Sweden: Complementary material - Structural setting of Iron oxide-apatite and Fe- Cu-sulphide occurrences in Kiruna, Northern
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Structural setting of Iron oxide-apatite and Fe-
Cu-sulphide occurrences in Kiruna, Northern
Sweden: Complementary material
Joel BH Andersson*, Leslie Logan, Tobias E Bauer, and Olof Martinsson
Division of Geosciences and Environmental Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden
* Corresponding author: joel.bh.andersson@ltu.se; +46 73 0821280
Background alization is present between the Loussavaara formation (Fig. 2C, D)
The Norrbotten lithotectonic unit (Stephens 2020) in northern Swe- and ignimbritic rhyolite (Frietsch 1979) of the Matojärvi formation
den hosts numerous iron and base metal deposits (Fig. 1), whereof (Fig. 2E). The Matojärvi formation is a heterogeneous and highly
the world-class Kiirunavaara iron oxide-apatite (IOA) deposit is the tectonized unit of ignimbritic rhyolite (2E), basaltic agglomerate and
most famous one. The area shares many geological characteristics to tuff (Fig. 2F), breccia-conglomerate (Fig. 2G), greywacke (Fig. 2H),
other IOA and iron oxide-Cu-Au (IOCG) prospective terrains around and phyllite (Fig. 2I). The uppermost unit, the Hauki quartzite, marks
the world. Similarities include a variable distributed sodic and po- the end of the Orosirian in the Kiruna area and constitutes cross-bed-
tassic alteration, bimodal character of host volcanic rocks, and the ded quartz-arenite (Fig. 2J) interrupted by breccia-conglomerate in a
deposits tend to be either hosted by, or spatially related, to crustal lower (Fig. 2K) and an upper horizon (Fig. 2L). The volcanic rocks,
scale deformation zones. Despite of the common structural control as well as the ores, were deposited during a short time interval of
on mineralized systems in Norrbotten, only few studies have been approx. 15 M. y. (Westhues et al. 2016).
performed focusing on the linkage between deformation, mineraliza-
tion, and hydrothermal alteration. In this predominantly field-based Structural geology and tectonic interpretation
project, we focus on the structural setting and evolution in Kiruna The stratigraphic column (Fig. 2M) indicates basin development
(Fig. 1) and the relation to iron oxide-apatite, Fe-Cu-sulphide and during the emplacement of the IOA deposits in Kiruna. During sub-
their associated hydrothermal alteration. sequent compression (D1-D2), the basin was inverted. During D1 a
heterogeneously developed continuous S1 cleavage was distributed
This complementary material to the EGU-abstract number EGU- regionally and shear zones were localized at lithological contacts and
2020-280 aims at providing an overview of the most recent geo- favorable volcanoclastic and sedimentary rocks. The same structures
logical research produced within the CAMM (Centre of Advanced were largely re-activated with differing kinematics under more brittle
Mining and Metallurgy)-project as well as preliminary results from conditions during D2 (Andersson et al. 2020). No regionally distrib-
the Horizon 2020 project “New Exploration Technologies (NEXT)” uted cleavage was produced during D2, instead F2 folds in low strain
at the Luleå University of Technology. We use results published in blocks adjacent to shear zones tend to lack axial planar cleavage and
Andersson (2019), Andersson et al. (2019, 2020), as well as new ma- when present it is spaced and brittle. West of Kiruna, the shear zones
terial in order to set these studies in a broader perspective. show reverse oblique (D1; Fig. 3A) and reverse dip-slip (D2; Fig.
3B) with an overall west-side-up sense-of-shear (Andersson et al.
Stratigtraphy 2020). Contrasting kinematics are shown by the shear zones in Kiru-
The Kiruna area constitutes the best-preserved continuous Rhya- na where reverse oblique and reverse dip-slip movements (D2; Fig.
cian-Orosirian stratigraphic sequence in the Norrbotten region. The 3C, D) give an overall east-side-up sense-of-shear (Andersson 2019).
supracrustal pile is dipping and younging towards east. The Orosirian Brittle components are uncommon in D1-structures whereas brittle
Kurravaara conglomerate marks the stratigraphic lowest unit and is and plastic structures formed simultaneous during D2 both in Kiruna
(Fig. 2A), interpreted as an alluvial fan deposit (Kumpulainen 2000) (Fig. 3E) and regionally to the west (Fig. 3F; Andersson et al. 2020)
overlying Rhyacian basaltic pillow lavas. Stratigraphically above the indicating higher crustal levels during D2. The contrasting kinemat-
Kurravaara conglomerate, volcanic and volcanoclastic rocks host ics from west to east results in reverse west-dipping shear zones to the
the largely concordant Kiirunavaara and Loussavaara IOA deposits west, and reverse east-dipping shear zones to the east (cf. Fig. 4, 5).
at the contact between basaltic-andesitic rocks (Hopukka formation; The result is a juxtaposition of different crustal levels with a central
2B) and rhyodacitic rocks (Loussavaara formation; 2C, D). A strati- block of lower metamorphic grade (central Kiruna) surrounded by
graphically higher, and largely concordant, horizon of IOA-miner- rocks of higher metamorphic grade.
1
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242 M. B. STEPHENS
300 500 700 900
Fe oxide dep
Base metal s
Au deposit
Other type of
7600
Mertainen Larger symbols i
Kirunavaara Leveäniemi
Kiruna Gruvberget
Kaunisvaara
NOR
N
NORRBOTTEN
OR
RR
RBOT
RBOTTE
TTTE N
Malmberget
Aitik
7400
Jokkmokk
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Bothnian
Maurliden Bay
GO
Björkdal
SK
LD
ELL
7200
Kristineberg EFT Kankberg
LI
Renström E
NE
Knaften
Caledonian orogen Base metal sulphide deposit
Bothnia-Skelleftå lithotectonic unit Deformation zones Base metal sulphide deposit
Örnsköldsvik
Norrobotten lithotectonic unit Fe-oxide deposit
Östersund
7000
Other types of deposit
Modified after Stephens (2020)
Figure 1: Geological map modified after Stephens (2020).
FINLAND
Hydrothermal alteration Discussion
The hydrothermal alteration linked to D1 is comprised of scapolite Two major deformation events in northern Norrbotten are fre-
Sundsvall
± albite ± sulphide that formed coeval with a magnetite + amphibole Hassela
quently reported regionally (e.g. Bergman et al. 2001, Bauer 2018,
alteration (Fig. 6A; Andersson et al. 2020). In thin section, annealed Bergman 2018, Andersson
Bothnian2019).
Sea The timing of these deformation
pyrite textures and porous rim-growth with albite inclusions indicate events is poorly constrained but thought to overlap with early and
pyrite recrystallization occurred after albitization and synchronous late syn-orogenic magmatism at c. 1.88 Ga and c. 1.78 Ga (Bergman
AY
with magnetite-actinolite alteration during D1 (Fig. 6B). The alter- et al, 2001, Sarlus et al. 2017) respectively. Ore formation associat-
RW
6800
ation styles characteristic for the D2-event is more diverse and po- ed to early magmatism comprises both IOA and IOCG formation
NO
tassic-ferroan in character and most often hosted by D2-structures. It even though geochronological data indicate that IOA emplacement
is comprised of K-feldspar ± epidote ± quartz ± biotite ± magnetite is marginally earlier than IOCG during this early tectono-magmat-
± sericite associated to Fe- and Cu-sulphide (Fig. 6C; Andersson et ic event (cf. Romer et al. 1994, Smith et al. 2009, Martinsson et al.
al. 2020). Both brittle and ductile D2-structures constitute effective 2016, Westheus et al. 2016). On the other hand, ore formation linked
traps for sulphide. Figure 6D shows a drill core sample of a F2-fold-N toGarpenbe rg is dominated by structurally controlled IOCG de-
late magmatism
AGE
ed sequence in a phyllite horizon of the Matojärvi formation. L X-ray posits hosted by D2 structures and it is likely that these ores represent
computed tomography imaging in combination with continuous remobilization of earlier and more significant base metal deposits.
S
RG
XRF-scanning of the same drill core sample reveal that pyrite was
BE
transported along brittle axial planar S2 and trapped in the F2 hinge The significance of the sulphide remobilization-entrapment in cm-
zone (Fig. 6E; Andersson et al. 2019). Lovisagruvanscale linked to D2-deformation in this study is ambiguous. However,
6600
we believe these small-scale key-observations will help understand
Stockholm
regional scale remobilization-entrapment mechanisms linked to
D2-deformation northern Norrbotten.
2
Zinkgruvan Baltic Sea
A B C
D E F
G H I
J K L
Figure 2: Field images showing key localities of the Oro-
Arenite sirian part of the stratigraphy. From bottom: A) Brec-
Breccia-Conglomerate Hauki cia-conglomerate of the Kurravaara conglomerate. B) Ba-
Quartzite
Arenite salt-andesite of the Hopukka formation. C) Rhyodacite of
Breccia-Conglomerate the Loussavaara formation. D) Breccia-conglomerate of
HIGH STRAIN
Phyllite
the Loussavaara formation. E) K-feldspar alterated rhy-
olite of the Matojärvi formation. F) Basaltic agglomerate
Greywacke
of the Matojärvi formation. G) Breccia-conglomerate of
Matojärvi the Matojärvi formation. H) Graywacke of the Matojär-
Breccia-conglomerate forma�on vi formation. I) Phyllite of the Matojärvi formation. J)
Basal�c lava, tuff, agglomerate Cross-bedded quartz-arenite of the Hauki quartzite. K)
Rhyolite tuff, ignimbrite Lower conglomerate of the Hauki quartzite. L) Upper con-
glomerate of the Hauki quartzite. M) Stratigraphic col-
umn of the Kiruna area showing the Orosirian part of the
Rhyodaci�c tuff, coherent stratigraphy.
volcanic, breccia conglomerate Luossavaara
forma�on
Basal�c, trachyandesi�c
sub-volcanic and lavas Hopukka
forma�on
Conglomerate
Kurravaara
Greywacke conglomerate
Conglomerate
M
3
WNW ESE W E
C
S
C
S
A 100µm B 100µm
NW SE NW SE
C 100µm D 100µm
W E W E
S2
Ri
ed
el
fra
tuc
re
s
D 1m E 100µm
Figure 3 Kinematic indications of microstructures and of a field locality. A) Asymmetric sigmoid and
SC-fabric indicating west-side-up. west of central Kiruna, Andersson et al. (2020). B) SC-fabric indicating
west-side-up, west of central Kiruna, Andersson et al. (2020). C) Oblique foliation in a calcite domain in-
dicating east-side-up, central Kiruna. D) Asymmetric sigma-clast indicating east-side-up, central Kiruna.
E) Riedel shear fractures developed in competent volcanic rocks indicting east-side-up. Ductile shearing at
ore contact, central Kiruna. F) Brittle fracturing of feldspar in a dynamically recrystallized quartz matrix.
4
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west-dipping reverse shear zones sub-paralell with the shear zones in central Kiruna.
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Ekströmsberg Fe-prospect
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Figure 4 Geological map of the Ekströmsberg area, west of Kiruna (Andersson et al 2020). The map shows steep
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Tectonic foliation
S0 n:19 L2
n: 146
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zones, saw theeth
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Figure 5 Structural map of the central Kiruna area. The map shows steep east-dipping reverse shear zones.
45 77 Overturned anticlinal F2 and on upthrown
block
40 High strain S2/bedding S0
3 km
Younging direction
Mag+Amp
act
Fe-oxide+Sul
S1 ab
Scp+Ab
act
py Kfs+Epi
Mag+Amp
A 2.5cm B 20 µm C 3cm
D 2 cm E 2 cm
Figure 6 Hydrothermal alteration, A, C from Andersson et al. (2020), D-E from Andersson et al. (2019). A) Magnetite +
amphibole alteration overprinted by scapolite + albite alteration, broadly D1-timing. B) BSE imaging of annealed pyrite
recrystallized in an albite + pyrite + magnetite + actinolite mineral association C) K-feldspar + epidote + Fe-oxide +
sulphide alteration of D2-timing overprinting magnetite + amphibole alteration. D) Drill core section showing an anti-
thetic flank fold in phyllite. E) CT-image of the same fold as in Fig. 6D. Red: pyrite, black: chlorite, dark grey: chlorite
+ white mica + quartz, light grey: white mica + quartz.
Acknowledgement en, C.: Metallogeny of the Northern Norrbotten Ore Province, North-
This study was financed by Centre of Advanced Mining and Metal- ern Fennoscandian Shield with emphasis on IOCG and apatite-iron
lurgy (CAMM). Loussavaara-Kiirunavaara AB is thanked for sup- ore deposits, Ore Geol. Rev., 78, 447–492, 2016.
porting the project.
Kumpulainen, R. A.: The Palaeoproterozoic sedimentary record of
northernmost Norrbotten, Sweden. Geol. Surv. of Swe, unpublished
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