An approximately 90 km long Palaeoproterozoic supracrustal
belt in the northwestern Norrbotten ore province (northernmost Sweden) was
investigated to characterize its structural components, assess hydrothermal
alteration–structural geology correlations, and constrain a paired
deformation–fluid flow evolution for the belt. New geological mapping of
five key areas (Eustiljåkk, Ekströmsberg, Tjårrojåkka,
Kaitum West, and Fjällåsen–Allavaara) indicates two major
compressional events (D1 and D2) have affected the belt, with each
associated with hydrothermal alteration types typical for iron oxide–apatite
and iron oxide Cu–Au systems in the region. Early D1 generated a
regionally distributed, penetrative S1 foliation and oblique reverse
shear zones that show a southwest-block-up sense of shear that formed in
response to NE–SW crustal shortening. Peak regional metamorphism at
epidote–amphibolite facies broadly overlaps with this D1 event. Based
on overprinting relationships, D1 is associated with regional scapolite ± albite, magnetite + amphibole, and late calcite alteration of
mafic rock types. These hydrothermal mineral associations linked to D1
structures may form part of a regionally pervasive evolving fluid flow event
but are separated in this study by crosscutting relationships.
During D2 deformation, folding of S0–S1 structures
generated F2 folds with steeply plunging fold axes in low-strain areas.
NNW-trending D1 shear zones experienced reverse dip-slip reactivation
and strike-slip-dominated movements along steep, E–W-trending D2 shear
zones, producing brittle-plastic structures. Hydrothermal alteration linked
to D2 structures is a predominantly potassic–ferroan association
comprising K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides. Locally, syn- or post-tectonic
calcite is the main alteration mineral in D2 shear zones that intersect
mafic rocks. Our results highlight the importance of combining structural
geology with the study of hydrothermal alterations at regional to
belt scales to understand the temporal–spatial relationship between
mineralized systems. Based on the mapping results and microstructural
investigations as well as a review of earlier tectonic models presented for
adjacent areas, we suggest a new structural model for this part of the
northern Fennoscandian Shield. The new model emphasizes the importance of
reactivation of early structures, and the model harmonizes with tectonic
models presented by earlier workers based mainly on petrology of the
northern Norrbotten area.
Introduction
The northern Norrbotten area of Sweden is an economically important
metallogenic province and mining district (Weihed et al., 2008). For example,
about 90 % of European iron ore is annually produced in the area from two
of the world's largest underground iron mines at Kiirunavaara and Malmberget
(LKAB, 2017; OECD, 2017). These world-class deposits (combined current
reserves of ca. 1051 Mt at 43.4 % Fe; LKAB, 2017) comprise iron
oxide–apatite (IOA) or “Kiruna-type” mineralization, with the Kiirunavaara
deposit representing the archetypal example (e.g. Geijer, 1910, 1930). The
Aitik Cu–Ag–Au deposit also occurs in northern Norrbotten and is one of the
largest open-pit copper mines in Europe (current resource of ca. 801 Mt at
0.22 % Cu, 1.3 g t-1 Ag and 0.15 g t-1 Au; New Boliden AB, 2017). Aitik
represents an enigmatic porphyry-style deposit with a protracted ore-forming
history that is thought to include an overprinting iron oxide–copper–gold
(IOCG)-style mineralization event (e.g. Wanhainen et al., 2005, 2012).
Beyond the active mines, numerous Fe and Cu ± Au prospects and
deposits occur, making the area one of the most prospective terrains in
Europe for IOA- and IOCG-style deposits (e.g. Carlon, 2000; Billström et
al., 2010; Martinsson et al., 2016).
Palaeoproterozoic supracrustal belts in Norrbotten are significant from a
metallogenic perspective as they preferentially host a variety of base and
precious metal ores and thus represent key exploration targets (e.g. Carlon,
2000; Martinsson, 2004). In detail, syn-orogenic “Svecofennian” sequences
deposited between ca. 1.90 and 1.87 Ga represent a key stratigraphic horizon
that locally hosts significant IOA- and IOCG-style deposits (Romer et al.,
1994; Edfelt et al., 2005; Smith et al., 2007; Wanhainen et al., 2012;
Westhues et al., 2016). Both deposit types commonly occur within or
immediately adjacent to large-scale deformation zones, which traverse and
follow the supracrustal belts, suggesting these zones and their contained
lithologies strongly influenced strain localization.
Palaeoproterozoic supracrustal rocks in northern Norrbotten also preserve
evidence of regional- and local-scale hydrothermal alteration and fluid–rock
interaction (e.g. Romer, 1996; Frietsch et al., 1997; Edfelt et al., 2005;
Bernal et al., 2017), and share broad lithological, structural, and
alteration characteristics with other IOCG and IOA prospective terrains
worldwide. For example, features such as variably distributed sodic and
potassic alteration, the bimodal character of host volcanic rocks, the
spatial proximity of deformation zones, and Cu–Au mineralization conform with the IOCG mineral system model defined by Skirrow et al. (2019) and mimic
the character of other IOCG-mineralized terrains in Brazil (e.g. deMelo et
al., 2017; Craveiro et al., 2019), Australia (e.g. Skirrow et al., 2019),
Canada (e.g. Corriveau and Mumin, 2010; Corriveau et al., 2016),and
Mauritania (e.g. Kolb et al., 2008). Therefore, further studies of IOA and
IOCG prospective terrains in northern Sweden may contribute to an improved
understanding of the formation of these deposit types, provide new insights
into the broader controls on mineralization, and help refine conceptual
ore-forming or exploration models applicable to geographically isolated and
underexplored supracrustal domains in northern Fennoscandia, or analogous
terrains elsewhere.
The deposition of Orosirian supracrustal rocks, collectively referred to as
“Svecofennian” (e.g. Gaal and Gorbatschev, 1987), marks the onset of the
Palaeoproterozoic Svecokarelian orogeny (ca. 1.96–1.75 Ga) in Sweden. Previous
studies of Svecofennian supracrustal rocks in northern Norrbotten have
included provincial compilations to ascertain lithostratigraphic and
petrogenetic insights (e.g. Frietsch, 1984; Pharaoh and Pearce, 1984;
Forsell, 1987; Perdahl and Frietsch, 1993; Bergman et al., 2001) and local
studies constraining the geological, geochemical, geophysical, and/or
structural character of sequences hosting Fe and Cu ± Au deposits
(e.g. Geijer, 1910, 1920, 1930, 1950; Parák, 1975; Edfelt et al., 2006;
Sandrin and Elming, 2006; Smith et al., 2007; Wanhainen et al., 2012;
Westhues et al., 2016, 2017; Bauer et al., 2018). With a few exceptions
(Wright, 1988; Bergman et al., 2001; Grigull et al., 2018; Luth et al., 2018; Lynch et al.,
2018a, b), regional compilations have tended to lack structural information.
Local studies with a structural component (e.g. Edfelt et al., 2006; Debras,
2010; Wanhainen et al., 2012) generally have not considered the broader
significance of deposit-proximal structures in reconstructing deformation
and/or fluid flow events for individual belts or the wider region. Thus,
deformation-zone- or belt-scale investigations that include a coupled
structural–alteration assessment may provide new insights into the number
and character of paired deformation–hydrothermal events affecting a given
supracrustal belt.
In this paper, we present new structural and alteration mapping of a
deformed and metamorphosed Orosirian supracrustal belt located about 40 km
to the west of Kiruna in northwest Norrbotten, northern Sweden (Figs. 1, 2,
and 3). The studied sequence, herein referred to as the Western Supracrustal
Belt (WSB; cf. Wright, 1988), extends for about 90 km in a NNW–SSE direction and
hosts several Fe and Cu ± Au occurrences (e.g. Offerberg, 1967;
Witschard, 1975; Edfelt et al., 2005; Frietsch, 1974). New geological
mapping of five key domains is used to ascertain the type, geometry,
kinematics, and interrelationships of various structural components within
the belt and thus constrain its structural evolution. Additionally, a
petrographic and paragenetic study of mappable hydrothermal alteration zones
associated with different lithotypes and/or structures is used to constrain
the character and number of fluid flow events within the supracrustal rocks
and attempts to link these hydrothermal events to specific phases of
deformation. Overall, this coupled structural–alteration approach provides a
new unifying deformation model for the Western Supracrustal Belt, identifies
key structural controls on hydrothermal alteration (and by inference Fe and
Cu ± Au mineralization), and establishes an updated space–time
framework for Svecokarelian deformation and hydrothermal fluid flow in this
sector of northern Fennoscandia.
Generalized geology of northern Norrbotten highlighting
Palaeoproterozoic supracrustal belts. Simplified and modified after Bergman
et al. (2001).
For simplicity, the prefix “meta” for various metamorphic rocks (e.g.
metarhyolite) is not used in this paper as all rocks have been metamorphosed
to some degree (Bergman et al., 2001). We also follow the recommendations of
the Committee for Swedish Stratigraphic Nomenclature for geological and
stratigraphic naming conventions (Kumpulainen, 2007).
Regional geological setting
The northern Fennoscandian Shield (Fig. 1) is underlain by a continental
nucleus of Archean (ca. 2.9–2.6 Ga) granitic, tonalitic, and amphibolitic
gneisses (Gaal and Gorbatschev, 1987; Lahtinen et al., 2008). Rifting of
this continental basement between ca. 2.5 and 2.1 Ga developed crustal-scale,
rift-parallel fault basins and voluminous tholeiitic mafic magmatism and
related sedimentation, producing an approximately NW-aligned large igneous
province extending from northern Norway to Russia (Pharaoh and Pearce, 1984;
Hanski et al., 2012; Melezhik and Hanski, 2012; Bingen et al.,
2015). In northern Sweden, Archean rocks belong to the Norrbotten Craton,
one of three continental nuclei that were rifted during the Rhyacian and
later reassembled during the Svecokarelian accretionary–collisional orogenic
cycle in the Orosirian (e.g. Lahtinen et al., 2005, 2008). In northern
Norrbotten (Fig. 1), Rhyacian rift basalt and related mafic igneous and
sedimentary rocks constitute the lowermost part of the Palaeoproterozoic
stratigraphy (Fig. 4) and occur within several NNE- and NNW-trending
greenstone belts (e.g. Martinsson, 1997; Melezhik and Fallick, 2010; Lynch
et al., 2018a).
Early Svecokarelian-cycle orogenic magmatism (ca. 1.90–1.86 Ga) in northern
Sweden generated two regional suites of co-magmatic volcano-plutonic rocks
that are divisible based on petrological, geochemical, and geographical
considerations (Fig. 4). In the east, calc-alkaline intermediate to felsic
volcanic–volcaniclastic rocks and co-magmatic dioritic to granodioritic
intrusions predominate (i.e. porphyrite group and Haparanda intrusive suites
in Fig. 4). In the west, mildly alkaline (shoshonitic), intermediate to
felsic volcanic–volcaniclastic rocks and co-magmatic monzonitic intrusions
occur (i.e. Kiirunavaara group and Perthite monzonite intrusive suites in
Fig. 4). Late Svecokarelian-cycle magmatism in northern Norrbotten
generated widespread I- to A-type granitic plutonism
(Edefors suite in Fig. 4) and coeval S-type granites (Lina suite in Fig. 4) ca. 1.81–1.78 Ga in response to eastward subduction as part of the Transscandinavian
Igneous Belt (Andersson, 1991; Åhäll and Larsson, 2000; Weihed et
al., 2002; Högdahl et al., 2004; Rutanen and Andersson, 2009).
In general, metamorphic facies and related pressure–temperature (P–T)
estimates are poorly constrained throughout northern Norrbotten (e.g.
Bergman et al., 2001; Skelton et al., 2018), but at least two regional
metamorphic events (Fig. 4) that broadly correspond to the early- and
late-orogenic cycles are reported (Bergman et al., 2001,
2006; Bergman, 2018; Sarlus et al., 2017). Based on metamorphic mineral
associations and microprobe data, Bergman et al. (2001) suggested that the
regional metamorphic grade increases from greenschist to amphibolite facies
conditions from west to east. East of the Western Supracrustal Belt
(Fjällåsen; Figs. 1, 2, 3), syn-orogenic volcanic rocks yielded P–T
values of 4.0–7.5 kbar and 630–805∘ (i.e. amphibolite to granulite facies; Bergman et al., 2001).
The uppermost granulite facies P–T estimate (7.5 kbar, 805 ∘C) was
determined for a sedimentary rock within a high-strain deformation zone and
bounded by ca. 1.8 Ga granites, and may represent contact metamorphic
conditions around the granite plutons and/or the effects of retrograde
reactions (Bergman et al., 2001).
Geological map of the Western Supracrustal Belt showing the key
areas mapped during this study. Lithological contacts simplified and
modified after Offerberg (1967), Witschard (1975), and Bergman et al. (2001). Coordinates: Sweref99.
First vertical derivative, 150 m upward continuation, aeromagnetic
map of the Western Supracrustal Belt overlain by first vertical derivative
ground magnetic map of Ekströmsberg. The aeromagnetic data were collected
using 200 m line spacing and 40 m down-line distance (Bergman et al., 2001).
The ground magnetic data were collected using ≤80 m line spacing and
≤20 m down-line distance (Frietsch et al., 1974). Outline of the key
areas, observation points, and dominant structures are the same as in Fig. 2. Data from Frietsch et al. (1974) and Bergman et al. (2001). Coordinates:
Sweref99.
In the Gällivare area (Fig. 1), shear-zone-hosted schists along the
Nautanen deformation zone (NDZ) have yielded P–T values of 2.5–4.3 kbar and 589–681 ∘C (i.e. amphibolite facies; Tollefsen, 2014). Also in the
Gällivare area, Romer (1996) reported a U–Pb age of 1730±6.4 Ma for fracture-hosted stilbite in volcanic rocks, suggesting this area has
remained below the closure temperature of stilbite (ca. 150 ∘C)
since ca. 1.73 Ga. A possible regional resetting of the Rb–Sr isotopic
systems at ca. 1.6–1.5 Ga (e.g. Welin et al., 1971; Skiöld, 1979) is
recorded by ca. 1.9–1.8 Ga magmatic rocks in northern Norrbotten (Fig. 4).
In central Sweden, ca. 1.6–1.5 Ga rapakivi intrusions occur (Andersson, 1991; Andersson, et al., 2002), but magmatic and/or hydrothermal ages in
Norrbotten are not known during this time span; hence, the geological
significance of the isotope resetting remains unclear.
Summary of the temporal relation between supracrustal/intrusive
rocks, metamorphism, deformation, and mineralization in northern Norrbotten.
Stratigraphic column of the supracrustal sequence relevant for this study is
included.
Palaeoproterozoic rocks in northern Norrbotten record evidence of a complex,
polyphase deformation history that evolved predominantly in response to
Svecokarelian orogenesis (D1 and D2 in Fig. 4) and involves at
least two regional deformation events (e.g. Vollmer et al., 1984; Forsell,
1987; Wright, 1988; Bergman et al., 2001; Bauer et al., 2018; Grigull et
al., 2018; Luth et al., 2018; Andersson, 2019). In the Kiruna area, Wright (1988) argued for an early D1 thrusting event overprinted by gentle
local folding and shear zone development (D1 to D4 in Wright
1988). For the same area, Andersson (2019) ascribed the earliest
Svecokarelian compressional deformation to basin inversion and proposed four
major deformation phases to explain the structural configuration in the
Kiruna area (D1–D4 in Fig. 4). Bergman et al. (2001) argued for
two regional deformation events (D1 and D2) in northern Norrbotten,
and Bauer et al. (2018) described a pronounced gneissic S1 cleavage
affected by F2 folding in the Gällivare area (Fig. 1), implying two
deformation events.
The maximum age of the earliest D1 event in Kiruna is constrained at
1880±3 Ma by Cliff et al. (1990) based on a zircon U–Pb TIMS (thermal ionization mass spectrometry) age for
an undeformed granophyre dyke cutting the Kiirunavaara IOA deposit. A
similar timing for D1 has been inferred based on deformation intensity
recorded by ca. 1.89–1.88 Ga as well as 1.88–1.86 Ga plutonic rocks in
northern Norrbotten (Bergman et al., 2001). The timing of the regional
D2 tectonic event is generally constrained by the emplacement of syn-
to late-orogenic, ca. 1.8 Ga granites and related hydrothermal activity (e.g.
Bergman et al., 2001; Smith et al., 2009; Bauer et al., 2018). The
D4 event in Fig. 4 corresponds to maximum ages of ca. 1740 and 1620 Ma for open fractures in the Gällivare area (Romer, 1996) and is the
last documented Proterozoic deformation in that area (Bauer et al., 2018).
Geology of the Western Supracrustal BeltSetting, extent, and lithotypes
The Western Supracrustal Belt refers to a discontinuous, ca. 6 km wide
by 90 km long, NNW-trending Orosirian (ca. 1.89–1.87 Ga) domain located to
the west of Kiruna in northwestern Norrbotten (Figs. 2, 3). In an earlier
study, Wright (1988) defined the WSB as a north–south-trending supracrustal
inlier zone immediately to the west of Kiruna (i.e. the Eustiljåkk key
area; Figs. 2, 3). However, this area represents the northernmost part of a
larger supracrustal terrain that extends further southward to the west and
southwest of the Kiruna and Gällivare mining areas. In this study, we
retain the original nomenclature of Wright (1988) but expand the term
“Western Supracrustal Belt” to include the areas from
Allavaara-Fjällåsen in the south to Eustiljåkk in the north that
are underlain by Orosirian supracrustal rocks (Figs. 2, 3). Similar
lithostratigraphic domains occur to the west of the WSB as relatively small
inliers surrounded by Palaeoproterozoic plutonic rocks or as tectonic windows
surrounded by Palaeozoic rocks (e.g. Angvik, 2014).
In general, the geology of the WSB is dominated by calc-alkaline to
alkaline, volcanic–volcaniclastic rocks with basaltic to rhyolitic
compositions that were metamorphosed at approximately peak
epidote–amphibolite facies conditions (Ros, 1979; Bergman et al., 2001;
Edfelt et al., 2006). Along its margins, the WSB is bound by subordinate ca.
1.88–1.86 Ga granodioritic to dioritic plutonic rocks and more abundant ca.
1.80 Ga granites (Bergman et al., 2001). The plutons intrude, truncate, and disrupt the supracrustal pile; this aspect, combined with the
polydeformed nature of the sequence, makes lateral stratigraphic
correlations difficult. In the Ekströmsberg area (Figs. 2, 3), Rhyacian
greenstones are found at the margin of the belt, providing a partly
persevered pre- to syn-orogenic stratigraphic record (Offerberg, 1967;
Witschard, 1975). In the Allavaara area (Figs. 2, 3), Witchard (1975)
indicated synclinal folds comprised Rhyacian greenstones on their flanks and
Orosirian felsic to intermediate volcanic rocks in their cores.
Structural geology
Previous studies incorporating parts of the WSB provide a partial and
somewhat contradictory assessment of its structural character and evolution
(cf. Wright, 1988; Bergman et al., 2001; Edfelt et al., 2006). At
Eustiljåkk in the northern WSB (named Ruojtatjåkka South in Wright, 1988), Wright (1988)
identified a steep, NW-trending mylonite zone that mimics the
NNW orientations of high-strain zones at Allavaara to the south (Figs. 2, 3).
The Eustiljåkk mylonite provides kinematic evidence for a
west-side-down, oblique normal sense of shear, based on rotated
porphyroclasts with asymmetric tails (Wright, 1988). In contrast, Bergman et
al. (2001) report overall west-side-up kinematics for the composite shear
zone within the WSB based on outcrop observations west of Kiruna and
Gällivare (Fig. 1). A set of ENE-trending dextral strike-slip shear
zones in the Eustiljåkk area (Ruojtatjåkka South in Wright, 1988) have also been reported
by Wright (1988), who suggested these structures post-date the dominant
NNW–SSE tectonic grain.
Based on airborne (Bergman et al., 2001) and ground magnetic data (Frietsch
et al., 1974), several prominent NNW-trending linear magnetic anomalies
occur along the WSB, or as splay anomalies extending NW to WNW towards the
Tjårrojåkka area (Fig. 3). These lineaments are assigned to an
unnamed, crustal-scale Palaeoproterozoic shear zone, analogous to the major
NE-trending Karesuando–Arjeplog deformation zone to the northeast and the
NNW-trending Nautanen deformation zone to the east (e.g. Bergman et al.,
2001; Sandrin and Elming, 2006). Moreover, the magnetically anomalous
character of the WSB mimics similar “striped” magnetic signatures
associated with intense magnetite alteration and mylonitic deformation
within the better-understood IOCG-mineralized Nautanen deformation zone to
the east (Fig. 1; e.g. Lynch et al., 2015, 2018b), giving
relevance to regional comparisons.
Mineralization and related alteration
Both iron oxide–apatite (IOA) and hydrothermal Cu ± Au mineralization
occurs along the WSB. The best-documented examples are the
Tjårrojåkka Fe–Cu system (e.g. Edfelt et al., 2005, 2006) in the
west and the Ekströmsberg IOA deposit (Frietsch, 1974) in the east (Figs. 2, 3). The Tjårrojåkka system (Edfelt et al., 2005, 2006) comprises
a western IOA deposit and an IOCG-style Cu ± Au orebody in the east.
The IOA deposit is primarily associated with pervasive albite + scapolite + magnetite ± amphibole alteration, while “red rock”-style
potassic–ferroan (K-feldspar + hematite ± albite) alteration is
mainly associated with the Cu deposit (Edfelt et al., 2005). The
Ekströmsberg deposit comprises several parallel NW-trending magnetite
and hematite orebodies. The orebodies are associated with sericite + quartz-altered host volcanic rocks and discordant calcite veining, as well
as muscovite, zircon, epidote, tourmaline, and allanite as probable secondary
accessory matrix minerals (Frietsch, 1974).
In general, Fe and Cu mineralization along the WSB appears to be partly
controlled by superimposed structures formed during polyphase deformation.
In the Tjårrojåkka area, Edfelt et al. (2006) report three main
deformation events; D1 and D2 generated cleavages in NE- and
E-oriented shear zones, respectively, and a later D3 event folded
D1 structures and produced shallow SE-striking cleavages dipping
towards the southwest. Additionally, the Fe and Cu orebodies at
Tjårrojåkka are aligned with D1 NE- to ENE-trending planar
structures (Edfelt et al., 2006). At the Ekströmsberg IOA deposit,
Frietsch (1974) reported several prominent structures, including NW-trending
schistosity parallel to the orientation of the main orebodies. Additional
structural components include locally developed folds, a major NW-trending
and smaller NE-trending fault zones, and brecciated phenocrysts (Frietsch,
1974). Overall, these features imply a polyphase structural evolution for
the Ekströmsberg IOA deposit based on plastic fabrics intersected by
brittle deformation zones. Further detailed descriptions of the structural
characteristics of the Ekströmsberg IOA deposit are presently lacking,
however.
Methods
In this study, five key areas were chosen to study the structural
differences and/or similarities along the WSB: Eustiljåkk,
Ekströmsberg, Tjårrojåkka, Kaitum West, and
Fjällåsen–Allavaara (Figs. 2, 3). Geological mapping with a
structural focus was conducted between 2015 and 2017. A total of 698 outcrop
observations were made, and 1079 structural measurements were collected. The
mapping campaign covered all outcropping areas between Allavaara in the
south and Eustiljåkk in the north (Figs. 1, 2), although it should be
noted that the total outcrop exposure for the WSB is estimated at ca. 1–3 % by surface area. All structural measurements were collected using
Brunton Geo Pocket Transits, and the data were digitized in the field on
ruggedized iPad mini devices using the application Field Move (formerly
Midland Valley Exploration Ltd., currently Petroleum Experts Ltd.).
Lineations were measured as the pitch on planes and recalculated into true
orientation using the software Geo Calculator (Holcombe, Coughlin, Oliver,
Valenta Global). For magnetite-rich rocks, structural measurements were
estimated using known distal points in the terrain. Structural analysis was
performed using the Move 2017 software package (formerly Midland Valley
Exploration Ltd., currently Petroleum Experts Ltd.), whereas maps were
constructed using ArcMap (ESRI). Stereographic plots were produced as
lower-hemisphere equal-area stereographic projections using Dips 7.0
(Rocscience). Forty-one oriented samples were collected throughout the study
area. The samples were cut across foliation and parallel to lineation and
sent to Vancouver Petrographics Ltd. for thin-section preparation, one thin
section per sample. Petrography and microstructural investigations were
performed using a conventional petrographic polarization microscope equipped
with a digital camera (Nikon ECLIPSE E600 POL).
Hydrothermal alteration mapping was conducted via field observations at the
outcrop to hand sample scale using 10× hand lenses. We focused on the
recognition of mappable alteration mineral associations to establish
possible links between certain structures and alteration styles, and
identify specific structural and/or alteration characteristics that may
prove useful as a vectoring tool toward Fe and/or Cu mineralization. The
purpose of this approach was to provide a holistic overview of paired
deformation–hydrothermal processes affecting the WSB, and this scientific
contribution offers a starting point for further studies on the evolution of
hydrothermal mineral alteration in this underexplored area.
Results
In general, the structural mapping results highlight two superimposed
deformation events that generated plastic and brittle-plastic structures
along the WSB that vary in terms of their character and intensity. Likewise,
variably developed hydrothermal alteration displays localized differences in
terms of type, style, and intensity.
Eustiljåkk area
The Eustiljåkk area provides a relatively well-exposed ca. E–W profile
across the northern WSB (Figs. 2, 5). The area predominantly comprises weakly
deformed porphyric volcanic rocks, along with subordinate sedimentary rocks
and mafic dykes. Steep, west-dipping shear zone structures occur in the
NE part of the area and impart a dominant N–S-trending structural grain
(Fig. 5). Beyond this area to the west, other large-scale ca. NW-trending
structures are interpreted from magnetic anomaly data (Bergman et al., 2001,
Fig. 3). Although ground truthing of these western structures was not
possible due to poor exposure, their continuity was verified by structural
measurements and thin-section analysis of similarly deformed rocks along
strike in key areas south of Eustiljåkk.
Geological map of the Eustiljåkk key area. Modified after
Offerberg (1967). Coordinates: Sweref99.
In general, a weakly developed penetrative cleavage is present throughout
the Eustiljåkk area. We designate this cleavage S1 because it is
folded into F2 folds with near-vertically plunging fold axes (Fig. 5).
NNW–SSE- or N–S-trending and west-dipping structural grains that are parallel
with magnetic lineaments appear to control the orientation of S1
cleavages (Figs. 5, 6). The magnetic map indicates that NNW–SSE-trending
planar structures are the more dominant trend compared to N–S-oriented
structures at Eustiljåkk (Figs. 3, 5), whereas stereographic projections
of S1 foliations indicate a N–S structural grain as dominant (Fig. 6).
This contradiction likely reflects a greater surface exposure in outcropping areas containing ca. N–S-oriented planar structures, giving a
greater number of structural measurements with N–S orientations as shown in
the S1 summary stereographic plot of the Eustiljåkk area (Fig. 6a–b).
Stereographic equal-area projections highlighting (a) low-strain
S1, (b) high-strain S fabrics, and (c) stretching and mineral lineations.
S1 foliation is defined by a preferred mineral orientation of feldspar + quartz ± biotite ± actinolite ± hornblende in
intermediate to felsic volcanic rocks (Fig. 7a) and adjacent granitoids. The
cleavage in the granitoids is unevenly distributed and is generally of low
intensity. In the area between Eustiljåkk and Ekströmsberg (Fig. 2),
calcite forms part of the foliated mineral association within porphyric
volcanic rocks. In mafic rocks, S1 is defined by actinolite + plagioclase ± epidote ± hornblende ± calcite that show a
preferred mineral orientation parallel to the ca. N–S-aligned S1 fabric.
Characteristics of the Eustiljåkk key area. (a) Low-intensity
S1 foliation, X688743 Y7542711. (b) Parasitic F1 folding of
sedimentary horizons, X687043 Y7539378. (c) Scapolite-altered mafic dyke
associated with N–S-directed high-strain zones, X687505 Y7543037. Coordinates:
Sweref99.
In general, bedding surfaces (S0) are only rarely preserved in the
Eustiljåkk area. In the southern part (Fig. 5), deformed arkose horizons
are interbedded with more competent volcanic porphyric rocks. The more
western of these horizons is slightly steeper and shows intense NNE-verging
parasitic F1 folds with M and Z geometry (Fig. 7b). Based on
bedding–bedding and bedding–cleavage relationships, combined with parasitic
fold geometries, we interpret the larger-scale structure as a NE-verging,
overturned synform. An axial planar S1 cleavage locally associated with
the F1 folds strikes subparallel to bedding but is slightly steeper
dipping (see inset stereoplots in Fig. 5). The calculated F1β axis
(β: 330/15) plunges gently towards the northwest, which is comparable
to the measured NW-plunging fold axes of parasitic F1Z folds (Fig. 5).
F1-fold limbs are transected by an E–W-trending high-strain zone in the
southern Eustiljåkk area as inferred from the aeromagnetic map and
supported by high-strain cleavage measurements near the magnetic lineament
(Fig. 5). Axial planar S1 appear to be locally transposed into this
high-strain zone (Fig. 5). We interpret this phenomenon as localized
transposition of S1 and S0 in response to D2 shearing. In the
northwestern part of the Eustiljåkk map sheet, a low-intensity S1
cleavage is folded into inferred F2 folds. Stereographic analysis
indicates slightly asymmetrical fold geometries with a calculated β axis
(β: 293/79) plunging steeply towards the northwest (Fig. 5).
Corresponding F2-fold hinges and axial planar cleavages (S2) were
not observed in the field or in thin sections.
The northeastern part of the Eustiljåkk area (Fig. 5) is characterized
by mainly N–S- and subordinate NW–SE- and NE–SW-trending sets of thin,
well-exposed high-strain zones. Stretching lineations show variable
orientations (Fig. 5). In relatively low-strain units, S1 cleavages
plot near the best-fit plane (Fig. 5), suggesting F2 folding around a
calculated β axis plunging steeply to the north (β: 357/79),
whereas high-strain fabrics (designated S2) are oriented consistently
N–S (Fig. 5). Mafic dykes are commonly encountered either within or at the
contacts of the high-strain zones (Fig. 7c) and are interpreted to reflect
magmatism during D2 deformation. Locally, mafic dykes show internal
folding of leucocratic material (possibly albititized dolerite), but
relatively undeformed dykes also occur. The mafic dykes are in most cases
not wider than a couple of metres and occur as swarms. The high-strain zones
and related mafic dykes transect the granitoid pluton bordering the volcanic
rocks in the east, implying granitoid emplacement occurred prior to
D2-related deformation and mafic magmatism.
Oriented thin sections did not yield any high-confidence kinematic
indicators, making the kinematic interpretation of Eustiljåkk uncertain.
Despite this, the east-verging F1 fold in the southern part of the area
(Fig. 5) suggests the Eustiljåkk area was affected by west-side-up
movements during D1.
Ekströmsberg area
The Ekströmsberg key area (Fig. 8) is dominated by felsic to
intermediate volcanic rocks that host the Ekströmsberg IOA deposit
(Frietsch, 1974). Dominant NNW–SSE-trending high-strain deformation zones
west of the Ekströmsberg IOA deposit (Fig. 9) are poorly exposed and are
mainly inferred from aeromagnetic and ground magnetic anomalies (Frietsch et
al., 1974; Bergman et al., 2001). These magnetic anomalies can be linked to
high-strain zones mapped just northwest of the Ekströmsberg IOA deposit
(Fig. 9) and further south in the Kaitum West area (Fig. 12, Sect. 5.4).
Geological map of the Ekströmsberg key area. Modified after
Offerberg (1967). Coordinates: Sweref99.
A penetrative, continuous cleavage is defined by a preferred orientation of
feldspar + quartz ± biotite ± amphibole (mean orientation:
251/50). We designate this fabric S1 because its orientation is partly
controlled by later D2 structures (see below). S1 fabrics are
developed in both supracrustal and plutonic rocks, although ductile fabrics
only sporadically occur in the latter. A mineral stretching lineation is
generally present in felsic–intermediate volcaniclastic rocks and is defined
by stretched feldspar and quartz. Occasionally, a well-developed mineral
lineation (Lm) in mafic rocks is defined by actinolite–tremolite
forming a distinct L tectonite (Fig. 9a). In general, however, LS tectonites
are more commonly developed at Ekströmsberg.
Field images and thin-section photographs of characteristics of
the Ekströmsberg area. (a) Actinolite–tremolite L tectonite overprinted
by calcite alteration, X689886 Y7530423. (b) Semi-conformable scapolite
replacement of magmatic bedding in Rhyacian basalt, X688163 Y7530345. (c) Magmatic bedding in a felsic volcanic rock resembling a weak tectonic
cleavage in outcrop, X691591 Y7527763. (d) Micrograph of the outcrop in (c). (e) Micrograph of crenulation from same locality as (b), X688163
Y7530345. (f) SCC′ fabric along north-plunging stretching
lineation in the NNW-directed grain, X688714 Y7530350. (g) SC fabric along
near-vertical stretching lineation in the NNW-directed grain, X688167
Y7530354. (h) SC fabric along shallow east-plunging stretching lineation
along the E–W-directed grain, X690276 Y7527251. (i) Brittle feldspar along
the E–W-directed grain, same location as (h). Scp: scapolite.
Coordinates: Sweref99.
Subordinate sedimentary rocks occur in the northwestern part of the
Ekströmsberg area and mainly comprise poorly sorted, polymict
conglomerate with poorly developed or indistinct bedding features (Fig. 8),
at a stratigraphic position corresponding to the Kurravaara conglomerate
(Fig. 4). Locally, pillow basalt provides reliable bedding markers,
especially where primary magmatic layering is indicated by stratiform
scapolite replacement (Fig. 9b). Additionally, lithological contacts
(Offerberg, 1967), structural lineaments derived from magnetic maps
(Frietsch et al., 1974; Bergman et al., 2001), and measured fold axes
(designated F1) were used to infer fold geometries at Ekströmsberg
(Fig. 8). In general, S1 foliations are axial planar to F1 folds
with steep, south-plunging fold axes (Fig. 8). A low-intensity foliation of
uncertain origin is commonly observed in felsic porphyric volcanic rocks
(Fig. 9c, d). This planar fabric is defined by a preferred orientation of
feldspar and quartz which locally resembles a tectonic cleavage, although
pressure shadows and/or recrystallization around feldspar phenocrysts were
not observed (Fig. 8d). We interpret this fabric as a primary (S0)
magmatic flow fabric which may correlate to what Frietsch (1974) described
as “fluidal banded” volcanic rocks at Ekströmsberg (see Fig. 9 in
Frietsch, 1974). In the southern Ekströmsberg area, the magmatic fabric
evident in felsic volcanic rocks is folded into a tight, upright, and
near-cylindrical F1 fold with a steeply plunging fold axis (β:
195/75). A NNE-oriented, subvertical foliation is parallel to the fold's
axial plane and is interpreted as an axial planar cleavage designated
S1.
Inferred S1 cleavages in felsic volcanic–volcaniclastic rocks appear to
be transposed into a set of subvertical, E–W-trending strike-slip- and
NNW–SSE-trending reverse dip-slip shear zones near the Ekströmsberg IOA
deposit (Fig. 8). We interpret this effect as the transposition of S1
cleavage into later-formed shear zones (S2), thus marking two
compressional deformation events (i.e. D1 and D2). A mafic
volcanic rock in the northwestern part of the key area shows a crenulation
cleavage within a NNW–SSE-trending reverse dip-slip mylonite zone, which
also suggests two fabric-forming events (Fig. 9e).
Shear sense determined from SCC′ fabrics in thin sections of
foliated volcanic rocks indicates oblique west- to southwest-side-up
kinematics for the main NNW–SSE-trending high-strain zones at
Ekströmsberg. Oblique sinistral (Fig. 9f) and reverse dip-slip (Fig. 9g)
movements are recorded along moderately N- to NNW-plunging and subvertical
stretching lineations respectively, with both suggesting overall
west-block-up kinematics. Approximately E–W-trending mylonite zones in
felsic volcanic rocks close to the Ekströmsberg IOA deposit (Fig. 8)
show strike-slip movement with a sinistral top-to-the-west sense of shear as
indicated by SC fabrics (Fig. 9h). No direct crosscutting relationships
between the E–W- and NNW–SSE-trending high-strain zones were observed at the
outcrop scale. However, E–W tectonic fabrics tend to offset
NNW–SSE-oriented lineaments on magnetic maps along the WSB (Fig. 3),
suggesting a later timing for E–W high-strain zones on a regional to
belt scale.
Close to the Ekströmsberg IOA deposit, feldspar porphyroclasts found in felsic
volcaniclastic rocks and transected by E–W high-strain deformation zones
record the effects of brittle deformation (Fig. 9i). In contrast, this type
of grain-scale brittle deformation was not observed in similar
feldspar-phyric lithologies deformed by NNW–SSE-trending mylonite zones
along the WSB. Rocks affected by both structural trends also show sub-grain
rotation (SGR) quartz recrystallization textures with local bulging (BLG)
recrystallization (cf. Fig. 9f, h, i). Overall, this textural evidence
suggests recrystallization proceeded at approximately 400 ∘C
(Passchier and Trouw, 2005), although a slightly lower temperature
(<400∘C) is indicated by the brittle character of
feldspar within the E–W-trending shear zones (Fig. 9i; cf. Passchier and
Trouw, 2005).
Tjårrojåkka area
The Tjårrojåkka area (Fig. 10) mainly comprises felsic and
intermediate volcaniclastic and volcanic rocks that host the
Tjårrojåkka Fe–Cu system (cf. Edfelt et al., 2005). Primary bedding
(S0) is locally visible in laminated volcanosedimentary rocks and
greywacke. Throughout the area, a visibly dominant penetrative planar
foliation occurs and is generally oriented subparallel to S0
bedding/laminae. This planar structure, here designated S1, is defined
by the alignment of amphibole, quartz, feldspar, and locally magnetite in the
volcanic–volcaniclastic rocks and is axial planar to meso-scale intrafolial
isoclinal folds (Fig. 11a). In the central part of the key area, S0
bedforms are folded by a major ENE–WSW-aligned, isoclinal F1-fold
sequence which, based on its surface trace, shows apparent re-folding (Fig. 10). Mineral lineations are generally observable as an alignment of
amphibole and quartz on S1-foliation planes. Both bedding and S1
foliations are overprinted by F2 folds with axial traces trending
approximately N–S to NE–SW (Fig. 10). F2-fold geometries are upright, open to
closed, and show a distinctive spaced cleavage that is parallel to their
axial surface traces and is here designated S2. The N- to NE-aligned
S2 cleavage is commonly defined by biotite alignment in felsic volcanic
rocks and also by brittle fractures. Spacing of the S2 cleavage ranges
from a few centimetres up to several tens of centimetres (Fig. 11b, c). Locally, S2 kink
bands are common features in volcaniclastic rocks (Fig. 11b). In general,
the dominate NE trend of S1 mimics the orientation of a NE–SW-striking
high-strain zone hosting the Tjårrojåkka Cu–Au deposit (cf. Edfelt
et al., 2005, 2006).
Geological map of the Tjårrojåkka key area. Modified
after Offerberg (1967). Coordinates: Sweref99.
Field images of key localities in the Tjårrojåkka key
area. (a) Isoclinal F1 folding, X675916 Y7515915. (b) Open chevron-style
F2 folding with spaced S2, X676285 Y7516107. (c) Open concentric
F2 folding with spaced S2, X675985 Y7516176. Coordinates:
Sweref99.
Kaitum West area
The Western Supracrustal Belt progressively widens southward toward the
Kaitum West area (Figs. 2, 12). Overall, the bedrock is dominated by felsic
to intermediate volcanic to volcaniclastic rocks with some additional basalt
sequences. Structural observations indicate the area constitutes several
low-strain zones that border a central high-strain block (Fig. 12). Most of
the strain is apparently accommodated by relatively narrow shear zones
transecting felsic volcaniclastic rocks in the central part of the area
(Fig. 12). Locally to the west, highly strained basaltic and andesitic rocks
do occur; however, these sequences generally show low strain intensity.
Geological map of the Kaitum West key area. Modified after
Offerberg (1967). Coordinates: Sweref99.
Bedding markers are rare in the Kaitum West area, and F1 folds were not
recognized. Within the high-strain central block (Fig. 12), a polymict and
poorly sorted clastic horizon occurs (Fig. 13a, b). This horizon contains a
penetrative S1 cleavage (266/79) defined by sericite + biotite + chlorite-dominated shear bands that trend subparallel to bedding and is
particularly well developed at the margins of compositional layers (Fig. 13a). At the outcrop scale, the S0/S1 composite fabric is
isoclinally folded (Fig. 13b) around a measured fold axis (335/60) plunging
moderately to the NW. Locally, a S1 high-strain cleavage is transposed
into later shear bands (possibly S2) that parallel the main shear zone
system (Fig. 13c, d). This indicates relatively high ductile strain was
localized both during D1 and D2 in the Kaitum West area.
Thin-section photographs and field images/sketches of key localities in the
Kaitum West key area. (a) Micrograph of S0/S1 composite fabric in
volcanosedimentary rock, X698952 Y7505460. (b) Isoclinal mesoscale folding of
the S0/S1 fabric (a). (c) Volcanosedimentary unit showing a
high-strain cleavage subparallel to S0. The S0/S1 fabric is
transposed into the direction of D2 shear bands, X700402 Y7504830. (d) Simplified sketch of (c). (e) SC fabric and asymmetric dextral
sigma-sigmoid viewed along shallow south-plunging stretching lineation,
X702488 Y7504983. (f) SC fabric indicating sinistral kinematics viewed along
steep, north-northwest-plunging stretching lineation, X693703 Y7519820.
Coordinates: Sweref99.
The easternmost low-strain block is affected by F2 folding around a
calculated β axis plunging steeply to moderately towards the south (β: 182/67; Fig. 12). Within the central high-strain block, several NW–SE-,
N–S-, and locally E–W-trending high-strain zones and shear zones are present
(Fig. 12). We interpret the central high-strain block as the northern
continuation of the shear zone system mapped in the
Fjällåsen–Allavaara area (see Sect. 5.5).
Kinematic indicators in the Kaitum West area suggest southwest-side-up and
shallow to steep oblique dextral displacement that is associated with a
south-plunging stretching lineation. This interpretation is based on
SCC′ fabrics and a asymmetric quartz sigmoid with stair-stepped pressure shadows (Fig. 13e). The same sense of shear is indicated by
a sinistral SC fabric (Fig. 13f) observed along a north-plunging stretching
lineation north of the Kaitum West area (south of Ekströmsberg; Figs. 2,
3) where the same shear zone system is related to a major fold structure
(Fig. 12). Locally, a contradicting sense of shear (east block up) is
indicated by asymmetric sigma clasts with poorly developed pressure shadows.
Fjällåsen–Allavaara area
The area between Fjällåsen and Allavaara is dominated by felsic,
intermediate, and mafic volcanic and volcaniclastic rocks. The dominant
structural grain is an approximately N–S-trending set of high-strain zones (Fig. 14) associated with a well-developed N–S-trending penetrative foliation. The
foliation is defined by the alignment of strained amphibole, biotite,
feldspar, quartz, and locally magnetite (Fig. 15a) and is here designated S1. Bedding is locally observable as compositional layering in
volcanosedimentary rocks and is typically subparallel to the
S1 foliation. Locally, isoclinally F1-folded quartz and amphibole
veins and bedding can be observed (Fig. 15b, c). Shearing is common and
localized in prominent high-strain zones that transpose S0 and S1
and form distinct mylonites (Fig. 15d). S0 and S1 are folded
openly to tightly into meso- and macro-scale F2 folds (Fig. 15e, f). An
axial surface parallel cleavage (S2) is locally observable as a weakly
developed, brittle, spaced cleavage (Fig. 15e, f). Locally, this S2
cleavage shows en echelon fracturing (Fig. 15b). Mineral lineations variably
plunge steeply to moderately towards the north and south. Based on
asymmetric sigma clasts and SC fabrics, a reverse west-block-up
sense of shear is interpreted for the majority of deformation zones in this
area. The central shear zone in Fjällåsen hosts the
Fjällåsen Cu prospect and shows oblique kinematics with a reverse
west-block-up and sinistral sense of shear along a mineral lineation
plunging 60∘ N (Fig. 14).
Geological map of the Fjällåsen–Allavaara key area.
Modified after Witschard (1975). Coordinates: Sweref99.
Field images of characteristics of the
Fjällåsen–Allavaara key area. (a) High-intensity foliation, X721253
Y7473543. (b) Isoclinally folded quartz and amphibole veins with related
axial planar S1 cleavage. Brittle-plastic S2 with dextral
sense of shear, X721192 Y7473733. (c) Tight F1 folding of S0,
X714642 Y7479559. (d) Asymmetric lithic sigma clast viewed along steep, north-plunging stretching lineation, X715483 Y7478182. (e) Isoclinal F1 gently
refolded by F2, X713553 Y7480026. (f) Gentle F2 folding of
S0/S1 with associated brittle-plastic S2, X721194 Y7473733.
Coordinates: Sweref99.
Metamorphism and hydrothermal alteration along the WSB
Mineral alteration associations identified along the WSB based on
mineralogical, textural, crosscutting, and/or overprinting relationships are
presented in this section. Although several associations likely formed in
response to a progressively evolving and likely protracted hydrothermal
system(s), the new alteration mapping results provide a field-based
classification/paragenetic scheme for the various alteration occurrences.
An epidote + plagioclase + hornblende association occurs parallel to
S1 fabrics in mafic volcanic rocks in the Ekströmsberg area and
forms a moderately intense and pervasive metamorphic mineral association
(Fig. 16a). Additionally, veins hosting hornblende + epidote locally occur
and preserve internal foliations that are laterally consistent with S1
fabrics in adjacent wall rocks (Fig. 16a), suggesting a relatively early
timing (i.e. pre- to syn-D1). However, hornblende shows syn-tectonic
growth (Fig. 16b, c), and the S1 foliation can be continuously traced
within the matrix and as a preserved S1 in the hornblende, indicating
that the vein mineralogy was altered during prograde metamorphism
approximately broadly coincident with D1. In the Eustiljåkk area
(Figs. 2, 5), epidote commonly developed as a retrograde product replacing
disseminated hornblende porphyroblasts, indicating retrograde metamorphic
processes affected the area.
(a) Albite + hornblende + epidote metamorphic fabric aligned with axial planar S1. Folded vein hosting hornblende + epidote. (b) Syn-tectonic growth of hornblende. (c) Needle-shaped mineral forming traces of
S1 in hornblende. All images from X690267 Y7529944, Ekströmsberg.
Coordinates in Sweref99. Hbl: hornblende; Ab: albite; Epi: epidote.
Scapolite ± albite hydrothermal alteration overprints pervasive
magnetite + amphibole alteration of basaltic rocks in the Kaitum West
area. However, scapolite ± albite alteration is also locally
overprinted by irregular and patchy amphibole + magnetite zones that tend
to be developed along S1 foliations (Fig. 17a). Thus, both associations
are interpreted to be broadly coeval and formed during D1. Discordant
magnetite + amphibole veins with white to buff albite haloes occur
locally in basaltic rocks with low strain intensities (Fig. 17b) and
are affected by more pervasive (disseminated) magnetite + amphibole
alteration.
Thin-section and field images of alteration styles throughout
the WSB. (a) Scapolite + albite overprinting magnetite + amphibole,
X695488 Y7507194, Kaitum West. (b) Vein-hosted magnetite + amphibole with
reddish albite haloes, X695459 Y7507412, Kaitum West. (c) Scapolite
porphyroblasts, X696049 Y7507684, Kaitum West. (d) Scapolite in veins and
patches transposed by later shear bands, X697177 Y7532766, Vieto (see Fig. 2). (e) Calcite overprinting actinolite–tremolite in L tectonite in Fig. 9a.
Calcite aligned with S1/S2 with undeformed granular epidote at
grain boundaries, X689886 Y7530423, Ekströmsberg. (f) Reddish calcite
overprinting ductile shear zone fabrics, X698369 Y7534383, Vieto (see Fig. 2). (g) K-feldspar + epidote concentrated along volcanosedimentary bedding
X721289 Y7471362, Fjällåsen–Allavaara. (h) K-feldspar + epidote + Fe-oxide + sulfide + malachite overprinting pervasive magnetite + amphibole, X694578 Y7506821, Kaitum West. (i) E–W-directed D2 shear zone
with magnetite bands, X690231 Y7527374, Ekströmsberg. (j) Shear-band-hosted biotite + magnetite + quartz + K-feldspar + muscovite from
the locality in (l), Ekströmsberg. (k) Selectively pervasive
K-feldspar replacing albite in same outcrop as Fig. 9c, X691491 Y7527635,
Ekströmsberg. (l) Selectively pervasive K-feldspar alteration overprinted
by epidote on a fracture plane, X696619 Y7508353, Kaitum West. Coordinates
in Sweref99. Scp: scapolite; Ab: albite; Mag: magnetite; Amp: amphibole;
Epi: epidote; Cal: calcite; Kfs: K-feldspar; Sul: sulfide; Mus: muscovite;
Bt: biotite.
In general, scapolite ± albite alteration is common throughout the WSB
and is mainly observed in compositionally mafic rocks (i.e. basalt and
dolerite) as a distinctive speckled (porphyroblastic) pale-grey to creamy-white granular discolouration on exposed surfaces (Fig. 17c). Disseminated
scapolite porphyroblasts are typically medium- to coarse-grained (1–8 mm) and irregular tabular to elongated prismatic, and represent ca. 10–35 vol %
of altered rock units (Fig. 17c). Weakly to strongly developed
porphyroblastic scapolite ± albite alteration is best preserved in
relatively low-strain areas adjacent to or within NNW- to N-trending shear
zones. In the Eustiljåkk area (Figs. 2, 3), porphyroblastic scapolite
alteration affects mafic dykes in zones with relatively high strain, while
it locally overprints inferred S0 bedding planes in basalt units in the
Ekströmsberg area (Figs. 2, 3, 8b). Discordant, vein-hosted scapolite ± albite alteration is widespread in the Vieto area (Figs. 2, 3). These
veins are affected by shearing with a probable D2 timing (Fig. 17d);
hence, the veins are interpreted as being formed pre-D2.
Deformed and discordant calcite veins are common throughout the WSB and are
mainly present in low-strain blocks dominated by mafic rocks. However,
pronounced calcite alteration also shows syn- to post-D2 timing since
it overprints clear S2 shear zone fabrics in high-strain zones.
Syn-tectonic (D2) calcite associated with sparse occurrences of sulfide
(pyrite + chalcopyrite) overprints relatively intense and pervasive
tremolite–actinolite alteration (Fig. 17e) at one locality in the northern
part of the Ekströmsberg area where it forms part of an amphibolitic
L tectonite within a steep, reverse dip-slip shear zone that transects mafic
rocks (Fig. 9a). Intense and pervasive calcite alteration shows a
post-D2 timing in the nearby Vieto area (Figs. 2, 3), where relatively
undeformed calcite overprints an S2 fabric in a moderately (ca. 285/75)
west-dipping shear zone intersecting Rhyacian basalt (Fig. 17f).
Hydrothermal mineral associations restricted to D2 structures are in
general potassic–ferroan in character comprising K-feldspar, quartz, and
epidote associated with Fe oxide and sulfide. The most prominent alteration
association in the Tjårrojåkka area is a relatively intense and
pervasive K-feldspar + epidote + quartz hydrothermal alteration
association that overprints S1 foliation. Locally, both K-feldspar and
epidote appear to be remobilized into S2 spaced cleavage domains as
well as volcanosedimentary bedding (Fig. 17g). In the Kaitum West area, a
relatively intense, selectively pervasive epidote + K-feldspar association
is spatially related to localized Cu-sulfide weathering and overprints a
weak, pervasive amphibole + magnetite alteration in a mafic volcanic rock
(Fig. 17h). In the Ekströmsberg area, a relatively intense, shear-band-hosted biotite + magnetite + K-feldspar + muscovite association
affects a rhyodacitic volcanosedimentary rock intersected by steep,
approximately E–W-trending sinistral strike-slip shear zones with a D2
timing (Fig. 17k, l). The biotite-bearing shear bands are oriented
subparallel with the volcanosedimentary bedding (S0).
Selectively pervasive K-feldspar alteration (replacing albite) is common in
intermediate to felsic volcanic rocks throughout the WSB (Fig. 17k) and is
typically accompanied by weak retrograde sericite and hematite staining of
secondary K-feldspar as well as epidote. K-feldspar alteration of albite of
this type is the only hydrothermal alteration with a syn- to post-D2 timing that is pervasive over large distances and not directly
associated with tectonic structures.
The paragenetically latest hydrothermal mineral alteration identified in
this study constitutes epidote forming patches on fracture planes
intersecting selectively pervasive K-feldspar alteration at
Tjårrojåkka. This association produces a distinctive reddish-green
rock (Fig. 17l) with a late timing relative to brittle deformation (veins
and fractures) of uncertain timing.
The hydrothermal mineral alteration associations identified in this study
are summarized as a simplified alteration map in Fig. 18. The map shows
that magnetite + amphibole alteration is spatially correlated to scapolite ± albite alteration, with the latter affecting large areas at some
distance from dominant structures. The intensity of the potassic alteration
seems to decrease towards the north and increases towards the south, where ca. 1.8 Ga granites are found. The uncertainty of the map is relatively high due to
the generally low rock exposure of the WSB.
Geological map showing the spatial distribution of identified
hydrothermal mineral associations and their relation to dominant structures
and aeromagnetic anomalies. The map is based on the observations performed in
this study together with a compiled database of recorded alteration minerals
at the Geological Survey of Sweden. Coordinates in Sweref99.
Overall, the identified metamorphic and hydrothermal mineral associations
are summarized as follows:
D1-related fluid flow (or marginally later):
regional epidote–amphibolite facies metamorphism, early to syn-D1;
regionally pervasive amphibole + magnetite alteration, early to
syn-D1;
discordant, vein-related amphibole + magnetite + albite alteration,
early to syn-D1;
selectively pervasive scapolite ± albite alteration or growth of
albite–scapolite porphyroblasts along S0 and S1 structures, syn-
to late D1;
shear-zone-hosted (selectively pervasive) actinolite + tremolite
alteration, late D1;
calcite in veins with an uncertain temporal relationship to D1, but
probably pre-D2;
scapolite in veins with an uncertain temporal relationship to D1, but
probably pre-D2.
local vein-related K-feldspar + epidote + iron oxide ± sulfide
alteration, syn-D2;
regionally distributed, patchy (selectively pervasive) K-feldspar alteration
of plagioclase, syn- to post-D2;
shear-zone-hosted calcite alteration, syn- and late/post-D2;
local fracture-filling and patchy epidote alteration, post-D2;
retrograde sericite alteration, post-D2.
DiscussionStructural evolution of the WSB
In general, the structural elements preserved within the WSB are consistent
with a complex, polyphase deformation history. Locally, S1 cleavages
are axial planar to F1 folds affecting S0 bedding planes and
magmatic flow structures associated with sedimentary, volcanosedimentary, or
volcanic rocks. Overall, F1 folds are generally poorly exposed and are
thus difficult to constrain due to the lack of clear bedding and/or
stratigraphic way-up indicators. Where observed, however, F1 folds are
tight to isoclinal and are either upright or overturned, with the latter
verging eastward (Figs. 5, 7). F1 folds in the southern Ekströmsberg
area (Fig. 8) have a relatively steep calculated β axis that plunges
towards the SSW (β: 195/75), which is a typical feature for F2
folds throughout the WSB and suggests some F1-fold axes were rotated
into a steep southward plunge during D2 transposition.
S1 cleavages throughout the WSB are best preserved in relatively
low-strain domains as a penetrative planar fabric in supracrustal rocks,
while they are only weakly developed in adjacent shoshonitic plutonic rocks
(i.e. Perthite monzonite suite intrusions; see Fig. 4). We interpret the
relative timing of the shoshonitic (PMS, perthite monzonite suite) plutonic rocks as syn- to
late D1. Reported igneous ages for shoshonitic (PMS) plutonic rocks
across northern Norrbotten range from ca. 1.88 to 1.86 Ga (Bergman et al., 2001;
Sarlus et al., 2017; Kathol and Hellström, 2018). Thus, we interpret the
absolute timing of syn- to late-D1 deformation that affected the WSB to
the later ages of the same time span, i.e. 1.88–1.86 Ga.
Mylonitization of volcanic-sedimentary rocks along the WSB frequently shows
subparallel orientations to the regionally extensive and laterally
continuous S1 cleavage. Although the level of exposure does not allow
for accurate estimations of the width of these zones, our mapping experience
combined with ground magnetic signatures in the Ekströmsberg area
(Frietsch et al., 1974) indicates that these zones are relatively thin
(metres to tens of metres), controlled by lithological contrasts (or primary
depositional features), and display sharp contacts to adjacent lower-strain
rocks. Overall, the mylonitic strain appears to be favourably partitioned by
volcaniclastic and sedimentary horizons that are sandwiched between more
competent volcanic rocks throughout the WSB.
Penetrative fabrics formed during D1 temporally coincide with the
metamorphic peak indicated by syn-tectonic growth of hornblende in the
Ekströmsberg area (Fig. 16b), implying temperatures >450∘C (e.g. Blatt et al., 2006). In comparison, similar
temperature conditions are suggested by GBM (grain boundary migration)-SGR dynamic quartz
recrystallization textures observed in the Kaitum West area indicating
temperatures of ca. 500 ∘C (Passchier and Trouw, 2005). However,
dynamic quartz recrystallization textures (SGR with minor BLG) observed in
D1 shear zones suggest lower syn-deformation temperatures of ca.
400 ∘C (Fig. 9f). This may indicate that shear zone activity
during D1 post-dates the metamorphic peak and that non-coaxial strain
dominated the deformation during the late stages of D1.
In general, S2 cleavages developed predominantly in NNW–SSE- and
approximately E–W-trending high-strain zones. Where S1 is not completely
transposed into S2 parallelism or overprinted by high-strain S2
fabrics, the S1 cleavage is locally overprinted by a S2
crenulation cleavage (cf. Fig. 9e). Dynamic quartz recrystallization
textures formed during D2 in the mylonite zones (SGR texture with minor
BLG) suggest ambient temperatures of ca. 400 ∘C; however, brittle
feldspar observed in association with SGR quartz textures (Fig. 9i) in an
E–W-trending mylonite indicates the temperature as slightly lower (<400∘C) in the E–W-trending structures during D2 (Passchier
and Trouw, 2005).
In low-strain blocks throughout the WSB, S1 planar structures are
folded into meso- to macro-scale F2 folds with fold axes generally
plunging moderately to steeply (60–80∘) northward or southward
(Figs. 5, 10, 11, 12, 13). Axial planar S2 cleavage related to the
F2 folds in the low-strain blocks are rare. Only a few examples in the
Tjårrojåkka and Fjällåsen areas as well as north of the Ekströmsberg
area have been observed as axial planar, brittle, spaced cleavages (Figs. 11b, c, 15e). This characteristic of D2 deformation is typical for
this part of Norrbotten and is also observed east of the WSB. For example,
in the Gällivare area (Fig. 1), Bauer et al. (2018) report folding of an
S1 gneissic fabric into F2 synformal structures without axial
planar S2 cleavage. In the Aitik Cu–Au–Ag deposit also near
Gällivare (Fig. 1), Wanhainen et al. (2005, 2012)
report lower amphibolite facies metamorphism and deformation at
500–600∘ and 4–5 kbar between ca. 1.89 and 1.87 Ga. This medium-grade
tectonothermal event was later overprinted by a hydrothermal event estimated
at 200–500 ∘C and 1–2 kbar at ca. 1.78 Ga based on fluid inclusion
data and geochronology (Wanhainen et al., 2012). The findings in the Aitik
Cu–Au–Ag and Malmberget IOA deposits may not be directly applicable to the
WSB in terms of metamorphic-hydrothermal PT conditions but are compatible
with a more intense earlier deformation event (regional D1) overprinted
by a weaker deformational event (regional D2), which we suggest
represents the overall regional deformation history.
While the identification of two generations of planar fabrics is relatively
straightforward in the WSB based on their orientation and
interrelationships, linear structures are more difficult to interpret due to
the lack of crosscutting relationships. Crenulation of mylonitic cleavage
has only been observed along near-vertical stretching lineation, which leads
us to interpret the well-clustered near-vertical stretching lineation in
Fig. 20 as L2. The sense of shear associated with subvertical
L2 lineation is reverse dip-slip (Fig. 9g) and is best explained by an
E–W compressional stress field. Sinistral strike-slip movement along steep,
approximately E–W-trending shear planes (Fig. 20b) offsetting the NNW–SSE-trending
structural grain in the Ekströmsberg area are D2 structures, and the
associated shallowly east-plunging stretching lineation is designated as
L2. Stretching lineations measured on S1 planes in low-strain
blocks are interpreted to be L1 structures. The orientation of L1
lineation varies considerably more than that of stretching lineation measured in
relatively high-strain zones (cf. Fig. 20a–b). We suggest that the
non-clustered shallowly to moderately north- and south-plunging stretching
lineation (Fig. 20b) of the NNW–SSE-trending mylonites might represent
traces of L1 lineation. The sense of shear associated with the inferred
L1 lineation is reverse oblique slip with the SW side up and best explained as
resulting from NE–SW-directed crustal shortening. This implies that the
kinematics of D1 and D2 are best explained by two compressional
events that deviate approximately 45∘ from each other. Based on the
assumption that traces of L1 can be identified, we argue that the steep
to near-vertical cluster in the low-strain L1 plot (Fig. 20a) represents
L1 lineation that was subsequently transposed during D2 in a
similar manner to how F1-fold axes were transposed in the Ekströmsberg
area.
Summary of mineral alteration associations in the WSB and their
inferred timings relative deformation. For comparison, the timing of
supracrustal/intrusive rocks as well as mineralization in northern
Norrbotten is included. Question mark indicates probable distribution.
Lower-hemisphere equal-area stereographic projections of
lineation throughout the WSB. Cones represent 30∘ circles. (a)L1 stretching and mineral lineation measured on S1 foliation
planes in low-strain blocks. (b) Stretching and mineral lineation measured in
high-strain zones.
The NNW–SSE-trending mylonites with moderately plunging stretching lineation
(L1) suggest oblique-slip SW-side-up kinematics based on
SCC′ fabrics and rotated porphyroclasts in oriented samples
from the Ekströmsberg, Kaitum West, and Fjällåsen–Allavaara key
areas. These kinematic indicators suggest that both sinistral and dextral
movements occurred, with dextral sense of shear observed along moderately
S-plunging lineation (Fig. 13e) and sinistral sense of shear observed along
moderately N-plunging lineation (Figs. 9f, 15d). Similar kinematics are
indicated during D1 by the east-verging F1 fold in the
Eustiljåkk area, implying consistent reverse oblique-slip
southwest-side-up movement during D1 throughout the WSB.
The kinematics derived from SC fabrics along the near-vertical lineations
(L2 generation) within the NNW–SSE-oriented mylonites in the
Ekströmsberg and Fjällåsen areas indicate reverse dip-slip, W-
to WSW-side-up sense of shear (Fig. 9g). This implies a reactivation of the
NNW–SSE-trending structures during an approximately E–W-directed D2
shortening. Additionally, sinistral strike-slip movement along E–W-trending
shear zones in the Ekströmsberg area (Fig. 9h) indicate they were active
during a ca. E–W compression coincident with reverse dip-slip movements along
the NNW–SSE-trending mylonites. A late timing for the E–W-trending
structures is supported by the consistent offset of NNW–SSE-trending grain
by E–W-trending structures throughout the WSB (Fig. 3).
Summary of major deformation events affecting the WSB
Based on the new structural data presented in this paper and with reference
to tectonic models proposed for surrounding areas (Wright, 1988; Talbot and
Koyi, 1995; Bergman et al., 2001; Angvik, 2014; Skyttä et al., 2012;
Andersson et al., 2017; Sarlus et al., 2017; Grigull et al., 2018; Luth et
al., 2018; Lynch et al., 2018b), we propose the following tectonic model that
utilizes two major deformation events for the WSB (Fig. 21).
Conceptual model of the structural development of the WSB.
Pre-D1 event
Syn-orogenic crustal thinning and mafic to felsic magmatism associated with
back-arc basin development has been inferred for the wider WSB–Kiruna area
based on regional petrological/geochemical studies (Perdahl and Frietsch,
1993; Martinsson, 2004; Sarlus et al., 2017, 2018). The Orosirian
stratigraphic record in the central Kiruna area (Figs. 1, 4) represents one
of the best-preserved conformable Svecofennian supracrustal sequences in
northern Norrbotten and comprises rock types that are correlative with those
within the WSB. This suggests basin development at Kiruna was coeval with
the deposition of the supracrustal rocks dominating the WSB (Andersson,
2019). Based on this lithostratigraphic correlation, our deformation model
for the WSB is predicated on the notion that the area initially developed within
an extensional, basin-type setting that underwent subsequent compression and
inversion.
D1 event
NE–SW- to ESE–WNW-directed crustal shortening (this study; Wright, 1988;
Talbot and Koyi, 1995; Lahtinen et al., 2005; Angvik, 2014) that was coeval
with syn-tectonic plutonism at 1.88–1.86 Ga (Bergman et al., 2001; Sarlus et
al., 2017; Kathol and Hellström, 2018) generated ENE-verging, tight to
isoclinal F1 folds with shallowly NNW-plunging fold axes. A WSW-dipping
axial planar S1 cleavage/foliation is locally associated with
these F1 folds in the WSB.
Oblique reverse dextral and sinistral shear zones with west-side-up
sense of shear developed during D1 and produced the dominant
NNW–SSE-trending, undulating, magnetic lineaments that characterize the WSB
(Fig. 3; Bergman et al., 2001). Deformation along these zones during D1
is indicated by the presence of crenulated S1 (Fig. 9e) and high-strain
S1 fabrics that are tightly folded by later D2 structures (Fig. 13c–d). Strain partitioning appears to have been controlled by lithological
contacts (or perhaps pre-existing discontinuities) as the highest strain is
recorded by favourable, less competent lithologies such as
volcanosedimentary rocks.
Based on the relatively steep dip of the NNW–SSE-aligned structural grain
and the broadly alkaline character of the affected volcanic rocks (Perdahl
and Frietsch, 1993; Bergman et al., 2001; Martinsson, 2004; Martinsson et
al., 2016; Sarlus et al., 2017, 2019), we favour a model involving inversion
of an evolving back-arc basin to account for D1-related structures in
the WSB. Back-arc basin inversion took place under epidote–amphibolite
metamorphic facies conditions (this study; Ros, 1979; Edfelt et al., 2005)
as recorded by syn-tectonic growth of hornblende in albite + hornblende + epidote metamorphic textures in volumetrically minor Rhyacian pillow
lavas in the Ekströmsberg area (Fig. 16a–c). This contrasts with models
that envisage the development of a classic fold-and-thrust belt in the
broader WSB–Kiruna area during D1 (Wright, 1988; Talbot and Koyi,
1995). Consistent evidence for the rotation of originally shallow-dipping
thrust-type structures into subvertical orientations was not found along
the WSB. Furthermore, no classic fold–thrust belt features involving shallow
thrusts and/or nappe stacks have been identified in nearby areas (Vollmer et
al., 1984; Wright, 1988; Talbot and Koyi, 1995; Grigull et al., 2018; Luth
et al., 2018). However, Angvik (2014) identified a series of fold–thrust
belts in the Rombak Tectonic Window, west of the WSB in Norway. It is
possible that the Rombak Tectonic Window represents a fundamentally
different setting and that the change from a classic fold–thrust belt to an
extensional back-arc setting is to be found between the WSB and the Rombak
Tectonic Window.
D2 event
A phase of ca. E–W compression caused meso- to macro-scale folding of S1
foliation and produced near-cylindrical, upright F2 folds with steep, N-
and S-plunging fold axes. Broadly similar F2-fold characteristics are
developed in all key areas throughout the WSB. Strong strain partitioning
focused D2 deformation into pre-existing NNW–SSE-trending oblique-slip
D1 shear zones, causing their reactivation with a reverse dip-slip,
west-side-up sense of shear. Synchronously, near-vertical E–W-trending
sinistral (and possibly dextral; Wright, 1988) brittle-plastic strike-slip
shear zones were active and locally off set the NNW–SSE structural grain.
Applying a basin inversion model to the WSB implies that the E–W-directed
structures might have originated as transfer faults between NNW–SSE-trending
normal faults and that the combined structural configuration was reactivated
together, first during D1 and later during D2.
D2-related kinematics are based on SC fabrics observed along steep to
near-vertically plunging L2 stretching lineations within NNW–SSE-trending
high-strain zones at Ekströmsberg, north of Kaitum West and the
Fjällåsen–Allavaara areas. Correlative microstructures are also
observed within shallowly E-plunging stretching lineations in E–W-trending
high-strain zones at the Ekströmsberg area. The E–W-directed offset of
NNW–SSE-trending high-strain zones is interpreted from magnetic maps
(Frietsch et al., 1974; Bergman et al., 2001). Joints and fracture planes
pre-date the latest epidote alteration in the area and are interpreted as
developed either during D2 or slightly thereafter.
Timing of D1–D2 deformation within the WSB, and comparative links with adjacent areas
Tectonic models for northern Norrbotten and the Skellefte district generally
include an early phase of deformation at approximately 1.88–1.86 Ga (e.g. Wright,
1988; Talbot and Koyi, 1995; Lahtinen et al., 2005; Skyttä et al.,
2012; Angvik, 2014). In the Skellefte district, the minimum timing of
crustal shortening with related folding and shearing was constrained at 1874±4 Ma (Skyttä et al., 2012), which is comparable to the maximum
ages at 1888±7 and 1865±8 Ma for NNW–SSE-trending shear
zones in the Kautokeino greenstone belt north of the WSB in Norway (Bingen
et al., 2015). Similarly to this study's interpretation, Allen et al. (1996), Bauer et al. (2011), and Skyttä et al. (2012) proposed that
shear zones in the Skellefte district formed during a phase of continental
arc extension and volcanic activity prior to 1.88 Ga, and were subsequently
reactivated ca. 1.87 Ga during accretion of the arc onto the Archean
continent and subsequent crustal shortening.
In terms of later deformation events, Bauer et al. (2017) and Lynch et al. (2018b) report west-side-up movements during a D2 phase of deformation
in the Cu–Au-mineralized NDZ near Gällivare,
which generated a duplex Riedel shear system within that composite zone.
Additionally, Bauer et al. (2018) argue for E–W compression during a D2
phase of deformation and link this event to the intrusion of
syn-tectonic granites 1.8 Ga (e.g. Öhlander et al., 1987; Bergman et al., 2001;
Sarlus et al., 2017). A similar timing of deformation is interpreted for the
Rombak Tectonic Window ca. 100 km west of the WSB in Norway, where U–Pb zircon
ages for syn-tectonic granites (Angvik, 2014) bracket the timing of a
comparable deformation event between 1778 and 1798 Ma (D3–D4 in
Angvik, 2014) . Based on the above studies, we suggest a similar timing for
D2 in this study, which includes folding, reverse dip-slip reactivation
of NNW–SSE-trending D1 shear zones, and strike-slip shearing along E–W-trending brittle-plastic structures (see Sect. 6.2 above).
Hydrothermal alteration, metamorphism, and their relationship to
deformational events
A preferentially aligned hornblende + epidote + plagioclase mineral
association defines S1 continuous cleavages in mafic volcanic rocks in
the Ekströmsberg area (Fig. 17g). A similar mineral association was used
by Ros (1979) and Edfelt et al. (2005) to define the metamorphic grade
affecting mafic rocks in the Tjårrojåkka area (Figs. 2, 3, 10).
According to Spear (1993), hornblende + epidote + plagioclase would
indicate a transition from greenschist to amphibolite facies metamorphic
conditions. Similarly, we interpret the hornblende + epidote + plagioclase association as a key metamorphic indicator mineral association
(Ros, 1979; Edfelt et al., 2005), which accords with the generally
accepted, but poorly constrained, low- to medium-grade low-P regional
metamorphism of northern Norrbotten (e.g. Frietsch et al., 1997; Bergman et
al., 2001; Skelton et al., 2018). In Fig. 16a, the hornblende + epidote + plagioclase S1 fabric forms axial planes to a folded hornblende + epidote vein fill, which possibly indicates a pre-compressional timing for
some hornblende + epidote and thus a pre-D1 commencement of prograde
metamorphism. However, hornblende in this vein displays syn-tectonic
characteristics (Fig. 16b, c), implying that metamorphism probably peaked
during the fabric-forming D1 event.
Regional scapolite alteration in northern Norrbotten is thought to be linked
to the formation of IOA and Cu–Au deposits there, having formed due to the
activity of relatively high-salinity ore-forming fluids (Martinsson et al.,
2016). In this respect, the widespread albite + scapolite alteration in
the WSB may partly represent the effects of hydrothermal fluid flow
associated with the formation of IOA and Cu–Au mineralization in the area.
Several generations of albite + scapolite alteration are present
throughout the WSB and in differing settings. For example, the
porphyroblastic and the semi-conformable (selectively pervasive) types (Fig. 17a, c) are commonly encountered in low-strain blocks close to shear zones
or in mafic dykes within or at the margins of metre-wide shear zones.
Scapolite porphyroblasts are often undeformed and tend to overprint
D1 fabrics. Hence, we infer the timing of the regional porphyroblastic
scapolite formation as syn- to late D1. This inference is broadly
consistent with metasomatic (titanite) U–Pb ages at 1851±6 and
1850±7 Ma reported for a scapolite-altered diorite east of the WSB
(Martinsson et al., 2016). However, at the same locality, Smith et al. (2009)
reported 1903±8 Ma for scapolitization, which indicates that
scapolite alteration in the Norrbotten area is likely polyphase and occurs
as several generations.
Pervasive magnetite + amphibole alteration (Fig. 17a, h) is commonly
distributed in the WSB, and it is locally overprinted and affected by S1
foliation and interpreted as early to syn-D1. Amphibole + magnetite
alteration also occurs as discordant veins with albite haloes (Fig. 17b).
The relative timing of these veins is difficult to resolve due to the lack
of an association with other discernible tectonic structures, but we
tentatively assign a D1 timing and a paragenetic link with the
pervasive amphibole + magnetite association based on their similar
mineralogy and often close spatial relationship (see Fig. 19). Pervasive
amphibole + magnetite occasionally shows an early timing relative to the
early selectively pervasive albite + scapolite alteration (Fig. 17a).
However, we have also observed the opposite relationship between pervasive
magnetite + amphibole and early selectively pervasive albite + scapolite
alteration. Thus, it is possible that both hydrothermal mineral
associations represent an evolving calcic–sodic–ferroan alteration system
similar to that suggested from other analogous IOA- and IOCG-mineralized terranes
(e.g. Corriveau et al., 2016; Montreuil et al., 2016).
However, in the WSB, a clear spatial zonation of hydrothermal alteration at
the exposed surface has not been observed (e.g. Fig. 18), but it is possible
that such a zonation is masked by overburden.
The hydrothermal mineral associations linked to D2 in this study are
potassic–ferroan in character and in most cases are hosted by
D2 structures. A late-D2 timing is interpreted for the biotite + magnetite + K-feldspar + muscovite alteration hosted by sinistral E–W
shear zones near the NNW–SSE-trending Ekströmsberg IOA deposit (Fig. 17k, l). A late-D2 timing for this potassic–ferroan alteration is
evident by offsetting relationships between the E–W structural grain and
more dominant NNW–SSE-trending D1 fabrics. Paragenetically late and
structurally controlled potassic alteration associated with epidote occurs
along volcanosedimentary bedding (Figs. 17g, 19) or as vein fills associated
with iron oxides and sulfide crosscutting magnetite-amphibole alteration at the outcrop scale (Fig. 17h). In the Tjårrojåkka area, Edfelt et al. (2005) suggest K-feldspar alteration is paragenetically late relative to
scapolite. Similarly, in the Gällivare area, K-feldspar alteration shows
a late-D2 timing and a close spatial relation to ca. 1.8 Ga granite and
pegmatite (Bauer et al., 2018), hence in agreement with our observations
from the WSB.
Locally, late-stage calcite associated with rare sulfide mineralization forms
part of (Fig. 17e) or overprints (Fig. 17f) D2 shear zone fabrics in
structures intersecting mafic rocks. Late-stage carbonate deposition has
been described for magnetite group IOCG in the Cloncurry district in
Australia (Corriveau and Mumin, 2010) as well as in Norrbotten close to
Kiruna (Fig. 1), where carbonates are associated with sulfides in a discordant
albite-carbonate mylonite zone (Bergman et al., 2001). It is possible that
the late-stage carbonate alteration reported in this study forms part of a
larger ore-forming system during D2. However, in the case where
carbonate alteration overprints D2 shear zone fabrics (Fig. 17f), the
possibility that ingress of retrograde meteoric fluid depositing carbonate
during downwelling as described by Kesler (2005) cannot be excluded at this
stage of research.
Hydrothermal alteration with an inferred D2 timing that does not show a
direct association with tectonic structures is regional selectively
pervasive (patchy) K-feldspar alteration (replacing albite), often
accompanied by sericite and epidote. This is an important alteration
component in the felsic volcanic rocks of the WSB (Fig. 17k), and it was
documented already during the early mapping campaigns in the area
(Offerberg, 1967). Few crosscutting relationships exists for this type of
alteration; however, fracture planes carrying epidote (Fig. 17i) post-date
this selectively pervasive K-feldspar alteration, implying the latter
developed before brittle deformation, probably syn- to late D2 and
broadly coeval with the structurally controlled potassic mineral
associations occurring along the WSB.
The spatial distribution of identified hydrothermal mineral associations and
their spatial correlation to dominant structures is summarized on the map in
Fig. 18. The map shows that D1-related mineral associations are
confined to geological structures and also overprint areas distal to
dominant D1 structures. Overall, scapolite ± albite is more
widespread than magnetite + amphibole alteration. The intensity of
potassic alteration seems to decrease towards the north, which can be
explained by the general absence of ca. 1.8 Ga granites in that part of the
WSB. Carbonate alteration is confined to structures with an inferred
D2 timing. Distribution uncertainties for the alteration associations
shown in Fig. 18 are high due to the scattered nature of the observations
and the glacial till covering most of the WSB.
The alteration styles identified throughout the WSB may represent important
vectors for both IOA and IOCG mineralization along the belt, in northern
Norrbotten (e.g. Martinsson et al., 2016) and worldwide (e.g. Corriveau
and Mumin, 2010). In our study, alteration styles typical for comparable
IOA and IOCG districts elsewhere (i.e. calcic–sodic and potassic ± ferroan
associations) are consistently developed along the WSB and show a spatial
association with certain generations of structures that can be correlated
with the position of known IOA- and/or IOCG-style mineralization (e.g. the
Ekströmsberg and Tjårrojåkka areas; Fig. 18).
Summary of hydrothermal alteration, metamorphism, and their relation to deformation
To summarize the relative timing of the various alteration styles and how
they relate to Orosirian magmatism and mineralization in the WSB and
northern Norrbotten in general, a summary sketch in Fig. 19 and the
following key points are presented based on this study's results, combined
with stratigraphic and geochronology data reported by Romer et al. (1994),
Bergman et al. (2001), Wanhainen et al. (2005), Edfelt (2007), Smith et
al. (2009), Westhues et al. (2016), Martinsson et al. (2016), Sarlus et
al. (2017), and Bergman (2018):
D1:
emplacement of calc-alkaline intrusions (Haparanda suite) and coeval
volcanic rocks (porphyrite group) as well as pre- to early D1 in
northern Norrbotten;
porphyry copper formation east of WSB, pre- to early D1;
emplacement of mildly alkaline intrusions (Perthite monzonite suite) and
coeval volcanic rocks (Kiirunavaara group), pre- to syn-D1 in northern
Norrbotten;
IOA mineralization east of the WSB (in central Kiruna) during pre- to
early D1;
regional pervasive amphibole + magnetite alteration in the WSB, early to
syn-D1;
discordant vein fill amphibole + magnetite + albite alteration, early to
syn-D1;
metamorphic peak in the WSB (epidote–amphibolite facies metamorphism),
syn-D1 (earlier metamorphic onset as well as prolonged metamorphic
conditions is possible);
conformable albite–scapolite alteration or growth of albite–scapolite
porphyroblasts in the WSB syn- to late D1;
IOCG mineralization east of WSB during late D1;
shear-zone-hosted pervasive actinolite + tremolite alteration,
late D1;
calcite in veins with unknown temporal relation to D1 but pre-D2;
scapolite in veins with unknown temporal relation to D1 but
pre-D2.
D2:
emplacement of Lina and Edefors intrusive suites in northern Norrbotten,
syn-D2;
IOCG mineralization confined to shear zones east of the WSB, syn-D2;
local shear-band-hosted biotite + magnetite + K-feldspar alteration in
the WSB, syn-D2;
local discordant vein-hosted K-feldspar + epidote + iron oxide + sulfide alteration, syn-D2;
local discordant vein/shear band fill scapolite alteration, syn-D2;
IOA and IOCG emplacement in the Tjårrojåkka area (WSB), late D2;
calcite alteration, syn- and late to post-D2;
regional selectively pervasive K-feldspar alteration, syn- to post-D2;
local fracture-fill epidote alteration, post-D2;
retrograde sericite alteration, post-D2.
Conclusions
Based on the new structural mapping and microstructural investigation
presented in this study, two major compressional events affecting the
Western Supracrustal Belt in northwestern Norrbotten have been
identified. These deformation events, D1 and D2, developed
pronounced, corresponding structures which can be correlated with different
types of mineral alteration associations within the belt. Shear zones
recording reverse oblique west-side-up kinematics were developed during
D1, giving rise to an undulating NNW-trending configuration of magnetic
lineaments. D1 produced a steep SW- to WSW-dipping, heterogeneously
developed, penetrative, and continuous S1 foliation related to F1
folds with either tight, east-verging symmetry with a shallow NW-plunging
fold axis or tight, upright symmetry with steep fold axes. Steep plunges of
F1-fold axes are interpreted to be a result of later
D2 transposition.
S1 foliation was folded during D2 into near-cylindrical F2
folds with steep, N- and S-plunging fold axes. Axial planar S2
foliations are rarely developed in relation to F2 folds; where
present, S2 is a spaced cleavage on the centimetre to several-decimetre scale. The
finite D2 strain was partitioned into pre-existing D1 shear zones,
reactivating these structures with reverse dip-slip, west-side-up
sense of shear. The D2 strain was also partitioned into rheologically
favourable lithotypes, such as volcanosedimentary rocks. Synchronously, E–W
trending sinistral strike-slip shear zones were active and partly displaced
earlier-formed NNW-trending structural grains.
D1 is associated with regional scapolite ± albite alteration that
is broadly coeval with regional magnetite ± amphibole alteration under
epidote–amphibolite facies metamorphic conditions. The hydrothermal
alteration that affected rocks during D2 is generally structurally
controlled and potassic ± ferroan in character and dominated by
K-feldspar ± epidote ± quartz ± biotite ± magnetite ± sericite ± sulfides, as well as calcite. D2 is also
associated with selectively pervasive K-feldspar alteration replacing albite,
affecting intermediate to felsic rocks without any direct spatial
correlation to structures. This implies that our field-based observations
support an early-D1 timing for calcic–sodic alteration, whereas a later
timing (syn-D2) is interpreted for potassic ± ferroan alteration
associations. In absolute terms, the timing of these fluid flow events may
have differed by as much as ca. 80 million years based on previously reported
geochronological data from northern Norrbotten.
Data availability
Structural field measurements and analysed thin sections are available from
the corresponding author.
Author contributions
JBHA mapped the Eustiljåkk area and the eastern part of the Kaitum West area.
JBHA and TEB mapped the Ekströmsberg area and the areas between
Ekströmsberg and Eustiljåkk. TEB and EPL mapped the areas
Fjällåsen–Allavaara and Tjårrojåkka. JBHA, TEB, and EPL
mapped the Kaitum West area and the areas between Kaitum West and
Ekströmsberg. JBHA analysed the structures of Eustiljåkk,
Ekströmsberg, and Kaitum West. TEB analysed the structures of the
Tjårrojåkka and Fjällåsen–Allavaara areas. All
microstructural analysis used in this paper was done by JBHA. The writing
was performed by JBHA with much help from EPL and TEB. Contributions are as
follows: JBHA (50 %), TEB (25 %), and EPL (25 %).
Competing interests
The authors declare that they have no conflict of interest.
Acknowledgements
Advanced Mining and Metallurgy (CAMM) was
thanked for financing this study. Parts of this work
were undertaken as part of the VINNOVA project “Multi-scale 4-dimensional
geological modelling of the Gällivare area”, the SGU-funded project
“Structural vectoring of mineralized systems in northern Norrbotten”, and
SGU's “Barents project”. Thorkild Maack Rasmussen is thanked for
processing of the magnetic data and for compiling the magnetic maps. Hugo Hedin Baastrup is thanked for his field assistance in the eastern part of
the Kaitum West and Eustiljåkk key areas. We thank Kunfeng Qiu and Jochen Kolb for constructive reviews of this study. Software from Midland Valley
was used for data collection and subsequent structural analysis.
Financial support
This research has been supported by the Centre of Advanced Mining and Metallurgy (CAMM) (grant no. 2450-2009).
Review statement
This paper was edited by Florian Fusseis and reviewed by Jochen Kolb and Kunfeng Qiu.
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