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Geological Survey of Finland, Bulletin 333
257
THE TELKKALA NICXNL DEPOSIT
T. A. HAKLI
REGIONAL GEOLOGY
The Telkkälä ultramafic intrusion, some l6
km northwest of Lappeenranta in SE Finland,
hosts a small nickel deposit embedded in Svecokarelian mica gneisses. The Telkkälä area is
part of the Saimaa basin, which is composed of
Svecokarelian supracrustal and plutonic rocks
and Postsvecokarelian rapakivi granites (Fig.
55).
The bulk of the supracrustal rocks are argillaceous sediments (Vorma 1965), although calcareous and arenaceous interlayers may also be
encountered. The supracrustal rocks are in-
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Fig. 55. Simplified geological map of the Telkkälä area after Häkli et al. (1975).1. Mica gneiss; 2. Gabbro or diorite; 3.
Granodiorite to quartz diorite; 4. Granite; 5. Rapakivi granite; 6. Lakes; 7. Haikkaanlahti Ni-showing; 8. Ahokkala
Ni-showine.
t7
258
Geological Survey of Finland, Bulletin 333
truded by plutonic rocks ranging from granites
to hornblendites and other ultramafics in composition.
The hornblende variety predominates among
the gabbros but norites are also met with, particularly west, north and east of Telkkälä. South
and southwest of Telkkälä large areas of bedrock are occupied by Postsvecokarelian anorogenic rapakivi granites.
Nickel showings, albeit small, occur in vari-
of the Saimaa area. Two of them are
close to the Telkkälä nickel deposit (Fie. 55):
One at Ahokkala in a noritic body some 5 km
east of Telkkälä and the other in a small mafic
ous parts
body at Haikkaanlahti, 2 km southwest of Telkkälä. There is a minor nickel deposit (Kitula) at
Puumala (Marmo 1955), about 35 km northnortheast of Telkkälä. The latter was mined by
Outokumpu Oy in 1970 at the same time as the
Telkkälä deposit.
PETROLOGY OF THE INTRUSION
The Telkkälä nickel deposit is associated with
a mafic intrusive measuring 50 x 150 metres in
size (Fig. 56). The major axis of the intrusive
parallels the NW-SE strike of schistosity of the
surrounding mica gneiss (Häkli et al. 1975).
The intrusive is differentiated, ranging from
peridotite to cummingtonite gabbro in composition. The distribution of the lithologic units ex-
hibits a concentric pattern, a peridotitic core
being enveloped by a narrow rim of perknite.
The outer portions are cummingtonite gabbro.
The geophysics and tectonic features suggest
that the intrusion plunges gently towards SE.
The mica gneiss in the contact with the mafic
body has frequently altered into garnet and
cordierite gneisses, presumably owing to the
thermal action of the intruding mafic magma.
The peridotites show a large variation in
mineral composition from olivine-rich variants
to subperknites. The olivine grains are often
fresh and coarse although in some places they
are altered into serpentine. The olivines average
15.5 9o iron and 600 ppm nickel.
The orthopyroxene, which assays 9 9o Fe and
155 ppm Ni, occurs as idiomorphic grains with
partly uralitizated borders but it is also encountered as fine-grained crystals that seem to be of
a
younger generation than the idiomorphic
ones. The amphibole, which crystallized later
than the olivine and pyroxene, occurs in the intercumulus. The amphibole of the peridotites
averages 4.4 tlo iron and 225 ppm Ni. The amphibole of the perknites is somewhat darker and
richer in iron than is that of the peridotites, assaying 7.3 vlo Fe and 320 ppm Ni. The orthopyroxene of the amphibole is also richer in iron
and nickel, averaging 13.4 ü/o Fe and 190 ppm
Ni. The orthopyroxene is often
altered into
cummingtonite. Monoclinic pyroxene is an accessory. The perknites grade into gabbros with
the appearance of plagioclase.
The gabbros are hypidiomorphic in texture,
with orthopyroxene, hornblende and cummingtonite as main minerals. Biotite may or may not
be one of the major constituents. The composition of the plagioclase varies from Anro to
Anuo, although in rocks with some quartz it is
andesine. The orthopyroxene averages 13.3 Vo
Fe and 155 ppm Ni. The corresponding figures
for the amphibole are 6.9 9o Fe and 245 ppm
Ni. Biotite, chlorite, apatite, quaftz, titanite,
sericite. carbonates and zircon are the accessories. The abundance of sulphides varies, but it
usually exceeds 5 90. The cummingtonite gabbro
at the NW end of the intrusion is, however, almost devoid of sulphides.
Geological Survey of Finland, Bulletin 333
259
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Fig. 56. Ceological plan and cross section of the Telkkälä deposit after Häkli et al. (1975\. l. Peridotire; 2. Perknite; 3
Norite;4. Cummingtonite gabbro; 5. Massive sulphides;6. Mica gneiss;7. Ultramafic fragments in ore;8. Garnet;9
Amphibole. The heavy line denotes the cut-off boundary.
The gabbros range in composition from the
plagioclase-poor perknitic types through pale
norites to cummingtonite gabbros. Orthopyroxene is altered to some extent into cummingtonite. whose abundance increases towards the
contact of the intrusion with the mica gneiss.
Adjacent to the mineralized portions, plagioclase, orthopyroxene and cummingtonite are alter-
ed into chlorite.
At the contact of the mafic intrusion with the
260
Geological Survey of Finland, Bulletin 333
mica gneiss, the cummingtonite gabbro grades
into
sulphide-poor cummingtonite gneiss as
qvartz and biotite increase. The transitional
rock was presumably formed when the mafic
magma interacted with the country rock.
The massive ore either favours the contact
with the mica gneiss or is associated with the
pegmatite veins. The high-grade ore often ex-
hibits sulphide network texture, in which garnet,
hornblende, quartz, oligoclase, biotite and
sometimes chloritized pyroxene and siderite are
embedded in a sulphide matrix. Narrow pegmatite veins with oligoclase, microcline and quartz
as main minerals and biotite, chlorite, apatite
and opaques as accessories crosscut the mafic
intrusion.
GEOCHEMISTRY
The compositional variation in the Telkkälä
intrusion is shown in the ternary diagrams in
Figs. 57 and 58. The most ultramafic members
of the body are high-magnesia rocks that plot
close to the MgO apex. The rock suite forms a
gapless series from the MgO apex towards the
AlrO, apex. The bulk of the samples are low in
alkalies. Few samples, however, have elevated
alkali values, presumably owing to contamination by the adjacent pegmatite veins or mica
gneiss country rock. The intrusive exhibits CaO
values that are low in relation to the MgO and
AlrO, values. In this respect, the Telkkälä intrusion differs from the Kylmäkoski mafic body
(Papunen 1980) and the Vammala, Stormi ultramafic intrusion (Häkli et ol. 1979). These three
intrusions are not, however, directly comparable, because at Telkkälä the rocks are heavily
minaralized, whereas at Vammala and Kylmäkoski large portions of the intrusion show only
slight,
if
any, mineralization.
Al2o3
Fig. 57. (NarO
+
KzO)
- AlO, - ofMgO
the
diagram showing the compositions
Telkkälä rocks.
NarO * KrO
Geological Survey of Finland, Bulletin 333
26r
Al2o3
Fig. 58. CaO
- Al:Or - MgO
diagram
showing the compositions of the Telkkälä
rocks.
ORE TYPES
The sulphides occur as three ore types: mas-
sive ores, matrix ores and disseminated ores.
The massive ores are mainly restricted to the
peridotites and perknites, whereas the matrix
and disseminated ores favour cummingtonite
gabbros.
Pyrrhotite, pentlandite and chalcopyrite are
the major primary sulphides. Sphalerite is occasionally encountered in abundances unusual for
a nickel deposit. Ilmenite, like ilmenomagnetite,
chromite and rutile, which are met with in peridotite, are primary oxides.
Monoclinic pyrrhotite, which is more abundant than the hexagonal variety, is usually
rather rich in nickel (average 0.35 go). pentlandite occurs as discrete grains together with and
as exsolution bodies in pyrrhotite. Chalcopyrite
often rims the pentlandite and pyrrhotite grains
but it also occurs as stringers in gangue and
massive ore.
Supergene processes have produced secondary
minerals that replace the primary sulphides.
The alteration is mainly restricted to the SE part
of the deposit, where pyrite, marcasite, bravoite, violarite, hematite, goethite and magnetite
are encountered.
The alteration that entails the depletion of
iron from the sulphides began in the pyrrhotite
with the formation of marcasite-pyrite lamellae
parallel to the basal parting. The nickel abundance of pyrite and marcasite thus formed is
only slightly higher than that of pyrrhotite.
When in larger cavities, however, these minerals
are richer in nickel; nickel values up to 3.5 go
have been recorded in marcasite, and as much
as 4.6 Vo in bravoitic pyrite.
Pentlandite is often altered into violarite, and
the process, which begins from the grain boundaries and cleavages, seems to precede the
alteration of pyrrhotite. In places violarite is
further replaced by chalcopyrite and marcasite;
in fact the nickel-rich marcasites probably owe
262
Geological Survey of Finland, Bulletin 333
s
{000
5O.O Vo
8000 pPm
Vo
10
30
20
10
30.0 10.0
50,0
c/o
600
000
1000 ppm
o/o
70
60
50
40
30
29
10
3.0
2.O
3.0
1,0
r.0
5.O Vo
3.o
Vo
1000 PPm
Fig. 59. Frequency distributions of S, Fe,
Cu, Ni, Co, Zn and Pb in the Telkkälä
deposit after Häkli et al. (1975).
Geological Survey of Finland, Bulletin
their nickel to disintegrated violarite.
With the exception of gossan, which partly
covered the deposit, the'amount of oxides pro-
333
263
duced by the alteration processes is surprisingly
small.
ORE GEOCHEMISTRY
In 1969-1970 about 200,000
tonnes of ore,
averaging 1.06 9o Ni and 0.29 Vo Cu, were
mined from the open pit. The frequency distributions of the elements incorporated in sulphides are positively skewed (Fig. 59). The large
variation in S, Fe, Ni and Co is caused by the
mode of occurrence of the sulphides, which
varies from low grade dissemination to massive
ores.
The factor analyses performed on the geo-
chemical data (Häkli et sl. 1975') demonstrates
that four mutually independent agents were ac-
tive during the formation of the sulphide deposit. They gave rise to the disseminated and
matrix ores in cummingtonite gabbro, deposited
the massive sulphides at the base of the intrusion, produced stringers and dissemination of
the second generation chalcopyrite in country
rocks, and finally generated the Zn-Pb mineralization.
CONCLUSIONS
A genetic model has been proposed by Häkli
et al. (1975) for the formation of the Telkkälä
nickel deposit, dated by the leadlead method at
1820 Ma. In this model, the concentric structure
copper-rich sulphide liquid, which segregated
from the bulk of the sulphide liquid after it had
of the mafic intrusion is attributed to the dis-
eration. The monosulphide solid solution then
recrystallized at lower temperatures as pyrite,
chalcopyrite, pentlandite and monoclinic pyrrhotite.
Somewhat later. the vent introduced more
charge through a vent of mafic silicate melt carrying appreciable amounts of sulphides in dispersed immiscible melt. On account of gravita-
tive settling, the sulphide liquid concentrated in
depressions at the base on the intrusion where it
remained mobile after the consolidation of the
magma. At a lower temperature the increased
water pressure brecciated the rocks. The open
spaces were then invaded by the sulphide liquid,
and breccia and massive ores were produced. A
been crystallized as a monosulphide phase, produced chalcopyrite stringers of the second gen-
sulphur and cobalt and probably also some
iron, zinc and lead, thus reducing the Ni/S and
Ni/Co ratios, generating disulphides and producing a low-grade Zn-Pb mineralization at the
base
of the intrusion.
ACKNOWLEDGEMENT
I thank the outokumpu Company
publish this paper.
for
permission to