Early deformation in the Svecokarelian greenstone belt of the Kiruna iron district, northern Sweden

Vollmer, Frederick W.; Wright, Stephen F.; Hudleston, Peter J.

doi:10.1080/11035898409454620

Cited by 12 publications

(16 citation statements)

References 21 publications

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“…Note also the attitude of the secondary dike, which may suggest that the primary body had already been tilted prior to its western limb of a south-southwest-plunging homoclinal anticline (Geijer, 1910;Parak, 1975). Vollmer et al (1984), however, interpreted the Kiirunavaara deposit as a megaboudin, and Forsell (1987) suggested a series of tight isoclinal folds in the district. These interpretations both implicitly assume that the district was subjected to intensive penetrative deformation.…”

Section: Fig 2 a Late-stage Magnetite Dike' Crosscutting Earlier Mamentioning

confidence: 96%

Isotope systematics of the Kiruna magnetite ores, Sweden; Part 1, Age of the ore

Cliff

Rickard²,

Blake³

1990

Economic Geology

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U-Pb isotope dating of zircons from a granophyric dike complex which crosscuts the Kiirunavaara magnetite-apatite body demonstrates that the ore was emplaced before 1.880 ___ 0.003 Ga. Whole-rock Sm-Nd isotope compositions for five out of six samples from the footwall and hanging-wall complexes lie on a 1.89 ___ 0.09-Ga isochron. The initial ratio of 0.50991 ___ 0.00007 corresponds to e•a(T) ---5.6 and suggests a substantial late Archean component in the igneous rocks.Since the ore cannot have been emplaced after the granophyre nor before the host rocks, ore formation is limited to the period between 1.90 and 1.88 Ga. However, the granophyre remains at present in the form of a crosscutting dike, largely unaffected by the ductile and brittle deformation that has affected both the ore and host rocks. This implies that ore formation, igneous activity, and deformation were all confined to a short time interval during this period and suggests that the Kiruna magnetite-apatite ores were generated during orogenesis.This association oforogenesis, emplacement temperatures up to 600øC, abundant evidence of hydraulic processes during ore formation, and extensive porphyries is reminiscent of other types of intrusion-related, high-temperature, ore-forming processes.

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Section: Fig 2 a Late-stage Magnetite Dike' Crosscutting Earlier Mamentioning

confidence: 96%

Isotope systematics of the Kiruna magnetite ores, Sweden; Part 1, Age of the ore

Cliff

Rickard²,

Blake³

1990

Economic Geology

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“…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 D 1 thrusting event overprinted by gentle local folding and shear zone development (D 1 to D 4 in Wright 1988).…”

Section: Regional Geological Settingmentioning

confidence: 99%

Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

Andersson

Lynch

2020

Solid Earth

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Abstract. 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.

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“…In the Kiruna area this deformation was multiphase, dominated by west-northwest-directed compression, and resulted in the formation of north-south-trending, western sideup, shear zones, steep to vertical dips of most of the strata, and southeast-plunging folds (Vollmer et al, 1984;Wright, 1988;Bergman et al, 2001). A second phase of granitoid magmatism, termed the Lina suite, intruded these rocks around 1.79 Ga (Skiöld, 1988;Bergman et al, 2001).…”

Section: Regional Geologymentioning

confidence: 95%

The Geology of the Rakkurijarvi Cu-(Au) Prospect, Norrbotten: A New Iron Oxide-Copper-Gold Deposit in Northern Sweden

Smith¹,

Coppard²,

Herrington³

et al. 2007

Economic Geology

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The Rakkurijärvi prospect consists of a group of mineralized magnetite and lithic breccias within the ca. 2.05-to 1.90-Ga Proterozoic supracrustal sequence of the Kiruna district, northern Sweden. Potentially economic grades of Cu and Au, largely in the form of chalcopyrite and other sulfide assemblages, are hosted in brecciated magnetite and metavolcanic rocks. The extent of the mineralization is currently open, both downdip and along strike. The deposit was discovered through an integrated geophysical and geochemical program focused on iron oxide-copper-gold (IOCG)-style mineralization. It is hosted by brecciated greenschist facies metavolcanic rocks within and adjacent to an east-northeast-trending shear zone. The dominant characteristics of the deposit are consistent with the IOCG class and include magnetite and lithic breccias hosted in a metavolcanic sequence, with matrices of albite, actinolite, and calcite surrounded by halos of sodic (albitescapolite) and potassic (scapolite-K-feldspar-biotite) alteration. A distinctive accessory mineral assemblage includes apatite, titanite, and allanite. The paragenesis and textural evolution of the deposit includes early Narich alteration accompanying massive magnetite alteration. The Na-rich alteration is overprinted by potassic alteration (also associated with magnetite), although the paragenesis is complex and multiple generations of both sodic and potassic alteration are recognized. Alteration of lithic clasts to magnetite confirms a metasomatic origin, as opposed to an orthomagmatic origin, for the magnetite mineralization. Re-Os analyses of two separates of molybdenite intergrown with magnetite, interpreted as cogenetic with the sulfide assemblage, yield mineralization ages of 1853 ± 6 and 1862 ± 6 Ma.Reconnaissance bulk-rock chemistry of the host volcanic rocks is consistent with an intermediate volcanic protolith, but much of the original character of the rocks is masked by albitization and incipient iron, sodic, and potassic alteration. The data also indicate significant element mobility during metasomatism and, in particular, the addition of Ti to the rock mass in biotite and as titanite. The compositions of secondary minerals are consistent with alteration and mineralization caused by highly saline fluids of relatively low F activity. The stable isotope characteristics of calcite, with δ 18 OSMOW ranging from 9.43 to 19.89 per mil and δ 13 CPDB ranging from -11.69 to +4.88 per mil, suggest that the fluids of the calcite and sulfide stage were derived from a magmatic source but had interacted extensively with local sedimentary and volcanic rocks. MethodsAll field and geophysical data were provided by Anglo American Exploration BV. Core logging was carried out onsite by the authors and Anglo American field staff. Preliminary sample characterization was by transmitted and reflected light microscopy and was followed by examination using either a Hitachi S2500 scanning electron microscope at the Natural History Museum, London, or a JEOL 6310 SEM at the Universit...

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Early deformation in the Svecokarelian greenstone belt of the Kiruna iron district, northern Sweden

Cited by 12 publications

References 21 publications

Isotope systematics of the Kiruna magnetite ores, Sweden; Part 1, Age of the ore

Isotope systematics of the Kiruna magnetite ores, Sweden; Part 1, Age of the ore

Evolution of structures and hydrothermal alteration in a Palaeoproterozoic supracrustal belt: Constraining paired deformation–fluid flow events in an Fe and Cu–Au prospective terrain in northern Sweden

The Geology of the Rakkurijarvi Cu-(Au) Prospect, Norrbotten: A New Iron Oxide-Copper-Gold Deposit in Northern Sweden

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