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Nautanen
Norbotten, Sweden
Main commodities: Cu Au Ag Mo


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The Nautanen (and Nautanen North) Cu-Au deposit is located in the Norrbotten region of northern Sweden, ~15 km NNW of the Aitik copper mine, and 7 km east of the Malmberget iron deposit at Gällivare.
(#Location: 67° 11' 38"N, 20° 52' 48"E).

Mineralisation at Nautanen was discovered in outcrop in 1898, and was initially worked by Nautanens Kopparfält AB in a series of small scale mines between 1902 and 1907. Despite a program of exploration drilling and trenching to outline additional resources, and consolidation of the mines with those in the Liikavaara field 15 km to the south, the company went bankrupt in 1907 (Geijer, 1917). These historic underground mines extracted a total of 71 835 t of ore producing 5746 t of copper concentrate, 4635 t of iron concentrate, and an unspecified amount of gold (Geijer, 1917). Exploration was conducted by SGAB (Sveriges Geologiska AB, the Swedish Geological Survey) from 1951 to 1985. Drilling was focused on shallow targets at Nautanen and culminated in a resource estimate around the historic mines in two zones of mineralisation, namely an A-Zone, rich in chalcopyrite-magnetite mineralisation with 0.63 Mt @ 2.36% Cu, 1.3 g/t Au, 11 g/t Ag; and a C-Zone, with characteristics more akin to the Aitik deposit, with 2.3 Mt @ 0.34% Cu, 0.3 g/t Au (Danielsson, 1985). The mineralised zones defined in this work were complicated and often truncated by faults. Exploration resumed in the late 1990's when North Atlantic Resources acquired the titles. However, whilst drill hole data including collars, surveys, geology and assay has been located, other details of work undertaken has been lost. The focus appears to have been on testing a geophysical anomaly (magnetic) looking for a near surface copper-gold resource. In early 2000 Phelps Dodge undertook field mapping, geophysical surveys, soil sampling and drilling at Nautanen, whilst Boliden were contracted as consultants to conduct a ground electromagnetic (EM) survey. The latter survey resulted in an EM anomaly being identified coincident with the historical mining area at Nautanen. Phelps Dodge drilled a total of 3071 m at Nautanen and Liikavaara in 2003 and 2004, with a further 524 m in 2005 when Teck Cominco joined them in a Joint Venture. Boliden acquired the titles in 2009 and subsequently conducted ground geophysics, field mapping, sampling and till drilling program over the area, prior to commencing diamond drilling in 2011. This work resulted in the discovery of an additional Cu-Au zone of mineralisation known as Nautanen North, ~1.6 km NNW of the old Nautanen mine along the trend of a major structure, the Nautanen Deformation Zone. Drilling indicated this zone contained an indicated resource of 9.6 Mt @ 1.7% Cu, 0.8 g/t Au, 5.5 g/t Ag, 73 ppm Mo, plus an inferred resource of 6.4 Mt @ 1.0% Cu, 0.4 g/t Au, 4.6 g/t Ag, 41 ppm Mo (New Boliden 2016). Subsequent drilling has connected and expanded the historic Nautanen and Nautanen North mineralised zones to yield a current resource with a strike length of >1.6 km, as listed in the Mineral Resources section below. Boliden has continued exploration and internal technical-, environmental- and economical studies of the Nautanen Project, with an ongoing internal pre-feasibility study (Boliden, 2021).

Regional Setting

The deposit lies within the Norrbotten Region and Fennoscandian Iron Oxide Copper Gold Province which includes deposits such as the iron oxide apatite deposits at Kiruna and Malmberget, and the copper deposits of Aitik, Phatohavare, Sahavaara, Tapuli and Gruvberget. These deposits are hosted in 2.2 to 1.8 Ga Karelian and Svecofennian units which overlie Archaean basement (Martinsson and Wanhainen, 2004).

The Nautanen deposit lies within the NNW-SSE trending, dextral displaced Nautanen Deformation Zone, a large-scale, highly Cu-mineralised, magnetite-rich, shear zone with a long-lived, ~100 m.y. history of deformation, hydrothermal alteration and mineralisation (Bauer et al., 2022). This composite structure also hosts the Aitik Cu-Au-Ag ±Mo deposit and a number of smaller Cu-Au ±Fe ±Ag ±Mo occurrences, interpreted to represent IOCG style mineralisation. It is interpreted to have been the locus for polyphase deformation and intermittent metasomatic-hydrothermal activity that overprinted 1.90 to 1.88 Ga middle Orosirian continental arc-related volcanoplutonic rocks. Deformation is characterised by intense shearing fabrics forming a series of subvertical to moderately west-dipping, NNW-SSE–trending, first-order shear zones with oblique reverse kinematics and related NNE-SSW oriented second-order shear zones that control hydrothermal alteration patterns and Cu-Au mineralisation (Bauer et al., 2022).

Several hydrothermal alteration events are recognised in association with the Nautanen Deformation Zone. To the east and west of the main deformation zone, volcanic to volcaniclastic rocks have been subjected to low to moderately intense, pervasive to selective pervasive alteration as two assemblages: i). sericite-(±feldspar)-amphibole-biotite-magnetite ±tourmaline, and ii). K feldspar-hematite alteration, both of which occur as patchy zones or bands and disseminations. Both assemblages occur adjacent to NNW- and NE-oriented deformation zones and locally have minor associated sulphides. Within the main deformation zone, a moderate to intense biotite-amphibole-garnet-magnetite-tourmaline-sericite alteration assemblage occurs, typically with associated chalcopyrite-pyrrhotite-pyrite mineralisation that forms linear, subparallel, mainly NNW-oriented seams, bands, and zones that appear to locally overprint possible earlier scapolite-sericite ±feldspar alteration. Late-stage retrograde epidote ±quartz ±feldspar alteration forms selectively pervasive zones and epidote veinlets across the area and appears to be partly related to brittle faulting (Bauer et al., 2022).

A penetrative S1 foliation accompanied by a magnetite-amphibole-biotite assemblage, and a potential initial Cu mineralising event, is interpreted to be related to shortening and basin inversion during early Svecokarelian D1 deformation and 1.88 to 1.87 Ga arc accretion. Folding and repeated shearing along the Nautanen Deformation Zone has been attributed to D2, a late-Svecokarelian deformation event at a higher crustal level, between ~1.82 and 1.79 Ga. D2 has been connected to processes occurring during the transition to post-orogenic extension and collapse, and it was accompanied by abundant intrusions. This D2 associated magmatism took place under high-temperature and low-pressure conditions, and represents a regional magmatic-hydrothermal event that controlled the recrystallisation/remobilisation of magnetite, biotite and amphibole. D2 associated shear zone reactivation suggests the Nautanen Deformation Zone was utilised as a fluid conduit, which preferentially controlled the location and formation of epigenetic Cu-Au mineralisation with distinctive IOCG characteristics within second-order shear zones (Bauer et al., 2022).

Geology

The Nautanen mineralisation and Nautanen Deformation Zone lie within the larger Aitik-Malmberget field. The Aitik deposit is interpreted to be a Palaeoproterozoic porphyry-style mineralised system that also exhibits the characteristics of a late-stage IOCG overprint. It lies within a Svecofennian supracrustal belt composed of largely intermediate volcaniclastic, volcanic and intrusive rocks that have been metamorphosed to amphibolites facies (Wanhaninen, 2005). These rocks, which are predominantly a sequence of folded, 1.96 to 1.85 Ga intercalated sedimentary and volcanosedimentary suites and volcanic and volcaniclastic units (Sarlus et al., 2018) belong to the regional Muorjevaara Group which constitutes the bulk of bedrock in the east of the Aitik-Malmberget field (Martinsson and Wanhainen, 2004). Schwarz, et al. (2019) divide the protolith lithologies of the Muorjevaara Group into i). predominantly intermediate metavolcaniclastic rocks; ii). volcanogenic (epiclastic) metasedimentary rocks; iii). mica schist horizons; and iv). amphibolitic schist after mafic metavolcaniclastic rocks. The first two units are the most common across the Nautanen deposit area (Sarlus et al., 2018).

The entire area have been overprinted by later intrusions which include, 1.88 to 1.86 Ga diorite and granodioritic rocks cutting the Muorjevaara Group as well as gabbro, gabbro-norite and troctolite of the 1.88 Ga Dundret complex to the west, south of Gällivare, all of which belong to the Haparanda Suite. These were followed by the ~1.80 Ga Vassaravaara complex mafic to intermediate intrusives and extrusives that included gabbro, gabbro-basalt and andesite. The entire Aitik-Malmberget field is surrounded and locally intruded, or 'sheeted-over', by the ~1.81 to 1.78 Ga Lina Granite and associated pegmatites which are ubiquitous throughout the region (Sarlus et al., 2018).

The Nautanen Deformation Zone cuts the Muorjevaara Group supracrustal rocks where it is typically inferred to be a steep, near-vertical structure with an undetermined degree of displacement, and a strongly developed fabric or foliation. It encloses numerous lithologies, complicated by intense shearing, transposition and pervasive metasomatic-hydrothermal alteration throughout the deformation zone, generally comprising mafic to intermediate volcaniclastics and volcanic rocks, with some minor intrusive units (McGimpsey, 2010). Lithologies hosting the Nautanen resource have been divided into four main groups by Årebäck et al. (2021):
Amphibole-biotite gneiss, which is the most abundant rock type at Nautanen, typically occurring in the hanging wall to mineralisation, i.e. to the east. It is interpreted to represent a zone of calcic alteration, dominated by hornblende and actinolite with pervasive magnetite.
Biotite gneiss, which is typically interleaved with the amphibole-biotite gneiss, but gradually comes to dominate in the central core of the Nautanen deposit where it forms a gradational series with banded biotite gneiss on its lower contact. This unit is found in the western footwall as rare lenses, and is interpreted to represent a zone of intense potassic alteration, dominated by biotite.
Banded biotite gneiss, which is a distinctive component of the sequence at Nautanen, and due to its proximity to mineralisation in the southern lens at Nautanen North, makes it a reliable marker horizon, despite being volumetrically relatively minor. It's distinctive appearance is the result of it's increased feldspar content and lenses of biotite. It's upper contact is, in nearly all cases, transitional into biotite gneiss, whilst the lower contact with garnet sericite schist is often sharp, and possibly structural.
Garnet sericite gneiss/schist represents the bulk of the footwall at Nautanen, and is probably the dominant rock type of the Nautanen hill. Its lower contact has not yet been observed, and it likely structural, juxtaposing it against amphibole-biotite gneiss. This unit is the main host to the high grade mineralisation, which is typically located adjacent to its upper contact. Locally the unit exhibits strong gneissosity and schistosity, which are inferred to represent zones of structural deformation. In places, a possible porphyritic texture can be observed but is not ubiquitous. The rock is interpreted to represent a zone of intense potassic-calcic alteration.

Mineralisation

The Nautanen deposit was formed as part of a suite of IOCG deposits in Norrbotten which all show a strong association with zones of structural deformation. Mineralisation at Nautanen is hosted within the volcanosedimentary units and has no apparent clear link to intrusions (Årebäck et al., 2021).
The mineralisation, which is distributed over a strike length of >1.6 km and to a depth of 700 m, has been sub-divided into five main styles/zones as follows (after Årebäck et al., 2021):
High Grade 1, which includes almost the entire resource, and is confined to a sub-vertical to steeply east dipping, NNE striking structure. Its mineralogy comprises disseminated chalcopyrite with minor magnetite and pyrrhotite that appears to be structurally controlled on a small scale by microfractures. Typically this mineralisation style contains a higher grade, 20 to 100 cm wide core of 'Pebble Breccia' with rounded clasts of replaced garnets. It can be divided in a 'southern lens' which appears to be lithologically controlled by the contact between the banded biotite gneiss and garnet sericite schist, and a 'northern lens' that is apparently structurally controlled within a dilational jog and associated sheeted vein system. The banded biotite unit in the 'southern lens' pinches out into a series of narrow splays closer to the 'northern lens', although alteration is continuous, even across the 200 m gap in mineralisation between the two lenses.
High Grade 2, located in the hanging wall of High Grade 1 'southern lens', ~100 to 150 m to the east. It comprises disseminated chalcopyrite-magnetite-pyrite, although massive magnetite also occurs locally. Only a few drillhole intersection of this ore style are included in the resource estimate. This ore style appears more structurally complex compared to the relatively planar High Grade 1.
Disseminated Cu, which occurs as an envelope of elevated Cu mineralisation surrounding both High Grade 1 and 2, and is relatively continuous along strike. It is composed of chalcopyrite, pyrite, magnetite and molybdenite, and in cross-section, appears forms a vertically tilted zone of Aitik-like mineralisation, which at shallower levels pinches out before reaching the surface. At depth, the zone can reach widths of >100 m and is a potential host to undiscovered high grade lenses.
Tourmaline vein system, which is found in the hanging wall to the High Grade 1 'southern lens' and is interpreted to overprint the High Grade 2 mineralisation. It occurs as either a series of sheeted quartz tourmaline veins or an en echelon vein array that stikes ~north-south and dips at 70°E. Individual veins can contain copper grades of up to 1%, but have not been found over economic widths.
Iron-'skarn' massive to semi-massive and disseminated magnetite, which has been occasionally encountered at Nautanen. The massive to semi-massive magnetite occurs in association with carbonate and a clinoamphibole-diopside skarn assemblage with minor epidote (Jansson, 2013). The massive magnetite zones can be up to 1 to 2 m in width and grade outward into a stockwork-like magnetite mineralisation with minor disseminated sulphides, which include molybdenite.

Overall, the historic Nautanen mineralised zone is characterised by almandine porphyroblasts with associated amphibole-biotite-magnetite-sericite ±K feldspar ±sulphide and tourmaline ±quartz ±sulphide banding, patches and veins. Garnet growth is suggested by textural relationships to slightly predate the main-stage alteration and mineralisation event, and is followed by late-stage epidote ±quartz ±carbonate alteration. Chalcopyrite with lesser bornite and chalcocite are the main copper-bearing minerals and are typically associated with pyrite, pyrrhotite, magnetite and tourmaline. Quartz-amphibole ±tourmaline veins containing pyrite and minor chalcopyrite post-date the main-stage disseminated and micro-fracture type sulphide mineralisation. Gold generally occurs as inclusions and segregations in pyrite, chalcopyrite, bismuth-bearing phases and locally galena (Sammelin 2011, Bark et al., 2013; Schwarz et al., 2019).

Schwarz et al. (2019) compared 'least altered' meta-volcanosedimentary rocks from the eastern volcano-sedimentary domain with pervasively altered, mylonitic rocks within the Nautanen Deformation Zone. The latter are relatively enriched in Cu, Ag, Au, Fe, Mo, Ba, Mn and W. Similarly, the tendency for K/Na ratios to increase inwards within the Nautanen Deformation Zone reflects the association between potassic alteration and Cu-Au-Fe enrichment (e.g., Lynch et al., 2015). These features are regarded as diagnostic of typical geochemical affinities and metal abundance correlations associated with IOCG-style mineralisation, particularly deposits hosted by intermediate to felsic igneous rocks in continental settings (e.g., Barton 2014).

U-Pb LA-ICP-MS titanite and allanite ages of hydrothermal alteration at the Nautanen copper-gold deposit range from ~1.79 to 1.78 Ga (Smith et al., 2009). These dates have been interpreted to provide a temporal and inferred genetic link between the mineralisation and deformation, fluid mobilisation and late granitic magmatism during orogenic collapse and extension.

Mineral Resources

The Mineral Resource at Nautanen at a cut-off of ≥ 0.9% Cu, as of 22 December, 2020 (Boliden Summary Report Mineral Resources and Mineral Reserves, 2021 Report) was:
  Indicated Resource - 12.70 Mt @ 1.54% Cu, 0.8 g/t Au, 6 g/t Ag, 100 ppm Mo, 2.8% S;
  Inferred Resource - 8.70 Mt @ 1.37% Cu, 0.6 g/t Au, 6 g/t Ag, 98 ppm Mo, 2.5% S;
  TOTAL Resource - 21.40 Mt @ 1.47% Cu, 0.7 g/t Au, 6 g/t Ag, 99 ppm Mo.

The most recent source geological information used to prepare this decription was dated: 2022.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


Nautanen

  References & Additional Information
   Selected References:
Areback, H., Dean, B., McGimpsey, I. and Drejing-Carroll, D.,  2021 - Boliden Summary Report Mineral Resources and Mineral Reserves, Nautanen: in    Boliden Summary Report    20p.
Bauer, T.E., Lynch, E.P., Sarlus, Z., Drejing-Carroll, D., Martinsson, O., Metzger, N. and Wanhainen, C.,  2022 - Structural Controls on Iron Oxide Copper-Gold Mineralization and Related Alteration in a Paleoproterozoic Supracrustal Belt: Insights from the Nautanen Deformation Zone and Surroundings, Northern Sweden: in    Econ. Geol.   v.117, pp. 327-359.
Drejing-Carroll, D., Hitzman, M.W. and Coller, D.,  2023 - Geology of the Nautanen North Cu-Au-Ag-(Mo) Deposit, Norrbotten, Sweden: in    Econ. Geol.   v.118, pp. 1765-1794. doi: 10.5382/econgeo.5019
Sarlus, Z., Martinsson, O., Bauer, T.E., Wanhainen, C., Andersson, J.B.H. and Nordin, R.,  2018 - Character and tectonic setting of plutonic rocks in the Gallivare area, northern Norrbotten, Sweden: in    GFF,    21p. doi.org/10.1080/11035897.2018.1526209.
Schwarz, G., Kathol, B., Ripa, M., Thunholm, B., Lynch, E.P. and Jonberger, J.,  2019 - Report on pilots: the Kristineberg and Nautanen mining areas in Northern Sweden: in    CHPM2030,   Deliverable D 6.2 Appendix 6.2.4, 72p.


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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