Geological Survey of Denmark and Greenland Bulletin 13, 2007
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Review of Survey activities 2006, 41-44 |
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A gold prospect on central Storø in the Nuuk region of south-
ern West Greenland is hosted by a sequence of intensely
deformed, amphibolite facies supracrustal rocks of late Meso-
to Neoarchaean age. The prospect is at present being explored
by the Greenlandic mining company NunaMinerals A/S.
Amphibolites likely to be derived from basaltic volcanic rocks
dominate, and ultrabasic to intermediate rocks are also inter-
preted to be derived from volcanic rocks. The sequence also
contains metasedimentary rocks including quartzites and
cordierite-, sillimanite-, garnet- and biotite-bearing alumi-
nous gneisses. The metasediments contain detrital zircon from
different sources indicating a maximum age of the minerali-
sation of c . 2.8 Ga. The original deposition of the various
rock types is believed to have taken place in a back-arc set-
ting. Gold is mainly hosted in garnet- and biotite-rich zones
in amphibolites often associated with quartz veins. Gold has
been found within garnets indicating that the mineralisation
is pre-metamorphic, which points to a minimum age of the
mineralisation of c . 2.6 Ga. The geochemistry of the gold-
bearing zones indicates that the initial gold mineralisation is
tied tOFluid-induced sericitisation of a basic volcanic pro-
tolith. The hosting rocks and the mineralisation are affected
by several generations of folding.
ern West Greenland is hosted by a sequence of intensely
deformed, amphibolite facies supracrustal rocks of late Meso-
to Neoarchaean age. The prospect is at present being explored
by the Greenlandic mining company NunaMinerals A/S.
Amphibolites likely to be derived from basaltic volcanic rocks
dominate, and ultrabasic to intermediate rocks are also inter-
preted to be derived from volcanic rocks. The sequence also
contains metasedimentary rocks including quartzites and
cordierite-, sillimanite-, garnet- and biotite-bearing alumi-
nous gneisses. The metasediments contain detrital zircon from
different sources indicating a maximum age of the minerali-
sation of c . 2.8 Ga. The original deposition of the various
rock types is believed to have taken place in a back-arc set-
ting. Gold is mainly hosted in garnet- and biotite-rich zones
in amphibolites often associated with quartz veins. Gold has
been found within garnets indicating that the mineralisation
is pre-metamorphic, which points to a minimum age of the
mineralisation of c . 2.6 Ga. The geochemistry of the gold-
bearing zones indicates that the initial gold mineralisation is
tied tOFluid-induced sericitisation of a basic volcanic pro-
tolith. The hosting rocks and the mineralisation are affected
by several generations of folding.
Regional geology
The Nuuk region is renowned for the presence of rocks rep-
resenting at least three major episodes of crustal accretion
during the first billion years of the Earth's history. Each
episode is characterised by early formation of supracrustal
rocks followed by intrusion of tonalites now occurring as grey
hornblende-biotite gneisses. The rock packages have subse-
quently been deformed and metamorphosed, resulting in
often very complex outcrop patterns. Hollis et al . (2006) out-
lined the following sequence of events. The Eoarchaean Isua
supra crustal belt (3.87-3.71 Ga) is cut by 3.85 to 3.65 Ga
tonalites deformed and metamorphosed at c . 3.65 Ga. The
next sequence of supracrustal rocks was formed at c . 3.1-3.07
Ga, intruded by tonalites at c . 3.07-3.05 Ga, and deformed
and metamorphosed at c . 2.98 Ga. The supracrustals linked
to the third episode were formed at c . 2.85 to 2.8 Ga,
intruded by tonalites at c . 2.83 to 2.75 Ga, and deformed and
resenting at least three major episodes of crustal accretion
during the first billion years of the Earth's history. Each
episode is characterised by early formation of supracrustal
rocks followed by intrusion of tonalites now occurring as grey
hornblende-biotite gneisses. The rock packages have subse-
quently been deformed and metamorphosed, resulting in
often very complex outcrop patterns. Hollis et al . (2006) out-
lined the following sequence of events. The Eoarchaean Isua
supra crustal belt (3.87-3.71 Ga) is cut by 3.85 to 3.65 Ga
tonalites deformed and metamorphosed at c . 3.65 Ga. The
next sequence of supracrustal rocks was formed at c . 3.1-3.07
Ga, intruded by tonalites at c . 3.07-3.05 Ga, and deformed
and metamorphosed at c . 2.98 Ga. The supracrustals linked
to the third episode were formed at c . 2.85 to 2.8 Ga,
intruded by tonalites at c . 2.83 to 2.75 Ga, and deformed and
metamorphosed at 2.7 to 2.6 Ga. The last major event in the
region was the intrusion of the Qorqut Granite at c . 2.53 Ga.
The rocks formed during these episodes crop out in a com-
plex pattern of blocks or terranes often only a few tens of kilo-
metres wide. Attempts have been made to map and name
these terranes (e.g. Nutman et al . 2004; Friend & Nutman
2005), but as the rock types belonging to the different epi -
sodes are very similar, it has proved to be very difficult to dis-
tinguish and map the terranes in the field (Hollis et al . 2006).
region was the intrusion of the Qorqut Granite at c . 2.53 Ga.
The rocks formed during these episodes crop out in a com-
plex pattern of blocks or terranes often only a few tens of kilo-
metres wide. Attempts have been made to map and name
these terranes (e.g. Nutman et al . 2004; Friend & Nutman
2005), but as the rock types belonging to the different epi -
sodes are very similar, it has proved to be very difficult to dis-
tinguish and map the terranes in the field (Hollis et al . 2006).
Gold-hosting supracrustal rocks on Storø, southern
West Greenland: lithologies and geological environment
West Greenland: lithologies and geological environment
Christian Knudsen, Jeroen A.M. van Gool, Claus Østergaard, Julie A. Hollis,
Matilde Rink-Jørgensen, Mac Persson and Kristoffer Szilas
Matilde Rink-Jørgensen, Mac Persson and Kristoffer Szilas
© GEUS, 2007.
Geological Survey of Denmark and Greenland Bulletin
13, 41-44. Available at:
www.geus.dk/publications/bull
41
Fig. 1. Map of supracrustal units in the central part of Storø. The supra -
crustal rocks are cut by numerous pegmatites not shown on this map.
The gold-hosting supracrustal sequence overlies the Storø
shear zone (Figs 1, 2), an oblique ductile thrust which juxta-
poses the supracrustal sequence against the c . 3050 Ma tona-
litic Nûk gneisses to the west. The gneisses to the south-east
are thought to be Palaeoarchaean tonalites. The thrusting
along the Storø shear zone occurred syn-to-post the latest
folding event and is broadly coeval with a phase of pervasive
intrusion by pegmatites at 2630 Ma.
poses the supracrustal sequence against the c . 3050 Ma tona-
litic Nûk gneisses to the west. The gneisses to the south-east
are thought to be Palaeoarchaean tonalites. The thrusting
along the Storø shear zone occurred syn-to-post the latest
folding event and is broadly coeval with a phase of pervasive
intrusion by pegmatites at 2630 Ma.
Lithologies
The supracrustal rocks on Storø comprise a range of litholo-
gies including quartzites, quartzo-feldspathic gneisses, alumi-
nous schists and gneisses, amphibolites and ultramafic rocks.
The distribution and relative abundance of rocks in the cen-
tral part of the island is shown in Fig. 1. The rocks are meta-
morphosed to amphibolite facies, and the dominant litho l ogy
is mafic amphibolite.
gies including quartzites, quartzo-feldspathic gneisses, alumi-
nous schists and gneisses, amphibolites and ultramafic rocks.
The distribution and relative abundance of rocks in the cen-
tral part of the island is shown in Fig. 1. The rocks are meta-
morphosed to amphibolite facies, and the dominant litho l ogy
is mafic amphibolite.
The amphibolites (mafic, foliated, amphibole-dominated
rocks) can be divided into a number of types. They are all
highly strained while compositional layering varies from sub-
tle variations on dm- to m-scale in dark amphibolite, to more
distinct cm-scale compositional layering in more leucocratic
amphibolites. The most common type is homogeneous am-
phibolite, a medium- to coarse-grained, black to dark grey
rock mainly composed of hornblende and plagioclase. The
proportion of plagioclase to hornblende varies, and minor
amounts of quartz, biotite, sphene, apatite and ilmenite occur.
Locally, near some large anorthosite bodies at the struc tural
base of the sequence, the hornblende or the plagioclase in the
amphibolite occurs as cm-sized aggregates, suggesting that
highly strained while compositional layering varies from sub-
tle variations on dm- to m-scale in dark amphibolite, to more
distinct cm-scale compositional layering in more leucocratic
amphibolites. The most common type is homogeneous am-
phibolite, a medium- to coarse-grained, black to dark grey
rock mainly composed of hornblende and plagioclase. The
proportion of plagioclase to hornblende varies, and minor
amounts of quartz, biotite, sphene, apatite and ilmenite occur.
Locally, near some large anorthosite bodies at the struc tural
base of the sequence, the hornblende or the plagioclase in the
amphibolite occurs as cm-sized aggregates, suggesting that
the amphibolite here may represent metamorphosed gabbroic
rock. Varieties of the homogeneous amphibolites include gar-
net amphibolites and diopside amphibolites.
rock. Varieties of the homogeneous amphibolites include gar-
net amphibolites and diopside amphibolites.
Several types and generations of quartz veins have been
observed in the amphibolites. These include (1) 1-2 cm thick
sheeted quartz veins, with sharp contacts to the host rocks,
concordant and generally isoclinally folded; (2) thin, spidery
quartz veins anastomosing through the rocks; (3) centimetre
to decimetre thick agmatitic quartzo-feldspathic veins that
anastomose within 0.5-2 m wide zones; and (4) generally
concordant quartzo-feldspathic veins, commonly with minor
garnet.
sheeted quartz veins, with sharp contacts to the host rocks,
concordant and generally isoclinally folded; (2) thin, spidery
quartz veins anastomosing through the rocks; (3) centimetre
to decimetre thick agmatitic quartzo-feldspathic veins that
anastomose within 0.5-2 m wide zones; and (4) generally
concordant quartzo-feldspathic veins, commonly with minor
garnet.
A suite of foliated felsic and aluminous gneisses is inter-
leaved with the amphibolites on Storø (Fig. 2). Near the
structural base of the supracrustal sequence, grey composi-
tionally layered felsic biotite-hornblende gneisses of interme-
diate composition are often seen. The layering is expressed by
variable proportions of hornblende, biotite, quartz and pla-
gioclase. Sphene, apatite and ilmenite are common accessory
minerals while garnet is uncommon. The aluminous gneisses
contain high and variable amounts of garnet, biotite, silli-
manite and cordierite, and minor amounts of staurolite and
tourmaline. These rocks are characterised by strong foliation
combined with compositional layering (cm- to dm-scale) to
lamination (mm-scale; Fig. 3A). Amongst these rocks, a gar-
net-biotite gneiss, a sillimanite-garnet-biotite gneiss and a
cordierite-garnet-biotite gneiss can be mapped as lithological
units. These gneisses are characterised by a rusty brownish
weathering colour (Fig. 2) mainly due to the high content of
biotite. Within the package of aluminous gneisses, a c . 1 m
thick characteristic and mappable unit with a very high con-
tent of magnetite and garnet occurs (Fig. 1). A magnetite-free
quartz-garnetite, consisting of up to 90% garnet is interlay-
ered with the magnetite-bearing rocks. Where the degree of
exposure is high and using the garnet-magnetite rock as a
marker horizon, a tight to isoclinal fold pattern with repeti-
tion of the tectono-stratigraphic units can be identified in the
biotite-garnet gneiss.
structural base of the supracrustal sequence, grey composi-
tionally layered felsic biotite-hornblende gneisses of interme-
diate composition are often seen. The layering is expressed by
variable proportions of hornblende, biotite, quartz and pla-
gioclase. Sphene, apatite and ilmenite are common accessory
minerals while garnet is uncommon. The aluminous gneisses
contain high and variable amounts of garnet, biotite, silli-
manite and cordierite, and minor amounts of staurolite and
tourmaline. These rocks are characterised by strong foliation
combined with compositional layering (cm- to dm-scale) to
lamination (mm-scale; Fig. 3A). Amongst these rocks, a gar-
net-biotite gneiss, a sillimanite-garnet-biotite gneiss and a
cordierite-garnet-biotite gneiss can be mapped as lithological
units. These gneisses are characterised by a rusty brownish
weathering colour (Fig. 2) mainly due to the high content of
biotite. Within the package of aluminous gneisses, a c . 1 m
thick characteristic and mappable unit with a very high con-
tent of magnetite and garnet occurs (Fig. 1). A magnetite-free
quartz-garnetite, consisting of up to 90% garnet is interlay-
ered with the magnetite-bearing rocks. Where the degree of
exposure is high and using the garnet-magnetite rock as a
marker horizon, a tight to isoclinal fold pattern with repeti-
tion of the tectono-stratigraphic units can be identified in the
biotite-garnet gneiss.
Within these aluminous gneisses, a unit dominated by
quartzite occurs. The composition varies from pure quartzite
(> 90% quartz) to a rock composed predominantly of quartz
with feldspar, sillimanite and light green muscovite, and
locally with minor fuchsite, biotite or garnet.
(> 90% quartz) to a rock composed predominantly of quartz
with feldspar, sillimanite and light green muscovite, and
locally with minor fuchsite, biotite or garnet.
At the other end of the spectrum, ultramafic lenses
(boudins) form a characteristic component in the supra-
crustal sequence. The size of the lenses varies from round
pods tens of metres in diameter to elongate lenses hundreds
of metres thick and up to 1 km long (Fig. 1). The modal com-
position of the ultramafic rocks varies substantially. In the
cores of the major bodies the rock is mainly composed of
olivine, pyroxene (both orthopyroxene and clinopyroxene)
crustal sequence. The size of the lenses varies from round
pods tens of metres in diameter to elongate lenses hundreds
of metres thick and up to 1 km long (Fig. 1). The modal com-
position of the ultramafic rocks varies substantially. In the
cores of the major bodies the rock is mainly composed of
olivine, pyroxene (both orthopyroxene and clinopyroxene)
42
Fig. 2. View of the Storø supracrustal units looking north-east from
Qingaq. To the left, the south-west-dipping Storø shear zone is visible.
The rusty weathering rocks are supracrustals mainly thought to be of
metasedimentary origin. The amphibolites are dark grey and the
anorthosite body light grey. Highest summit at left is 1425 m.
and amphibole (hornblende and tremolite). The margins of
these bodies consist commonly of tremolite-phlogopite schist
with variable chlorite and actinolite. This rock is locally
affected by prograde breakdown of chlorite producing a con-
spicuous texture where large aggregates of parallel platy
olivine crystals are intergrown with large chlorite crystals at
high angles to the foliation (Fig. 3B). The ultramafic rocks
are locally altered to serpentinite.
these bodies consist commonly of tremolite-phlogopite schist
with variable chlorite and actinolite. This rock is locally
affected by prograde breakdown of chlorite producing a con-
spicuous texture where large aggregates of parallel platy
olivine crystals are intergrown with large chlorite crystals at
high angles to the foliation (Fig. 3B). The ultramafic rocks
are locally altered to serpentinite.
Gold mineralisation
Gold occurrences are located in the amphibolite-dominated
parts of the supracrustal sequence. They are often found in
zones where the amphibolite is enriched in biotite and garnet
and often associated with pyrrhotite, arsenopyrite and
loellingite (FeAs
parts of the supracrustal sequence. They are often found in
zones where the amphibolite is enriched in biotite and garnet
and often associated with pyrrhotite, arsenopyrite and
loellingite (FeAs
2
). Gold is found both in sericitised plagio-
clase and within garnet (Juul-Pedersen
et al
. in press), indi-
cating that the gold mineralisation predates the metamorphic
event at c . 2700-2630 Ma (Hollis 2005). The gold is also
often associated with quartz veins, and in these visible gold is
locally found.
cating that the gold mineralisation predates the metamorphic
event at c . 2700-2630 Ma (Hollis 2005). The gold is also
often associated with quartz veins, and in these visible gold is
locally found.
Geochemistry
The amphibolites have a composition equivalent to tholeitic
basalts and are interpreted as metamorphic equivalents of
basalts. Based on their geochemistry, Polat (2005) stated that
the amphibolites have a subduction zone geochemical signa-
ture. The common occurrence of calc-silicates is taken as a
sign of hydrothermal alteration in a sea-floor environment
(Polat et al . 2007). The grey biotite hornblende gneiss of
intermediate composition could be the metamorphic equiv-
alent of an intermediate volcanic rock. The ultramafic rocks
have MgO in the range of 20 to 40 wt%, high Cr and Ni
(1500 to 4000 ppm) and CaO/Al
basalts and are interpreted as metamorphic equivalents of
basalts. Based on their geochemistry, Polat (2005) stated that
the amphibolites have a subduction zone geochemical signa-
ture. The common occurrence of calc-silicates is taken as a
sign of hydrothermal alteration in a sea-floor environment
(Polat et al . 2007). The grey biotite hornblende gneiss of
intermediate composition could be the metamorphic equiv-
alent of an intermediate volcanic rock. The ultramafic rocks
have MgO in the range of 20 to 40 wt%, high Cr and Ni
(1500 to 4000 ppm) and CaO/Al
2
O
3
ratios around 1, sug-
gesting a komatiitic protolith. The garnet-biotite gneiss,
cordierite-garnet-biotite gneiss and sillimanite-garnet biotite
gneiss are generally characterised by high Al contents consis-
tent with a sediment protolith, which is supported by the
observation of detrital zircon grains in these rocks (Hollis et
al . 2006). The gneisses of supposed sedimentary origin have
distinctly different Zr/TiO
cordierite-garnet-biotite gneiss and sillimanite-garnet biotite
gneiss are generally characterised by high Al contents consis-
tent with a sediment protolith, which is supported by the
observation of detrital zircon grains in these rocks (Hollis et
al . 2006). The gneisses of supposed sedimentary origin have
distinctly different Zr/TiO
2
ratios compared to the amphi-
bolites (Fig. 4). There are, however, some extremely alumi-
nous rocks (consisting primarily of garnet and sillimanite
together with quartz, feldspar and biotite) in contact with the
amphibolite on Qingaq, and these rocks have Zr/TiO
nous rocks (consisting primarily of garnet and sillimanite
together with quartz, feldspar and biotite) in contact with the
amphibolite on Qingaq, and these rocks have Zr/TiO
2
ratios
similar to the amphibolites (Fig. 4). As neither Zr nor Ti is
likely to be mobile in a non-alkaline environment they have
probably been derived from basalt by sericitisation caused by
hydrothermal alteration (cf. Garde et al . 2007 - this volume).
This process can lead to volume loss due to loss of elements
likely to be mobile in a non-alkaline environment they have
probably been derived from basalt by sericitisation caused by
hydrothermal alteration (cf. Garde et al . 2007 - this volume).
This process can lead to volume loss due to loss of elements
such as Ca, Si, Fe and Mg, and accordingly to increased con-
centrations of Al, Ti and other immobile elements together
with K (stable in the sericite). This rock type contains gold
likely to have been introduced during the intense (premeta-
morphic) hydrothermal alteration. The more common gold-
bearing zones within the amphibolites are characterised by
elevated contents of biotite and garnet. These rocks have
increased concentrations of Al and K likely to have been
caused by a similar process.
centrations of Al, Ti and other immobile elements together
with K (stable in the sericite). This rock type contains gold
likely to have been introduced during the intense (premeta-
morphic) hydrothermal alteration. The more common gold-
bearing zones within the amphibolites are characterised by
elevated contents of biotite and garnet. These rocks have
increased concentrations of Al and K likely to have been
caused by a similar process.
Ages and structural relations
Detrital zircon from a garnet-biotite-sillimanite-cordierite
gneiss and from a quartzite has been analysed (Hollis 2005;
Rink-Jørgensen 2006) using laser ablation techniques (cf.
Frei et al . 2006). The results indicate that there are two dis-
tinct sources for these rocks (Fig. 5). The garnet-biotite-silli-
manite-cordierite gneiss contains detrital zircon distributed
gneiss and from a quartzite has been analysed (Hollis 2005;
Rink-Jørgensen 2006) using laser ablation techniques (cf.
Frei et al . 2006). The results indicate that there are two dis-
tinct sources for these rocks (Fig. 5). The garnet-biotite-silli-
manite-cordierite gneiss contains detrital zircon distributed
43
Fig. 3. Appearance of selected lithologies.
A
: Thinly laminated, rusty,
garnet-sillimanite-biotite gneiss with tight to isoclinal folds.
B
: Ultramafic
rock with a conspicuous texture consisting of sets of parallel oriented, up
to 10 cm large, platy, olivine crystals together with large chlorite crystals.
in two age populations around 2880 and 2830 Ma. We sug-
gest that the rock had an immature sedimentary precursor,
possibly derived from erosion of a relatively young volcanic
arc formed in two episodes at 2880 and 2830 Ma. The detri-
tal zircon in the quartzite, which is likely to represent a
mature sediment, forms populations with main peaks around
2960 and 3010 Ma and a minor peak at 3180 Ma. This rock
must have had a distinctly different source and probably rep-
resents the erosional products of an older continent (e.g. the
Nûk gneisses). This difference in sources is consistent with a
model of formation of the supracrustal rocks in a back-arc
environment. The immature sediments are likely to have
been derived from the relatively young rocks in the arc,
whereas the mature quartzites are likely to originate from the
older rocks located on the continent side of the back-arc
basin. This is supported both by the observation that the
amphibolites formed in an arc environment and by the suite
of rocks present. This environment is very similar to that
envisaged for Canadian Neoarchaean supracrustal belts
(Sandeman et al . 2006).
gest that the rock had an immature sedimentary precursor,
possibly derived from erosion of a relatively young volcanic
arc formed in two episodes at 2880 and 2830 Ma. The detri-
tal zircon in the quartzite, which is likely to represent a
mature sediment, forms populations with main peaks around
2960 and 3010 Ma and a minor peak at 3180 Ma. This rock
must have had a distinctly different source and probably rep-
resents the erosional products of an older continent (e.g. the
Nûk gneisses). This difference in sources is consistent with a
model of formation of the supracrustal rocks in a back-arc
environment. The immature sediments are likely to have
been derived from the relatively young rocks in the arc,
whereas the mature quartzites are likely to originate from the
older rocks located on the continent side of the back-arc
basin. This is supported both by the observation that the
amphibolites formed in an arc environment and by the suite
of rocks present. This environment is very similar to that
envisaged for Canadian Neoarchaean supracrustal belts
(Sandeman et al . 2006).
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Rink-Jørgensen, M. 2006:
In situ
U-Pb datering af zirkoner i teori og prak-
sis til bestemmelse af alder og populationer af detritale zirkoner, med
anvendelse på en prøve fra Nuuk regionen i det sydlige Vestgrønland.
Unpublished B.Sc. project, Københavns Universitet, Danmark.
anvendelse på en prøve fra Nuuk regionen i det sydlige Vestgrønland.
Unpublished B.Sc. project, Københavns Universitet, Danmark.
Sandeman, H.A., Hanmer, S., Tella, S., Armitage, A.A., Davis, W.J. &
Ryan, J.J. 2006: Petrogenesis of Neoarchaean volcanic rocks of the
MacQuoid supracrustal belt: a back-arc setting for the northwestern
Hearne subdomain, western Churchill Province, Canada. Precambrian
Research 144 , 140-165.
MacQuoid supracrustal belt: a back-arc setting for the northwestern
Hearne subdomain, western Churchill Province, Canada. Precambrian
Research 144 , 140-165.
44
Authors' addresses
C.K. & J.A.M.v.G.,
Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
E-mail:
ckn@geus.dk
C.Ø.,
NunaMinerals, Vandsøvej 5, P.O. Box 790, DK-3900 Nuuk, Greenland.
J.A.H.,
Northern Territories Geological Survey, P.O. Box 3000, Darwin NT 0801, Australia.
M.R.-J.,
M.P. & K.S., Institute of Geography and Geology, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark.
Fig. 4. Plot of Zr ppm vs. TiO
2
wt% in different rock samples from Qingaq
on Storø.
Fig. 5. Detrital zircon populations from two rock samples from Storø.
Sample 481465 is a garnet-biotite-sillimanite-cordierite gneiss and 481283
is a quartzite.
