are not tectonic.
Furthermore, a duplicate sample collected at the locality of
Fig. 3A by the first author in 2006 yielded a metamorphic U-
Pb zircon age of 2986 ± 3.6 Ma (13 stubby, very indistinctly
zoned zircon grains with Th/U <0.01; unpublished ion probe
data, May 2007). Hence, the zircon grains allegedly extracted
from sample 479745 seem to have been incorrectly labelled
during the sample preparation.
Volcaniclastic and volcano-sedimentary
rocks
rocks
The relict volcano-sedimentary arc has been intensely de -
formed and metamorphosed at middle to upper amphibolite
formed and metamorphosed at middle to upper amphibolite
grade and now mostly consists of monotonous, fine-grained,
schistose andesitic rocks besides intrusive bodies of
mafic-ultramafic rocks. However, small pockets of low-strain
rocks with distinct volcaniclastic textures are locally well pre-
served on central Bjørneøen and in the Qussuk area, particu-
larly in the cores of fold hinges. Andesitic rocks containing
volcanic clasts with fiamme textures have been identified on
central Bjørneøen and north-eastern Qussuk (Fig. 3), as well
as rare, more or less undeformed volcano-sedimentary depos -
its a few tens of metres across with well-preserved sedimen-
tary structures (Fig. 4). The recognition of the volcaniclastic
environments is important, because they document wide-
spread explosive volcanism and hence shallow subaqueous or
subaerial volcanic activity in an island arc. Unfortunately, it
has not been possible to reconstruct individual volcanic edi-
fices due to the general poor state of preservation.
schistose andesitic rocks besides intrusive bodies of
mafic-ultramafic rocks. However, small pockets of low-strain
rocks with distinct volcaniclastic textures are locally well pre-
served on central Bjørneøen and in the Qussuk area, particu-
larly in the cores of fold hinges. Andesitic rocks containing
volcanic clasts with fiamme textures have been identified on
central Bjørneøen and north-eastern Qussuk (Fig. 3), as well
as rare, more or less undeformed volcano-sedimentary depos -
its a few tens of metres across with well-preserved sedimen-
tary structures (Fig. 4). The recognition of the volcaniclastic
environments is important, because they document wide-
spread explosive volcanism and hence shallow subaqueous or
subaerial volcanic activity in an island arc. Unfortunately, it
has not been possible to reconstruct individual volcanic edi-
fices due to the general poor state of preservation.
Synvolcanic hydrothermal alteration
Hydrothermally altered aluminous and siliceous rocks, com-
monly associated with disseminated iron sulphides and gold
mineralisation, occur in several parts of the relict arc (Hollis
2005; Garde 2007). Unequivocal evidence that the hydro -
thermal alteration took place in unconsolidated volcaniclas-
tic rocks prior to deformation and metamorphism was found
monly associated with disseminated iron sulphides and gold
mineralisation, occur in several parts of the relict arc (Hollis
2005; Garde 2007). Unequivocal evidence that the hydro -
thermal alteration took place in unconsolidated volcaniclas-
tic rocks prior to deformation and metamorphism was found
38
Fig. 2. Intrusive contacts at the footwall (
A
) and hanging wall (
B
) of the
supracrustal belt on central Bjørneøen (see Fig. 1 for locations).
Fig. 3. Volcaniclastic rocks on central Bjørneøen (
A
) and at north-east-
ern Qussuk (
B
) with fiamme textures. Pen 8 mm thick. For locations see
Fig. 1.
in 2006 within the hinge zone of a large fold on the north-
eastern coast of Qussuk (Fig. 1). The lower part of Fig. 5 dis-
plays several nearly undeformed beds of fine- to coarse-
grained tuff (or tuffite) with granitic-pegmatitic partial melt
veins. The thickest bed displays right way-up graded bedding.
The contact to the overlying bed is rusty weathering and con-
tains sporadic iron sulphides and small garnets ( <5 mm), and
has clearly been affected by hydrothermal activity. The very
localised alteration furthermore indicates that the percolating
fluid used the unconsolidated bedding contact as a conve-
nient passageway.
eastern coast of Qussuk (Fig. 1). The lower part of Fig. 5 dis-
plays several nearly undeformed beds of fine- to coarse-
grained tuff (or tuffite) with granitic-pegmatitic partial melt
veins. The thickest bed displays right way-up graded bedding.
The contact to the overlying bed is rusty weathering and con-
tains sporadic iron sulphides and small garnets ( <5 mm), and
has clearly been affected by hydrothermal activity. The very
localised alteration furthermore indicates that the percolating
fluid used the unconsolidated bedding contact as a conve-
nient passageway.
There are many other examples of rocks with more wide-
spread and more intense alteration. Most of these are also
intensely deformed and difficult to recognise as hydrother-
mally altered, but exceptions do occur. Figure 6 shows
smooth, grey metatuff hosting an irregular, interfingering,
vein-like system of coarser, rusty weathering and crumbling
metatuff. This is hydrothermally altered and now contains
abundant quartz, biotite, garnet and iron sulphide.
intensely deformed and difficult to recognise as hydrother-
mally altered, but exceptions do occur. Figure 6 shows
smooth, grey metatuff hosting an irregular, interfingering,
vein-like system of coarser, rusty weathering and crumbling
metatuff. This is hydrothermally altered and now contains
abundant quartz, biotite, garnet and iron sulphide.
Rusty weathering and variably gold-mineralised rocks,
altered by synvolcanic, hydrothermal fluids and now rich in
quartz, biotite, garnet and commonly sillimanite, also occur
east and north of Qussuk in tracts up to several kilometres
long, which have previously been mapped as aluminous
metasedimentary rocks. Figure 7 shows a football-sized
enclave of unaltered and undeformed amphibolitic metatuff,
which is preserved inside a lens several metres across of gar-
net-rich rock (hydrothermally altered and metamorphosed
tuff ). The most intensely altered andesitic rocks now consist
of massive sillimanite-fuchsite quartzite, from which most
major and trace elements have been leached out except Si, Al
and Ti, and very immobile elements such as Ga, Zr and rare-
earth elements. Such rocks form the top of Ivisaat mountain
north of Qussuk (Fig. 8), where they form a layer c . 5-10 m
thick in the core of an isoclinal fold, surrounded by garnet-
rich and sulphidic, less altered andesitic metavolcanic rocks
with widespread gold mineralisation in the ppb range (the
authors' unpublished data). This very specific type of hydro -
quartz, biotite, garnet and commonly sillimanite, also occur
east and north of Qussuk in tracts up to several kilometres
long, which have previously been mapped as aluminous
metasedimentary rocks. Figure 7 shows a football-sized
enclave of unaltered and undeformed amphibolitic metatuff,
which is preserved inside a lens several metres across of gar-
net-rich rock (hydrothermally altered and metamorphosed
tuff ). The most intensely altered andesitic rocks now consist
of massive sillimanite-fuchsite quartzite, from which most
major and trace elements have been leached out except Si, Al
and Ti, and very immobile elements such as Ga, Zr and rare-
earth elements. Such rocks form the top of Ivisaat mountain
north of Qussuk (Fig. 8), where they form a layer c . 5-10 m
thick in the core of an isoclinal fold, surrounded by garnet-
rich and sulphidic, less altered andesitic metavolcanic rocks
with widespread gold mineralisation in the ppb range (the
authors' unpublished data). This very specific type of hydro -
Fig. 4. Graded volcano-sedimentary rock with early brittle/ductile defor-
mation. Central Bjørneøen (see Fig. 1 for location).
Fig. 5. Bedded metatuff at north-eastern Qussuk (see Fig. 1 for location)
showing a primary contact between mafic and intermediate beds (red
arrow) and a narrow rusty hydrothermal alteration zone along primary
bedding contact (blue arrow).
Fig. 6. Interfingering system of early hydrothermal alteration in andesitic
(meta)tuff at north-eastern Qussuk (see Fig. 1 for location). Glacial stri-
ations have produced a faint oblique `foliation' visible in the lower left
of the exposure.
Fig. 7. Enclave of unaltered and undeformed metavolcanic amphibolite
(arrow), surrounded by a lens of hydrothermally altered, now garnet-
rich rock of similar origin. Peninsula east of Qussuk (see Fig. 1 for loca-
tion). Hammershaft is
c
. 85 cm long.
39
thermal alteration points to acid leaching under low pressure,
a process which is very characteristic of epithermal, high-level
hydrothermal systems in modern andesitic arcs, which are
commonly associated with gold and copper mineralisation
(e.g. Sillitoe & Hedenquist 2003). A low-pressure hydrother-
mal system such as found today near the tops of andesitic arcs
is a prerequisite for wholesale leaching by strong acids, be -
cause higher pressures prevent the dissociation of their hydro-
gen ions (Sillitoe & Hedenquist 2003).
a process which is very characteristic of epithermal, high-level
hydrothermal systems in modern andesitic arcs, which are
commonly associated with gold and copper mineralisation
(e.g. Sillitoe & Hedenquist 2003). A low-pressure hydrother-
mal system such as found today near the tops of andesitic arcs
is a prerequisite for wholesale leaching by strong acids, be -
cause higher pressures prevent the dissociation of their hydro-
gen ions (Sillitoe & Hedenquist 2003).
Conclusions
The
c
. 3070 Ma andesitic arc in the eastern Akia terrane and
its slightly younger intrusive counterparts of tonalitic ortho -
gneisses, combined with previous structural evidence of early
crustal shortening in most of the Akia terrane, point to the
existence of a convergent plate-tectonic system in the North
Atlantic craton, where subduction of oceanic crust and par-
tial melting of the subducted slab occurred at least 3070 Ma
ago. The identification of the arc complex substantiates pre-
vious ideas that the orthogneisses in the Akia terrane are
products of slab melting in a convergent plate-tectonic set-
ting. In a wider context this also implies that the typical
Archaean high-grade orthogneiss-amphibolite associations in
its slightly younger intrusive counterparts of tonalitic ortho -
gneisses, combined with previous structural evidence of early
crustal shortening in most of the Akia terrane, point to the
existence of a convergent plate-tectonic system in the North
Atlantic craton, where subduction of oceanic crust and par-
tial melting of the subducted slab occurred at least 3070 Ma
ago. The identification of the arc complex substantiates pre-
vious ideas that the orthogneisses in the Akia terrane are
products of slab melting in a convergent plate-tectonic set-
ting. In a wider context this also implies that the typical
Archaean high-grade orthogneiss-amphibolite associations in
West Greenland may not represent plate-tectonic environ-
ments distinct from the granite-greenstone associations found
in most other Archaean cratons, but simply expose deeper
sections of the same convergent systems.
ments distinct from the granite-greenstone associations found
in most other Archaean cratons, but simply expose deeper
sections of the same convergent systems.
The relict arc hosts widespread hydrothermally altered
rocks and associated gold (-copper) mineralisation. Newly
discovered field relationships show that the hydrothermal
systems predated deformation and metamorphism. Further -
more, the peculiar mineralogical composition of the altered
rocks combined with their gold mineralisation suggest that
the hydrothermal alteration was synvolcanic and epithermal,
and characteristic of the arc itself.
discovered field relationships show that the hydrothermal
systems predated deformation and metamorphism. Further -
more, the peculiar mineralogical composition of the altered
rocks combined with their gold mineralisation suggest that
the hydrothermal alteration was synvolcanic and epithermal,
and characteristic of the arc itself.
References
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grey gneiss-amphibolite complex: the Fiskefjord area, southern West
Greenland. Geology of Greenland Survey Bulletin 177 , 114 pp.
Greenland. Geology of Greenland Survey Bulletin 177 , 114 pp.
Garde, A.A. 2007: A mid-Archaean island arc complex in the eastern Akia
terrane, Godthåbsfjord, southern West Greenland. Journal of the
Geological Society (London) 164 , 565-579.
Geological Society (London) 164 , 565-579.
Garde, A.A., Friend, C.R.L., Nutman, A.P. & Marker, M. 2000: Rapid matu -
ration and stabilisation of middle Archaean continental crust: the Akia
terrane, southern West Greenland. Bulletin of the Geological Society
of Denmark 47 , 1-27.
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of Denmark 47 , 1-27.
Hollis, J.A. (ed.) 2005: Greenstone belts in the central Godthåbsfjord
region, southern West Greenland: Geochemistry, geochronology and
petrography arising from 2004 field work, and digital map data.
Danmarks og Grønlands Geologiske Undersøgelse Rapport 2005/42 ,
215 pp.
petrography arising from 2004 field work, and digital map data.
Danmarks og Grønlands Geologiske Undersøgelse Rapport 2005/42 ,
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Hollis, J.A., Frei, D., van Gool, J.A.M., Garde, A.A. & Persson, M. 2006:
Using zircon geochronology to resolve the Archaean geology of
southern West Greenland. Geological Survey of Denmark and
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Knudsen, C., van Gool, J.A.M., Østergaard, C., Hollis, J.A., Rink-
Jørgensen, M., Persson, M & Szilas, K. 2007: Gold hosting supracrustal
rocks on Storø, West Greenland: petrogenesis and structural setting.
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rocks on Storø, West Greenland: petrogenesis and structural setting.
Geological Survey of Denmark and Greenland Bulletin 13 , 17-20.
Sillitoe, R.H. & Hedenquist, J.W. 2003: Linkages between volcanotectonic
settings, ore-fluid compositions, and epithermal precious metal
deposits. In: Simmons, S.F. & Graham, I. (eds): Volcanic, geothermal,
and ore-forming fluids; rulers and witnesses of processes within the
Earth. Society of Economic Geologists Special Publication 10 , 315-343.
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and ore-forming fluids; rulers and witnesses of processes within the
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Smith, G.M. 1998: Geology and mineral potential of the Bjørneøen
supracrustal belt, Nuukfjord, West Greenland. Unpublished report,
Nunaoil A/S, 13 pp. (in archives of Geological Survey of Denmark and
Greenland, GEUS Report File 21649).
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Greenland, GEUS Report File 21649).
40
Author's address
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
aag@geus.dk
Fig. 8. Folded sillimanite-fuchsite quartzite with disseminated iron sul-
phides, an intensely hydrothermally altered and metamorphosed rock of
andesitic origin. Ivisaat mountain north of Qussuk (see Fig. 1 for loca-
tion). Hammershaft is
c
. 45 cm long.
