Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara layered complex, northern Finland

240 Pages·14.769 MB·English

by  Mutanen Tapani.

G Geological Survey
e
o
lo
g
ic of Finland
a
l S
u
r
v
e
y
o
f F
in
la
n
d
, B Bulletin 395
u
lle
tin
3
9
5
- M
U
T
A
N
E
N
Tätä julkaisua myy Denna publikation säljes av This publication can be obtained , TA
from P
A
GEOLOGIAN GEOLOGISKA GEOLOGICAL SURVEY N
I
TUTKIMUSKESKUS (GTK) FORSKNINGSCENTRALEN (GFC) OF FINLAND (GSF)
Julkaisumyynti Publikationsförsäljning Publication sales
PL 96 PB 96 P.O. Box 96
02151 Espoo 02151 Esbo FIN-02151 Espoo, Finland
Geology and ore petrology of the Akanvaara
0205 50 11 0205 50 11 [phone]
Telekopio: 0205 50 12 Telefax: 0205 50 12 Telefax: [phone] and Koitelainen mafic layered intrusions and the
Keivitsa-Satovaara layered complex, northern Finland
GTK, Väli-Suomen GFC, Distriktsbyrån för GSF, Regional office for
aluetoimisto Mellersta Finland Mid-Finland
Kirjasto Biblioteket Library
PL 1237 PB 1237 P.O. Box 1237
70211 Kuopio 70211 Kuopio FIN-70211 Kuopio, Finland by Tapani Mutanen
0205 50 11 0205 50 11 [phone]
Telekopio: 0205 50 13 Telefax: 0205 50 13 Telefax: [phone]
GTK, Pohjois-Suomen GFC, Distriktsbyrån för GSF, Regional office for
aluetoimisto Norra Finland Northern Finland
Kirjasto Biblioteket Library
PL 77 PB 77 P.O. Box 77
96101 Rovaniemi 96101 Rovaniemi FIN-96101 Rovaniemi, Finland
0205 50 11 0205 50 11 [phone]
Telekopio: 0205 50 14 Telefax: 0205 50 14 Telefax: [phone] Geological Survey of Finland
Espoo 1997
ISBN 951-690-652-4
ISSN 0367-522X
E-mail: [email protected]
WWW-address: http://www.gsf.fi
9
X
Geological Survey of Finland, Bulletin 395
GEOLOGY AND ORE PETROLOGY OF THE AKANVAARA
AND KOITELAINEN MAFIC LAYERED INTRUSIONS AND THE
KEIVITSA-SATOVAARA LAYERED COMPLEX, NORTHERN FINLAND
by
TAPANI MUTANEN
with 233 pages, 88 figures, 11 tables in the
text and 5 appended maps
ACADEMIC DISSERTATION
in Geology and Mineralogy
To be presented, with permission of the Faculty of
Science, University of Helsinki, for public criticism in Lecture Room 1
of the Department of Geology, on May 23rd, 1997, at 12 noon
GEOLOGIAN TUTKIMUSKESKUS
ESPOO 1997
Mutanen, Tapani., 1997. Geology and ore petrology of the Akanvaara and
Koitelainen mafic layered intrusions and the Keivitsa-Satovaara layered com-
plex, northern Finland. Geological Survey of Finland, Bulletin 395, – pages,
88 figures, 11 tables and 5 appended maps.
The Akanvaara and Koitelainen intrusions, ca 2440 Ma old, represent a
group of cratonic mafic layered intrusions (aged from 2500 to 2440 Ma) of
the Fennoscandian (Baltic) Shield. These intrusions, which show all the stand-
ard types of igneous layering, host deposits of chromite, vanadium, titanium,
platinum-group elements (PGE) and gold.
The cumulate stratigraphy in Akanvaara and Koitelainen reflects both
normal tholeiitic fractionation and superimposed intermittent selective and
wholesale contamination by anatectic salic melt (now granophyre) from the
floor and the roof, and by refractory residual phases after anatexis of the roof
rocks. Cumulus phases separated mainly in a hybrid melt between the mafic
main magma and the overlying buoyant acid melt. The contaminants are found
in cumulates as fossil melt and magma inclusions in olivine, chromite, or-
thopyroxene and plagioclase, and as “granitic” intercumulus material. The
composition of the intercumulus material in the most heavily contaminated
cumulates deviates strongly from what one would expect of the mafic main
magma. Unusual minerals (loveringite, chlorapatite. zircon, zirconolite, bad-
deleyite, thorite, allanite, galena, perrierite, a Ti-Th-REE-P silicate) are com-
mon in the intercumulus of the ultramafic cumulates; ilmenite, chlorapatite
and zirconolite occur as daughter and occluded minerals in melt and magma
inclusions in olivines and orthopyroxene. Chlorapatite, ilmenite and loverin-
gite are the most common Doppelgänger phases (Mutanen, 1992), of which
only the apatite phase (as a fluorapatite) appears as a late cumulus mineral.
The melting occurred both early and late in the crystallization history,
corresponding to the water-saturated and dry melting of the granitic constit-
uents of the surrounding crustal rocks. There is no evidence for, nor need of,
multiple magma pulses; the reversals can readily be explained by contami-
nants transmitted by two-phase convection. Contamination of the magma by
crustal material shifted the liquidus phase boundaries, resulting in excessive
separation first of olivine and, subsequently of orthopyroxene. Refractory
aluminous phases after partial melting of pelitic roof rocks were responsible
for the anorthositic cumulates.
Chromitite layers occur from near the base of the intrusions to the 86 PCS
level. Almost all the chromitites have elevated concentrations of PGE. The
gangue is generally rich in biotite-phlogopite. All chromites are very low in
MgO (from 0.0% to 1.2%). These features imply that chromite crystallized and
was equilibrated in a melt poor in magnesium and rich in potassium. It is
suggested that the chromium of the uppermost chromitites (UC layers) was
exotic: it was liberated from melted high-aluminous schists averaging 500
ppm Cr. Exotic chromium may well have contributed to the genesis of some
other anorthosite-associated chromitites, too (e.g., Fiskenaesset, Sittampundi,
Soutpansberg, Bushveld, Stillwater).
In the Akanvaara and Koitelainen intrusions the PGE show little respect for
sulphides, and are typically associated with oxides (chromite, ilmenomagnet-
ite, ilmenite). Evidently the PGM phases crystallized from the silicate liquid
direct.
The Keivitsa intrusion (age 2057 Ma) is located south of the Koitelainen
intrusion. Isotopic (Sm-Nd, S-isotopes, Re-Os), geochemical (Ni/Co, Ni/S,
PGE/S, S/Se, S/Te, Re/PGE, among others) and mineralogical data all point
to significant exotic contamination by surrounding carbonaceous, sulphide-
rich sediments and by solid refractory debris from disaggregated komatiitic
rocks. The addition of exotic sulphur (as anatectic immiscible sulphide melt)
and olivine debris was decisive for the genesis of the low-grade Keivitsansarvi
Cu-Ni-PGE-Au deposit, which is located far above the basal contact of the
intrusion. Other contaminants acquired from surrounding rocks were water,
reduced carbonaceous material and chlorine, all of which affected the crys-
tallization of the magma. Mineralogical evidence from Keivitsa and from
other PGE-bearing magmatic sulphide deposits together with our experimental
work (Mutanen et al., 1996) suggest strongly that immiscible sulphide melt
either acted as a phase boundary collector for platinum-group mineral (PGM)
particles already crystallized from melt or that the supersaturated PGM nucle-
ated on sulphide droplets.
Key words (GeoRef Thesaurus, AGI): layered intrusions, contamination, de-
gassing, petrology, chlorapatite, Cl, ultramafics, chromitite, gabbros, geo-
chemistry, copper ores, vanadium ores, nickel ores, platinum ores, gold ores,
Proterozoic, Finland, Lappi Province, Savukoski, Akanvaara, Sodankylä,
Koitelainen, Keivitsa, Keivitsansarvi, Satovaara
Tapani Mutanen
Geological Survey of Finland
P.O. Box 77
FIN-96101 ROVANIEMI, FINLAND
ISBN 951-690-652-4
ISSN 0367-522X
Vammalan Kirjapaino Oy 1997
4 Geological Survey of Finland, Bulletin 395
Tapani Mutanen
ABBREVIATIONS AND DEFINITIONS
ab – albite
an – anorthite
APB – Archaean-Proterozoic boundary
bi – biotite
CC model – convection-contamination model
CN – chondrite-normalized
cp – chalcopyrite
cpx – clinopyroxene (augite, diopside, hedenbergite)
crt – chromite
CuEQ – copper equivalent, wt% (includes Cu, Ni and part of Pd and Au)
cumulus crystal – settled (or floated) crystal
DDH – diamond (core) drill hole
D-value – simple ratio; distribution coefficient of an element between phases
Doppelgänger – a premature magmatic phase, opposite to reversed phase (in phase reversal; (see
(doubleganger) Mutanen, 1992, p. 26)
economic – pertaining to the production, distribution, and use of income, wealth, and commodi-
ties — pertaining to an economy, or system of organization or operation, esp. of
the process of production — pertaining to one’s personal resources of money
(Webster’s Encyclopedic Unabridged Dictionary of the English Language. Portland
House, New York, 1989). ‘Economic’ is misleadingly used in Finland as a synonym
with ‘profitable’.
false ore – Ore type in Keivitsa. Analogous to false money: rock with massive or rich dissemi-
nated sulphides but low in Ni. Metal analysis (as with false coins) is needed to
distinguish false from genuine (viable) ore. No economic connotation!
Fe(tot) – Total iron calculated as FeO
GSF – Geological Survey of Finland
hb – hornblende
ilm – ilmenite
ilmenomagnetite – magnetite with exsolved ilmenite
KEI – type intrusion – Keivitsa-type intrusion
KOI – type intrusion – Koitelainen-type intrusion
KSC – Keivitsa-Satovaara Complex
LC – Lower Chromitite
LIB – Lapland Intrusion Belt
LLC – Lowermost Lower Chromitite
LZ – Lower Zone
mg# – atomic Mg/Mg+Fe ratio
MZ – Main Zone
Ni(100S), Ni(S) – Ni concentration (in wt%) in calculated 100% sulphides
ol – olivine
opx – orthopyroxene (Ca-poor pyroxene)
P(fl) – fluid pressure
P(lit) – lithostatic pressure
PCS – per cent solidified
PGE – platinum-group elements
PGE(100S), PGE(S) – PGE concentration (in ppb or ppm) in 100% sulphides (similarly, Pd(S), Au(S) etc.)
PGM – platinum-group minerals
phl – phlogopite
pl – plagioclase
pn – pentlandite
po – pyrrhotite
px – pyroxene
py – pyrite
q – quartz
RONF – Regional Office for Northern Finland
SHMB – siliceous high-Mg basalt
SLI – silicate liquid immiscibility
sp – spinel (denotes the spinel group, unless stated otherwise)
titanomagnetite – homogeneous (high-T) titaniferous magnetite
UC – Upper Chromitite
ULC – Uppermost Lower Chromitite
UZ – Upper Zone
VTT – Valtion Teknillinen Tutkimuskeskus (Technical Research Centre of Finland)
wr – whole-rock
% – weight per cent (unless stated otherwise)
Geological Survey of Finland, Bulletin 395 5
Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...
A1: View east of Koitelainen hill. Photo by TM.
A2: View south of Lakijänkä towards Koitelainen fell
6 Geological Survey of Finland, Bulletin 395
Tapani Mutanen
B1: Frosty bog, western part of Koitelainen
B2: Bog, north of Koitelainen fell
Geological Survey of Finland, Bulletin 395 7
Geology and ore petrology of the Akanvaara and Koitelainen mafic layered intrusions and the Keivitsa-Satovaara...
C1: Spruce swamp with Russki head tussocks,
Koitelainen
C2: Piiku, field assistant in Koitelainen, 1985

CONTENTS
Abstract........................................................................................................................................... 2
Abbreviations and definitions...................................................................................................... 4
Contents.......................................................................................................................................... 9
INTRODUCTION.......................................................................................................................... 11
General location and physiography of the study areas........................................................ 11
Preglacial weathering, glacial geology and bedrock exposure........................................... 11
Previous work and work in progress...................................................................................... 13
Materials and methods............................................................................................................. 14
GEOLOGICAL SETTING............................................................................................................ 17
Archaean basement complex .................................................................................................. 17
Late Archaean to early Proterozoic volcanism..................................................................... 17
MAFIC LAYERED INTRUSIONS OF THE FENNOSCANDIAN SHIELD......................... 19
PREVIEW OF THE CONVECTION-CONTAMINATION MODEL...................................... 24
THE AKANVAARA INTRUSION ............................................................................................. 25
General geology....................................................................................................................... 25
Igneous stratigraphy and petrography ................................................................................... 27
Chemistry and fractionation ................................................................................................... 31
Chromitite layers...................................................................................................................... 40
Behaviour of PGE.................................................................................................................... 49
Some petrological aspects....................................................................................................... 56
THE KOITELAINEN INTRUSION............................................................................................ 59
General geology....................................................................................................................... 59
Emplacement and fractionation.............................................................................................. 61
Igneous stratigraphy and petrography ................................................................................... 64
Chromitite layers...................................................................................................................... 80
Occurrences of PGE and Au................................................................................................... 88
Ultramafic pegmatoid pipes.................................................................................................... 93
PETROLOGY OF THE KOITELAINEN INTRUSION............................................................ 95
On reversals and multiple magma pulses (MMP)................................................................ 95
Melting of country rocks......................................................................................................... 98
Convection................................................................................................................................ 103
Contamination.......................................................................................................................... 106
Genesis of the Upper Chromitite............................................................................................ 114
Genesis of pegmatoids............................................................................................................. 119
THE KEIVITSA – SATOVAARA COMPLEX ......................................................................... 128
Location and exploration history ........................................................................................... 128
General geology....................................................................................................................... 129