Skellefte Group Research Articles

2D and 3D MT in the central Skellefte Ore District, northern Sweden

New broadband magnetotelluric (MT) data have been acquired along two parallel profiles in the central part of the metallogenic Skellefte district in northern Sweden. The data were recorded as part of the Swedish 4D modelling of mineral belts project and cover an area with several economical and sub-economical deposits. The dimensionality and quality of the data were carefully analyzed and new error floors were systematically determined prior to inverse modelling in 2D and 3D. The algorithms used were EMILIA and WSINV3DMT. For the 2D inversion, only the determinant of the impedance tensor was used, while for the 3D inversion all elements were considered. The obtained models fit the inverted data, and image the main regional features. A detailed comparison reveals the superiority of the 3D model, both in model structures and data fit. After assessing the main features in the model, an interpretation is proposed and refined with the support of previous geophysical studies. The most interesting features are large and medium-sized conductors associated with crustal-scale shear zones and faults within the Skellefte Group rocks. These may be depicting a network of fossil pathways for hydrothermal fluid transport and as such, provide new insight into past processes in the area.

Deep massive sulphide exploration using 2D and 3D geoelectrical and induced polarization data in Skellefte mining district, northern Sweden

ABSTRACTGeoelectrical and induced polarization data from measurements along three profiles and from one 3D survey are acquired and processed in the central Skellefte District, northern Sweden. The data were collected during two field campaigns in 2009 and 2010 in order to delineate the structures related to volcanogenic massive sulphide deposits and to model lithological contacts down to a maximum depth of 1.5 km. The 2009 data were inverted previously, and their joint interpretation with potential field data indicated several anomalous zones. The 2010 data not only provide additional information from greater depths compared with the 2009 data but also cover a larger surface area. Several high‐chargeability low‐resistivity zones, interpreted as possible massive sulphide mineralization and associated hydrothermal alteration, are revealed. The 3D survey data provide a detailed high‐resolution image of the top ∼450 m of the upper crust around the Maurliden East, North, and Central deposits. Several anomalies are interpreted as new potential prospects in the Maurliden area, which are mainly concentrated in the central conductive zone. In addition, the contact relationship between the major geological units, e.g., the contact between the Skellefte Group and the Jörn Intrusive Complex, is better understood with the help of 2010 deep‐resistivity/chargeability data. The bottommost part of the Vargfors basin is imaged using the 2010 geoelectrical and induced polarization data down to ∼1‐km depth.

Likely “mantle plume” activity in the Skellefte district, Northern Sweden. A reexamination of mafic/ultramafic magmatic activity: Its possible association with VMS and gold mineralization

Most attention has been given to the geology of the extensive VMS and subordinate precious metals mineralization in the Skellefte district. Less attention has been given to indications of deep-seated origins of felsic and mafic/ultramafic volcanic rocks; of VMS and precious metals mineralizing fluids; and the primary origins of these metals. A holistic view of the significance of mafic/ultramafic volcanic rocks to both the geotectonic evolution of the area and the existence of its important base and precious metals deposits has never been presented. These subjects are discussed in this investigation.Primitive mantle normalized spider diagrams of rare-earth-elements (REE) distinguish two groups of mafic/ultramafic volcanic rocks, each with distinct geochemical characteristics: a mid-ocean-ridge “MORB”-type, and a geochemically unusual and problematic calc–alkaline–basalt “CAB”-type which is the main subject of this investigation. The “MORB”-type mafic volcanic rocks are mostly older than the Skellefte Group felsic volcanic rocks hosting the VMS deposits, whereas the more primitive “CAB”-type mafic/ultramafic volcanic rocks are mostly younger.A common source for these “CAB”-type, mafic-(MgO wt.%<14%) and ultramafic-(MgO wt.% >14%) volcanic rocks is suggested by their similar and distinctive geochemical features. These are near-chondritic (Al-undepleted) Al2O3/TiO2 ratios; moderate to strong high-field-strength-element (HFSE) depletion; light-rare-earth-element (LREE) enrichment and moderate heavy-rare-earth-element (HREE) depletion. They outcrop throughout an area of at least 100×100km. Gold mineralization is spatially associated with ultramafic volcanic rocks.Zr and Hf depletion has been shown to be associated with Al-depletion in mafic/ultramafic volcanic rocks elsewhere, and has been attributed to deep-seated partial melting in ascending mantle plumes. Zr and Hf depletion in “CAB”-type Al-undepleted mafic/ultramafic volcanic rocks is therefore unusual. The solution to this dilemma is suggested to be contamination of an Al-depleted mantle plume by felsic crustal rocks whereby Al-depleted ultramafic magmas become Al-undepleted. It will be argued that this model has the potential to explain previous observations of deep-seated origins; the spatial association of ultramafic volcanic rocks with occurrences of gold mineralization; and even the primary origin of metals in VMS deposits.

Skellefte mining district in 3D; results from integrated interpretation of potential field, resistivity/IP and reflection-seismic data

Located in northern Sweden, the Skellefte mining district has been subject to several geological and geophysical investigations, as it is hosting abundant volcanic-hosted massive sulfide deposits. The importance of mineral exploration at greater depths in the Skellefte district has been increased since most of mineralization at shallow depths are already discovered and exploited. Therefore, geophysical methods become particularly important as they can improve our knowledge about spatial relationship between geological features at the depth. In the first part (local-scale), we used resistivity/IP data to map the subsurface geometry down to 430m depth. Furthermore, the results of the resistivity/IP studies were constrained with potential field data down to 1.5 km depth. In the second part (Regional-scale), potential field data were used to constrain the interpretation of the reflection-seismic data down to 5 km depth. The result from the first part indicated a good correlation between the initial resistivity model and the magnetic and gravity field calculated from that model. The volcano sedimentary contact between the Skellefte and Vargfors group rocks and three proposed locations for sulphide mineralization were identified along the resistivity/IP profile. In second part, the gravity and magnetic data were investigated to better understand the contact between the Skellefte group, volcanic rocks and the Bothnian Basin sedimentary rocks. Furthermore the data was used to constrain the geometry of late-orogenic gabbro-diorite and granite intrusions, which occur along inferred shear zones that are only poorly indicated, or not visible at all in the reflection-seismic data. As the main outcome, the proposed integrated 3D model of the central Skellefte district (CSD) revealed crucial information about the spatial relationship between key lithologies, which will be further used to understand the evolution of CSD in the 4th dimension, time.

The Boliden gold-rich volcanogenic massive sulfide deposit, Skellefte district, Sweden: new U–Pb age constraints and implications at deposit and district scale

The Boliden deposit (8.3 Mt at 15.9 g/t Au) is interpreted to have been formed between ca. 1894 and 1891 Ma, based on two new U–Pb ID-TIMS ages: a maximum age of 1893.9 + 2.0/−1.9 Ma obtained from an altered quartz and feldspar porphyritic rhyolite in the deposit footwall in the volcanic Skellefte group and a minimum age of 1890.8 ± 1 Ma obtained from a felsic mass-flow deposit in the lowermost part of the volcano-sedimentary Vargfors group, which forms the stratigraphic hanging wall to the deposit. These ages are in agreement with the alteration and mineralization being formed at or near the sea floor in the volcanogenic massive sulfide environment. These two ages and the geologic relationships imply that: (1) volcanism and hydrothermal activity in the Skellefte group were initiated earlier than 1.89 Ga which was previously considered to be the onset of volcanism in the Skellefte group; (2) the volcano-sedimentary succession of the Vargfors group is perhaps as old as 1892 Ma in the eastern part of the Skellefte district; and (3) an early (synvolcanic) deformation event in the Skellefte group is evidenced by the unconformity between the ≤1893.9 + 2.0/−1.9 Ma Skellefte group upper volcanic rocks and the ≤1890.8 ± 1 Ma Vargfors sedimentary and volcanic rocks in the Boliden domain. Differential block tilting, uplift, and subsidence controlled by synvolcanic faults in an extensional environment is likely, perhaps explaining some hybrid VMS-epithermal characteristics shown by the VMS deposits of the district.

Pre-1.87 Ga development of crustal domains overprinted by 1.87 Ga transpression in the Palaeoproterozoic Skellefte district, Sweden

The complex structural evolution within the VMS-hosting Skellefte district, Sweden, has been investigated to provide a solid structural framework for the known mineral deposits in the area. The area occurs in a transition zone between dominantly N–S to NNE–WSW striking structures in the north and approximately WNW–ESE oriented structural trends in the south. The presence of high-strain zones with both the above orientations in the Skellefte district allows constraining their mutual relationship, as well as their significance for the build-up of the Svecokarelian orogen at around 1.89Ga and for the following tectonic overprint between 1.87 and 1.80Ga. The methods used in this study include structural analysis complemented by potential field modeling and SIMS U–Pb geochronology on zircon. Based on the results of this study, the earliest deformation (D1) is constrained at 1.89–1.88 (1.87)Ga and tentatively attributed to crustal extension occurring synchronously with volcanism. Deposition of the Skellefte Group metavolcanic rocks is inferred to have occurred in a pull-apart basin developed due to dextral strike-slip shearing along approximately N–S striking regional-scale shear zones. Variations in the development of deformation fabric across the district indicate that the crust was divided into an upper, un-metamorphosed domain and a lower, strongly metamorphosed domain during D1. We further infer that the transition from the upper to lower crust was locally coupled with development of low-angle crustal-scale detachment zones during D1. The heterogenous crust was subsequently overprinted by transpressional deformation which may be explained by two alternative models. According to the first model, one single SSE–NNW transpressional event with distinct strain partitioning between the coaxially deformed upper crust and the non-coaxially deformed lower crust is largely responsible for the present-day structural geometry. A post-folding rhyolite dyke, here dated at 1871±4Ma, constrains the minimum age of this event (D2). The alternative model includes two separate transpressional events: a SW–NE one at (1.88–) 1.87Ga, followed by SSE–NNW transpression at 1.86Ga. Recognition of the early-orogenic detachment zones allow us to suggest that many of the major crustal-scale shear zones in the central Fennoscandian Shield have originated as 1.89–1.87Ga crustal detachment zones, i.e. earlier than typically considered.

Geophysical modelling of the central Skellefte district, Northern Sweden; an integrated model based on the electrical, potential field and petrophysical data

Abstract The central Skellefte district (CSD) is a part of a major ore-bearing district in northern Sweden. Studying the depth and patterns of the contact relationship between the two major stratigraphic units of the CSD, the Skellefte Group and the Vargfors Group, is a key issue to understand the geometry and structure of the area and to guide exploration of base metals. In this study, we interpret geoelectrical data collected along two profiles and magnetic and gravity data obtained from the database of the Swedish Geological Survey (SGU) and Boliden Mineral, to reveal contact relationship and depth extension of the major geological structures. Petrophysical analyses of the different lithologies were conducted on samples from the database of the SGU. Electric resistivity, induced polarisation (IP), magnetic susceptibility and density were determined on 154 core samples representing the different lithologies of the area. The resistivity/IP data were acquired to define structural relations down to a maximum depth of ~ 430 m. The major contact between sediments of the Vargfors basin and volcanic rocks of the Skellefte Group were outlined from the inversion of the resistivity/IP sections, suggesting a synform boundary between the Vargfors Group and Skellefte Group. The contact relationship between the felsic and mafic volcanic rocks of the Skellefte Group is also understood with the help of the resistivity/IP data. The resistivity models were tested using the magnetic data and magnetic susceptibility inferred on the resistivity bodies. The result suggests a good correlation between the initial resistivity model and the magnetic and gravity field calculated from that model. The integration and interpretation of geological and geophysical data improved the basic understanding of the geometry of CSD. Based on previous geological investigations, the potential ore deposits are believed to be found along the volcano-sedimentary contact. The result from this study can thus be used for the base metal exploration, finding the locations of potential sulphide deposits and give a better understanding about spatial relationship between different geology structures in the CSD.

Crustal geometry of the central Skellefte district, northern Sweden – constraints from reflection seismic investigations

The Palaeoproterozoic Skellefte mining district in Sweden is one of the most important mining districts in Europe. As a part of a 4D geologic modeling project, three new sub-parallel reflection seismic profiles, with a total length of about 95km, were acquired in the central part of the district. Processed seismic data reveal a series of gentle- to steeply- dipping reflections and a series of diffraction packages. The majority of reflections that extend to the surface can be correlated with geological features either observed in the field or interpreted from the aeromagnetic map. A set of south-dipping reflections represent inferred syn-extensional listric extensional faults that were inverted during subsequent crustal-shortening. Cross-cutting north-dipping reflections are correlated to late-compressional break-back faults. Flat-lying reflections in the central parts of the study area could represent lithological contacts within the Skellefte Group, or the contact between Skellefte Group rocks and their unknown basement. Flat-lying reflections occurring further north are inferred to originate from the top of the Jörn intrusive complex or an intrusive contact within it. So far unknown south- and north-dipping faults have been identified in the vicinity of the Maurliden deposit. Based on the seismic results, a preliminary 3D-model has been created in order to visualize the fault pattern and to provide a base for future 3D/4D modeling in the Skellefte district.

New zircon data supporting models of short-lived igneous activity at 1.89 Ga in the western Skellefte District, central Fennoscandian Shield

Abstract. New U-Th-Pb zircon data (SIMS) from three intrusive phases of the Palaeoproterozoic Viterliden intrusion in the western Skellefte District, central Fennoscandian Shield, dates igneous emplacement in a narrow time interval at about 1.89 Ga. A locally occurring quartz-plagioclase porphyritic tonalite, here dated at 1889 ± 3 Ma, is considered the youngest of the intrusive units, based on the new age data and field evidence. This supports an existing interpretation of its fault-controlled emplacement after intrusion of the dominating hornblende-tonalite units, in this study dated at 1892 ± 3 Ma. The Viterliden magmatism was synchronous with the oldest units of the Jörn type early-orogenic intrusions in the eastern part of the district (1.89–1.88 Ga; cf. Gonzàles Roldán, 2010). A U-Pb zircon age for a felsic metavolcanic rock from the hanging-wall to the Kristineberg VMS deposit, immediately south of the Viterliden intrusion, is constrained at 1883 ± 6 Ma in this study. It provides a minimum age for the Kristineberg ore deposit and suggests contemporaneous igneous/volcanic activity throughout the Skellefte District. Furthermore, it supports the view that the Skellefte Group defines a laterally continuous belt throughout this "ore district". Tentative correlation of the 1889 ± 3 Ma quartz-plagioclase porphyritic tonalite with the Kristineberg "mine porphyry" suggests that these units are coeval at about 1.89 Ga. Based on the new age determinations, the Viterliden intrusion may equally well have intruded into or locally acted as a basement for the ore-hosting Skellefte Group volcanic rocks.

Palaeoproterozoic mineralized volcanic arc systems and tectonic evolution of the Fennoscandian Shield: Skellefte District Sweden

The Skellefte 1.9 Ga volcanic arc in northern Sweden is one of the most mineralized (VMS, orogenic gold, mafic hosted Ni, porhyry style Cu-Au) Palaeoproterozoic arc systems in the world. The Skellefte District is interpreted to have accreted, or formed as a continental volcanic arc system, during accretionary processes related to the Svecokarelian Orogeny. Based on Sm-Nd isotope studies it has been concluded that the basement to the ore-bearing Skellefte Group cannot be much older than the volcanic arc and was thus probably juvenile Palaeoproterozoic crust. The basement is not known to outcrop and recently it was speculated, based on high resolution seismic work in the western part of the district, that the basement is dipping gently northwards beneath the ore-bearing Skellefte Group. It was further postulated from these studies that the basement could at least partly constitute the Bothnian supergroup, metasedimentary rocks that outcrop south of the Skellefte District. Part of this supergroup has been dated at 1.95 Ga. For economic reasons it is extremely important to understand the 3-dimensional extent of the Skellefte Group and this constitutes one direct aim of a future deep drilling proposal. The basic scientific aim for the drilling project is to better understand the accretionary processes that constitute the Svecokarelian Orogeny. One of the best places to study these processes is the Skellefte District where well preserved volcanic rocks form an arc system on the older Karelian Craton margin. A drilling programme in the Skellefte District will thus benefit the exploration and mining industry directly and at the same time address fundamental questions related to the tectonic processes that built the Fennoscandian Shield during one of the most intense orogenic periods in the evolution of the earth, between 1.95 and 1.80 Ga.

Successive ∼1.94 Ga plutonism and ∼1.92 Ga deformation and metamorphism south of the Skellefte district, northern Sweden: Substantiation of the marginal basin accretion hypothesis of Svecofennian evolution

In contrast to the prevailing arc-accretion hypothesis, it has recently been argued that the most extensive deformation and metamorphism (D 1) in the Skellefte region of north-central Sweden took place in older metasedimentary sequences before the eruption of the ∼1.90–1.87 Ga volcanic sequences. The new data presented here provide a basis for a refined hypothesis involving accretion of a pre-1.92 Ga marginal basin during an Early Svecofennian orogenic episode between ∼1.92–1.91 Ga. To the south of the Skellefte district, in a part of the metasedimentary Robertsfors Group dominated by NE (D 1) structural trends, 40 SIMS analyses of zircon from a metasediment (Robertsfors-1) have been made. Fourteen analyses of overgrowths and structureless zircon grains provide strong evidence for a post-depositional metamorphic event dated to 1916 ± 5 Ma , which we correlate with the observed D 1 metamorphism and migmatisation. This age is indistinguishable from that of a corresponding event in the Vammala Migmatite Belt (VMB) in Finland. This is the most important crustal thickening episode in the evolution of the Svecofennian province, here distinguished as the early Svecofennian orogenic episode. Nine Archaean and 15 older Palaeoproterozoic (OP) detrital zircon grains also yielded data comparable to that from the VMB. The age of 1980 ± 6 Ma obtained from the total OP population possibly indicates derivation from a single magmatic source area, with similarly aged metamorphic overgrowths on some of the grains. Two sampled localities to the NE show D 1 schistosity overprinted by D 2 shears and granitic veining. Sample Rönnskär-3, from a relatively homogeneous granitoid, and sample Rönnskär-2, from a garnetiferous felsic layer in well-banded layered gneisses, both contain igneous zircon populations. These have been affected by D 1 and D 2 events but the SIMS data allow the age of magmatism in both rocks to be placed between 1940 and 1950 Ma, thus reinforcing the links to the Knaften/Storuman areas to the WNW. In the granitoid, six analyses from highly luminescent rims, yielding an imprecise age of 1856 ± 25 Ma, are consistent with the earlier more precise 207Pb/ 206Pb dating of the D 2 event at 1858 ± 2 Ma. The D 2 event is also recorded by ID-TIMS analyses of monazite from five widely spaced localities in the Skellefte structural zone with ages ranging from 1849 ± 3 to 1866 ± 2 Ma. The D 2 deformation has similar characteristics in the Skellefte/Robertsfors and Tampere/Vammala areas. We suggest that it can be allocated to a second and Middle Svecofennian orogenic episode between 1.88 and 1.85 Ga, affecting a wide area. It has been demonstrated that the protoliths of the Robertsfors Group were deposited before ∼1.95 Ga; the group was intruded by granitoids between ∼1.94 and 1.95 Ga; and it was strongly deformed and metamorphosed between ∼1.92 and 1.91 Ga. Proximity to the post-1.90 Ga Skellefte Group during the deposition of the latter is indicated for both Robertsfors and Knaften/Storuman areas by the presence of ∼1.90 Ga granitoids and ∼1.88 Ga conglomerates like those in the Skellefte district. The regional evidence for the continuity of metamorphic complexes over wide areas, and the similarity in pre- and post-depositional histories of the Robertsfors Group and the Vammala Migmatite Belt 300 km apart, support the hypothesis that accretion of a large (Svionian) marginal basin at ∼1.92–1.91 Ga provided the basement for the subsequent volcanism.

Reflection Seismic Investigations in the Western Part of the Paleoproterozoic VHMS-Bearing Skellefte District, Northern Sweden

The Skellefte district forms part of the Svecofennian ca. 1.90 to 1.80 Ga, supracrustal sequence and associated intrusive rocks in the northern part of Sweden. The western part of the Skellefte district, which is the most important metallogenic province in northern Sweden today, hosts major volcanic-hosted massive sulfide (VHMS) deposits (e.g., the 23 million metric tons (Mt) Kristineberg Cu-Zn-Pb-Ag-Au deposit). In order to obtain a better understanding of the VHMS ore potential at depth, new seismic reflection data were acquired along two parallel and 25-km-long profiles in the Kristineberg area in 2003. The data were collected with the purpose of obtaining high-resolution images of the top 10 km of the crust and are presented here for the first time. Although the structural setting is very complex, the stacked sections reveal numerous reflections that can be correlated with surface geology. Visible on both profiles is a pronounced north-dipping band of reflections marking a boundary between relatively transparent crust above and significantly more reflective crust beneath it. We interpret this reflective crust to represent a structural basement to the ore-bearing Skellefte Group, possibly constituting Bothnian basin metasedimentary rocks bordering the Skellefte district to the south. This new interpretation is important for the understanding of the tectonic evolution of the Skellefte district and for defining exploration strategies in the area. The seismic results suggest that the Kristineberg and Ravliden deposits occur on the northern limb of a kilometer-scale local second-order syncline within the hinge zone of a major antiform. Results from a profile located approximately 8 km to the west of the Kristineberg mine indicate that the Revsund granitoid has a thickness of about 3 to 3.5 km. Ultramafic rocks are also imaged clearly. Diffraction patterns and bright-spot reflectivity is interpreted as originating from either mafic to ultramafic intrusions or a mineralization zone at 3- to 5-km depth. These results help to identify new prospective areas and mineral potential, both downplunge from known ores and on the same stratigraphic horizon on the southern limb of the ore-bearing syncline. The seismic reflection profiling has been effective in imaging the major structures around the Kristineberg orebody, demonstrating that this technique can be used for delineating complex structures significant for mineral exploration.

The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden. Part II: chemostratigraphy and alteration

The Kristineberg massive sulfide deposit is hosted by metamorphosed volcanic and subvolcanic rocks of the Palaeoproterozoic Skellefte Group. The deposit consists of: (1) two main massive sulfide horizons, the A-ores and B-ores, which dip steeply southwards and are separated by 100–150 m; and (2) the Einarsson Zone, a complex interval of Cu–Au-rich ‘stockwork‘ sulfides and small massive sulfide lenses in altered and deformed rocks near the 1,000 m level. The Einarsson Zone occurs some 20–100 m south of the B-ores. There are no definite younging indicators in the mine sequence. In many areas of the mine, the original host rocks are impossible to identify petrographically due to the abundance of secondary minerals such as quartz, chlorite, muscovite, cordierite, andalusite, phlogopite, pyrite and talc, combined with variably schistose fabrics. Application of immobile-element methods to 600 recent whole-rock chemical analyses has, however, allowed the original rock types to be identified and correlated. Rhyolite X lies immediately north of the A-ore, while andesitic to dacitic to rhyodacitic rocks make up the 100–150 m interval between the A-ore and B-ore, and massive rhyolite A lies immediately south of the B-ore. The felsic rocks are mostly of calc-alkaline affinity, excluding rhyolite X, which is transitional. The mine porphyry, which lies north of the A-ore and forms the marginal phase of the synvolcanic Viterliden Intrusive Complex, is compositionally similar to dacite and rhyodacite. Mass changes calculated for all rock types indicate that most of the volcanic rocks in the mine area are strongly depleted in Na and Ca, and have gained variable amounts of Mg and Fe, whereas Si changes range from negative to positive. Gains in Fe and changes in Si are largest within 5–10 m of the massive sulfide lenses. Cordierite-bearing schists of andesitic to felsic compositions that lie between massive sulfide lenses A and B are not as altered. The Einarsson Zone commonly shows large gains in Fe and Mg, while Si shows large gains to large losses. Immobile-element ratios indicate that very different secondary assemblages in the mine, e.g. andalusite–quartz–muscovite and cordierite–chlorite–talc, can be produced from the same precursor volcanic unit, e.g., rhyolite. Conversely, the same secondary mineral assemblage can be produced from different rocks, e.g. weakly altered andesite and strongly altered rhyolite. The common presence of cordierite + andalusite in the mine area, without anthophyllite, is unusual in the alteration systems of volcanic-hosted massive sulfide deposits, and is proposed to have formed by the metamorphic reaction of the synvolcanic alteration minerals kaolinite and chlorite to produce cordierite. Where kaolinite was in excess of chlorite, andalusite was also formed. We propose that highly acidic alteration fluids locally produced high-Al minerals such as kaolinite that either overprinted, or occurred in place of, a more typical sericite–chlorite–quartz alteration assemblage that otherwise formed near the massive sulfide lenses. Application of lithogeochemical methods to the altered, deformed and metamorphosed Kristineberg rocks has identified specific volcanic contacts with massive sulfide potential, and quantified the effects of synvolcanic hydrothermal alteration. Such an approach can increase the effectiveness of mineral exploration in metamorphosed terrains.

The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden, part I: geology

The Kristineberg volcanic-hosted massive sulphide (VMS) deposit, located in the westernmost part of the Palaeoproterozoic Skellefte district, northern Sweden, has yielded 22.4 Mt of ore, grading 1.0% Cu, 3.64% Zn, 0.24% Pb, 1.24 g/t Au, 36 g/t Ag and 25.9% S, since the mine opened in 1941, and is the largest past and present VMS mine in the district. The deposit is hosted in a thick pile of felsic to intermediate and minor mafic metavolcanic rocks of the Skellefte Group, which forms the lowest stratigraphic unit in the district and hosts more than 85 known massive sulphide deposits. The Kristineberg deposit is situated lower in the Skellefte Group than most other deposits. It comprises three main ore zones: (1) massive sulphide lenses of the A-ore (historically the main ore), having a strike length of about 1,400 m, and extending from surface to about 1,200 m depth, (2) massive sulphide lenses of the B-ore, situated 100–150 m structurally above the A-ore, and extending from surface to about 1,000 m depth, (3) the recently discovered Einarsson zone, which occurs in the vicinity of the B-ore at about 1,000 m depth, and consists mainly of Au–Cu-rich veins and heavily disseminated sulphides, together with massive sulphide lenses. On a regional scale the Kristineberg deposit is flanked by two major felsic rock units: massive rhyolite A to the south and the mine porphyry to the north. The three main ore zones lie within a schistose, deformed and metamorphosed package of hydrothermally altered, dominantly felsic volcanic rocks, which contain varying proportions of quartz, muscovite, chlorite, phlogopite, pyrite, cordierite and andalusite. The strongest alteration occurs within 5–10 m of the ore lenses. Stratigraphic younging within the mine area is uncertain as primary bedding and volcanic textures are absent due to strong alteration, and tectonic folding and shearing. In the vicinity of the ore lenses, hydrothermal alteration has produced both Mg-rich assemblages (Mg-chlorite, cordierite, phlogopite and locally talc) and quartz–muscovite–andalusite assemblages. Both types of assemblages commonly contain disseminated pyrite. The sequence of volcanic and ore-forming events at Kristineberg is poorly constrained, as the ages of the massive rhyolite and mine porphyry are unknown, and younging indicators are absent apart from local metal zoning in the A-ores. Regional structural trends, however, suggest that the sequence youngs to the south. The A- and B-ores are interpreted to have formed as synvolcanic sulphide sheets that were originally separated by some 100–150 m of volcanic rocks. The Einarsson zone, which is developed close to the 1,000 m level, is interpreted to have resulted in part from folding and dislocation of the B-ore sulphide sheet, and in part from remobilisation of sulphides into small Zn-rich massive sulphide lenses and late Au–Cu-rich veins. However, the abundance of strongly altered, andalusite-bearing rocks in the Einarsson zone, coupled with the occurrence of Au–Cu-rich disseminated sulphides in these rocks, suggests that some of the mineralisation was synvolcanic and formed from strongly acidic hydrothermal fluids.

Post-deformation, sulphide-quartz vein hosted gold ore in the footwall alteration zone of the Palaeoproterozoic Långdal VHMS deposit, Skellefte District, northern Sweden

The Palaeoproterozoic, c. 1.88 Ga old Långdal VHMS deposit is situated in the eastern part of the Skellefte District, northern Sweden. In the stratigraphic footwall to the VHMS ore a sulphide-quartz vein system with high gold grades was mined in the second half of the 1990′s. The Långdal VHMS ore is hosted by the uppermost part of the Skellefte Group volcanic rocks, close to the contact with an overlying fine-grained sedimentary unit. Regional structural studies indicate that bedding surfaces in volcanic rocks are parallel to the contact between the volcanic and the sedimentary rocks. Based on the differences in structural style on each side, the contact is interpreted as a major structural break. The Långdal ore is situated close to this break that may have focussed fluid flow during metamorphism and deformation. The orientation of the contact indicates that it either is a D2 structure or that it was at least active during D2. The structural development in the altered footwall rocks to the Långdal VHMS ore indicates that gold-bearing sulphide and sulphide-quartz veins both pre- and post-date the first deformation. Gold associated with the vein system can thus not only be attributed to syngenetic exhalative or replacement processes. The close spatial relationship with the massive sulphide deposits suggests, however, that the gold was remobilized from these syngenetic systems. It is concluded that sulphides were introduced at several stages during the geological evolution of the area as: a) syngenetic disseminations of sulphide and folded, pre-S1 stringer sulphide±quartz veins in the footwall related to the syngenetic VHMS deposit, b) syn-S1 sulphide veins in the footwall gold ore, c) main, post-S1, sulphide-quartz veins associated with the gold ore in the footwall rocks to the Långdal VHMS deposit, and d) post-S1 to pre-S2 galena and sphalerite rich veins post-dating the main, post-S1, sulphide-quartz vein system in the footwall to the Långdal ore. From these relationships it is suggested that gold was re-mobilized from the sulphide rich parts of the VHMS system into post-D1 structures during or slightly after the peak metamorphism. The timing of this event is poorly constrained to post-date the syngenetic ore emplacement by 20–80 m.y.

Mineral-chemical studies of metamorphosed hydrothermal alteration in the Kristineberg volcanogenic massive sulfide district, Sweden

The massive sulfide deposits of the Kristineberg area, Sweden, occur within a 2- to 3-km-thick succession of felsic volcaniclastic rocks belonging to the Skellefte Group. The volcanic pile is intruded by a synvolcanic Jorn-type granitoid (Viterliden intrusive complex) and is overlain by a thick sequence of metasedimentary rocks (Vargfors Group). Mineralization occurs at two main stratigraphic levels, at the base of the felsic volcanic succession and at the contact with the metasedimentary rocks of the Vargfors Group. The Kristineberg Cu–Zn mine is the largest deposit (approximately 21 Mt) and occurs at the base of the volcanic pile, close to the contact with the Viterliden intrusive complex. Four smaller deposits (Ravliden, Ravlidmyran, Horntrask and Nyliden) occur along the upper ore horizon. These deposits are thought to be related to a late intrusive phase of the Viterliden complex which cuts the altered volcanic rocks at the Kristineberg deposit. Within an area of about 50 km2 surrounding the Kristineberg deposit, felsic volcanic rocks between the two ore horizons are affected by extensive albite-destructive alteration (sodium depletion) and development of chlorite and muscovite (strong co-enrichment in magnesium and potassium). The Kristineberg deposit is enveloped by a large and partly transposed quartz–chlorite alteration zone, approximately 2 km in diameter, and a distal but coherent pyrite–quartz–muscovite alteration zone extending as far as 4 km from the deposit. Chlorite(±talc) in the mine area is notably magnesium-rich and contains anomalous F, Ba, Zn and Mn. High fluorine is also present in coexisting muscovite and phlogopite. The magnesium-rich chlorite alteration contrasts sharply with the iron enrichment observed in many other felsic, volcanic-hosted Precambrian massive sulfide deposits. This may indicate fixation of iron by large amounts of pyrite in the section or entrainment of large amounts of seawater in the hydrothermal upflow zones. Kyanite is developed locally in the chlorite-rich pipe at Kristineberg in response to regional thermal metamorphism of highly aluminous alteration in the immediate foot-wall rocks. Spectacular, andalusite-bearing quartz–muscovite schists and quartz–biotite–cordierite schists also occur where the altered felsic volcanic rocks are intruded by the late Revsund granite. However, similar metamorphic mineral growth is not observed where the volcanic rocks at the contact are less altered. Deposits near the top of the felsic volcanic succession are characterized by magnesium-rich chlorite alteration in the foot wall and proximal calc-silicate assemblages (dolomite, calcite, tremolite, ±garnet, ±margarite) where the host sedimentary rocks are carbonate-rich. In general, the calc-silicate alteration is restricted to the immediate hanging wall and zones lateral to the deposits and does not represent a regionally extensive exploration target. The two main ore horizons in the Kristineberg area are not linked by any obvious discordant structures or alteration zones. However, mineral-chemical studies highlight several possible fluid flow pathways leading from the Kristineberg deposit to the Ravliden ore horizon, more than 2 km upsection. Overprinting regional metamorphic minerals have inherited the hydrothermal signature of the ore-related alteration. Electronic Supplementary Material is available at http://dx.doi.org/10.1007/s00126-002-0299-y. On that page (frame on the left side), a link takes you directly to the supplementary material.

Nature of a major tectonic discontinuity in the Svecofennian province of northern Sweden

The volcanic rocks of the Skellefte ore district (ca. 1.89–1.87 Ga), like other volcanic complexes of similar age in the Svecofennian orogenic province, have been regarded as juvenile arc material developed over a contemporaneous subduction zone, and subsequently accreted to the continent, during a period of very rapid crustal growth. A modelled geoelectric anomaly, dipping NE beneath the district from the southern province, has provided support for the subduction/ collision hypothesis. We have used recently acquired geological and geophysical evidence to suggest an alternative to this model. Regional aeromagnetic and gravity data indicate that the Skellefte district is the site of a major tectonic discontinuity. The province to the north of this discontinuity provides evidence of an Archaean basement to within 100 km of Skellefteå. The Skellefte Group, on the southern margin of this province, displays a narrow belt of strong upright folding, trending W to WNW. We correlate this folding with the second major deformation episode (D 2), of similar trend, in the metamorphic complex of the southern province (here informally called the Robertsfors Group). The family of D 2 structures, on both sides of the province boundary, defines the 80 km wide Skellefte Zone. D 2 in the Robertsfors Group consists of discrete ductile shear zones and associated upright folds. These zones are consistently steep, display southside up kinematics, and are superposed on earlier NE trending folding (D 1) and migmatisation in the Robertsfors Group. We infer from the style of D 2 that it could not be responsible for the accretion of an exotic volcanic arc to the continent. Upper crustal thickening was probably largely achieved during D 1. We conclude from the style and orientation of the D 1 structures in the Robertsfors Group, their absence from the Skellefte Group, and their relation to granitoid intrusions, that D 1 pre-dates deposition of the Skellefte Group. We infer that the Robertsfors Group forms part of the previously unknown basement of the Skellefte Group. The latter is therefore regarded as a supracrustal sequence, deposited in a rift setting, rather than a juvenile volcanic arc. It is also concluded that the basement stratigraphic package, which also occurs to the southwest of the Skellefte ore district, and has variable internal structures, is responsible for the modelled geoelectric anomaly. We therefore interpret the tectonic discontinuity as the boundary between pre- and post-1.9 Ga rock groups, both of which have been intruded by post-1.9 Ga granitoids and affected by the localised upright D 2 (post-1.87 Ga) deformation in the Skellefte Zone. The volcanism of the Skellefte ore district and the Ni bearing mafic intrusions of the southern province are attributed to an episode of extension, between D 1 and D 2, which was probably related to a contemporaneous active margin lying west of the present exposure of Svecofennian rocks. D 1 in the Robertsfors Group is attributed to the closure of an extensive marginal basin, possibly involving subduction, prior to Skellefte Group deposition (i.e. before 1.9 Ga). Geochronological data from the Robertsfors Group on the ages of D 1 and D 2, and the regional significance of pre-1.9 Ga metamorphic complexes, are discussed in a companion paper.

Age of deformation episodes in the Palaeoproterozoic domain of northern Sweden, and evidence for a pre-1.9 Ga crustal layer

It is proposed in a companion paper that the volcanic rocks of the Skellefte Group in northern Sweden were deposited on a pre-1.9 Ga basement, rather than formed as a juvenile volcanic arc and subsequently accreted to the continent. It is suggested that the proposed basement, the Robertsfors Group to the south of Skellefteå, was already deformed during an early episode of deformation (D 1), prior to the deposition of the Skellefte stratigraphic sequence during an extensional tectonic episode. A younger episode of deformation, D 2, then affected both areas. This hypothesis has important implications for the evolution of the Svecofennian Province. To test it, by constraining the ages of the principal deformation episodes, we have undertaken both isotope dilution and ion microprobe studies of zircon and monazite from three localities in the Robertsfors Group. A phase of migmatisation directly related to D 2 shear zones allows the dating of D 2 at ≈1860 Ma. Other planar granitoid bodies, which cut the D 1 structures, confirm the age of this event. In the context of earlier published data, we conclude that the correlation of D 2 in the Robertsfors Group with the main folding in the Skellefte Group is valid. Another granitoid body, folded by D 2 but emplaced after D 1, contains a suite of zircons of apparent magmatic aspect, but with a spread of ages from 1870 to 1900 Ma. Because of the high MSWD, the suite was divided into two groups with ages of ≈1896 and 1874 Ma. Recognising the possibilities of inheritance and of updating by younger events, we have preferred to interpret the younger age as a minimum age for magmatic intrusion. This is consistent with the basement hypothesis but is our only definite constraint on the age of D 1. However, we infer, using other evidence, that D 1 is probably older than 1900 Ma. Xenocrysts, interpreted as detrital zircons derived from the associated metasediments, give ages between 2716 and 1941±20 Ma. The latter age provides a maximum age of deposition for part of the Robertsfors Group and a maximum age for D 1. Other sequences, to the south and west, considered broadly equivalent to the Robertsfors Group, are intruded by granitoids dated between ≈1920 and 1960 Ma, providing local minimum ages of deposition for the sequences involved. Regional correlation with respect to age and deformation style can be made with the pre-1.9 Ga Kalevian rocks to the NE, deposited on Archaean basement on the SW margin of the Karelian Province. We suggest that pre-1.9 Ga complexes, equivalent to the Robertsfors Group, are widespread in the Svecofennian Province, and were deposited in a marginal basin which may have been initiated as early as 2.2 Ga ago. This hypothesis of an extensive basement crustal layer evidently requires that a substantial volume of crustal growth occurred prior to 1.9 Ga, in contrast to previous hypotheses. Erosion of the pre-1.9 Ga basin rocks after D 1 is also the likely source of the 2.1–1.9 Ga detrital zircons in younger sediments associated with the overlying post-1.9 Ga volcanics. The source of these detrital zircons has previously been problematical.

Age and provenance of host rocks and ores in the Paleoproterozoic Skellefte District, northern Sweden

The Skellefte district in northern Sweden is a ca. 1.9 Ga, extensively mineralized, mainly felsic, submarine volcanic belt. Within the district, the volcanic rocks (Skellefte Group) are overlain by ...

The geochemistry of early Proterozoic metavolcanic rocks hosting massive sulphide deposits in the Skellefte district, northern Sweden

The Skellefte district, an early Proterozoic massive sulphide ore province in Northern Sweden, contains a well-preserved volcano-sedimentary rock association, referred to as the Skellefte Group, metamorphosed predominantly under low-grade conditions. The volcanic rocks in the lower part of the succession host the massive sulphide deposits and comprise a bimodal sequence, composed of a calc-alkaline group of felsic rocks and a tholeiitic group of mafic rocks displaying a basaltic to andesitic composition. Derivation of the felsic rocks by fractional crystallization from the mafic rocks is not supported by the geochemical data. Application of tectonic environment discriminant diagrams involving the elements Ti, Zr, Y and Cr suggests that these rocks were deposited in a volcanic arc setting. Stratigraphically above the volcanic rocks hosting the massive sulphide deposits, there follows a sequence of metamorphosed greywackes and pelitic sediments with intercalation of primitive basaltic to ultrabasic volcanic and high-level intrusive rocks. The volcanic rocks show geochemical similarities to both Archaean basaltic komatiites and Phanerozoic boninites. The primitive nature of these rocks present difficulties in assessing their palaeotectonic setting. A volcanic arc or transition to an inter- or back-arc rift situation is suggested.