| 1. |
- Allen, Ann, et al.
(författare)
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Geochemistry as a Tool for Exploration at the Renstrom Zn-Pb-Cu-Au-Ag VMS Camp, Skellefte District, Sweden
- 2015
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Ingår i: Mineral Resources in a Sustainable World. - 9782855550664 ; , s. 2047-2050
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Konferensbidrag (refereegranskat)abstract
- The Skellefte mining district in northern Sweden contains over 85 pyritic Zn-Cu-Au-Ag massive sulphide deposits. The Renstrom area, one of the most intensely mineralized parts of the Skellefte district, contains five zinc-and gold-rich deposits, three of which are confined to a specific continuous stratigraphic unit, the "Renstrom ore host unit". The great structural complexity of the area made it difficult to locate and follow the ore horizon to generate new exploration targets. A new study in the Kyrkvagen area, based on stratigraphic correlations, structural interpretations and lithogeochemical and geophysical data interpretation, revealed several NW-trending faults which separate five structural blocks. The rocks of the area could be characterized in terms of geochemistry, stratigraphy and their position in the hanging or footwall with respect to the ore horizon. Moreover, alteration patterns allowed predictions of possible extensions of the ore horizon. This increased knowledge of the Kyrkvagen area led to the identification of five new drilling targets for further exploration in one of Boliden's most important mining areas.
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| 10. |
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| 11. |
- Allen, Rodney, et al.
(författare)
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Global comparisons of volcanic-associated massive sulphide districts
- 2002
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Ingår i: The timing and location of major ore deposits in an evolving Orogen. - London : Geological Society of London. - 186239122X ; , s. 13-37
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- Although volcanic-associated massive sulphide (VMS) deposits have been studied extensively, the geodynamic processes that control their genesis, location and timing remain poorly understood. Comparisons among major VMS districts, based on the same criteria, have been commenced in order to ascertain which are the key geological events that result in high-value deposits. The initial phase of this global project elicited information in a common format and brought together research teams to assess the critical factors and identify questions requiring further research. Some general conclusions have emerged. (1) All major VMS districts relate to major crustal extension resulting in graben subsidence, local or widespread deep marine conditions, and injection of mantle-derived mafic magma into the crust, commonly near convergent plate margins in a general back-arc setting. (2) Most of the world-class VMS districts have significant volumes of felsic volcanic rocks and are attributed to extension associated with evolved island arcs, island arcs with continental basement, continental margins, or thickened oceanic crust. (3) They occur in a part of the extensional province where peak extension was dramatic but short-lived (failed rifts). In almost all VMS districts, the time span for development of the major ore deposits is less than a few million years, regardless of the time span of the enclosing volcanic succession. (4) All of the major VMS districts show a coincidence of felsic and mafic volcanic rocks in the stratigraphic intervals that host the major ore deposits. However, it is not possible to generalize that specific magma compositions or affinities are preferentially related to major VMS deposits world-wide. (5) The main VMS ores are concentrated near the top of the major syn-rift felsic volcanic unit. They are commonly followed by a significant change in the pattern, composition and intensity of volcanism and sedimentation. (6) Most major VMS deposits are associated with proximal (near-vent) rhyolitic facies associations. In each district, deposits are often preferentially associated with a late stage in the evolution of a particular style of rhyolite volcano. (7) The chemistry of the footwall rocks appears to be the biggest control on the mineralogy of the ore deposits, although there may be some contribution from magmatic fluids. (8) Exhalites mark the ore horizon in some districts, but there is uncertainty about how to distinguish exhalites related to VMS from other exhalites and altered, bedded, fine grained tuffaceous rocks. (9) Most VMS districts have suffered fold-thrust belt type deformation, because they formed in short-lived extensional basins near plate margins, which become inverted and deformed during inevitable basin closure. (10) The specific timing and volcanic setting of many VMS deposits, suggest that either the felsic magmatic-hydrothermal cycle creates and focuses an important part of the ore solution, or that specific types of volcanism control when and where a metal-bearing geothermal solution can be focused and expelled to the sea floor, or both. This and other questions remain to be addressed in the next phase of the project. This will include in-depth accounts of VMS deposits and their regional setting and will focus on an integrated multi-disciplinary approach to determine how mineralisation, volcanic evolution and extensional tectonic evolution are interrelated in a number of world-class VMS districts.
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| 12. |
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| 13. |
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| 14. |
- Allen, Rodney, et al.
(författare)
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Links between volcanism and the distribution and timing of massive sulphide (VMS) deposits
- 2006
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Ingår i: The 27th Nordic Geological Winter Meeting, January 9-12, 2006, Oulu, Finland. - Helsinki : Geological Society of Finland. ; , s. 7-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The links between volcanism and massive sulphide deposits are being studied as part of the "Global Volcanic-hosted Massive Sulphide (VMS) Project", which is IGCP project 502. Different types and settings of VMS deposit show different degrees of influence from volcanic or magmatic processes, with the most distinct genetic connection shown by some felsic-hosted deposits. These influences include:(1) Basin-wide volcano-tectonic events cause deposition of VMS on specific time-stratigraphic horizons. (2) With the exception of mid-ocean ridge settings, major VMS deposits are mainly associated with felsic volcanic rocks, even where felsic rocks form a minor component of the region. (3) Most VMS deposits form in proximal volcanic settings. (4) Most VMS deposits form at a particular stage in the evolution of their host volcanoes, typically late in the magmatic-hydrothermal cycle following a significant felsic eruptive event. The specific relationship in time and place implied by these last two points indicate that either the magmatic-hydrothermal cycle creates an important part of the ore solution, or controls when and where a metal-bearing geothermal solution can be focused and expelled to the sea floor, or both.(5) VMS deposits occur preferentially at times and places where both felsic and mafic magmas were erupted. In felsic-dominated regions, eruption of the mafic rocks commonly closely followed deposition of the ore-host felsic package. (6) Volcanic host rocks influence the morphology and stratigraphic position of VMS. Volcaniclastic and especially pumiceous strata promote deposition of VMS below the sea floor via replacement, whereas coherent lava flows and intrusions promote deposition of VMS on the sea floor. (7) Volcanic rocks and/or magmas are probably the source of metals in most VMS deposits.
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| 15. |
- Allen, Rodney, et al.
(författare)
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Setting of Zn-Cu-Au-Ag massive sulfide deposits in the evolution and facies architecture of a 1.9 Ga marine volcanic arc : Skellefte district, Sweden
- 1996
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - : Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 91:6, s. 1022-1053
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Tidskriftsartikel (refereegranskat)abstract
- Skellefte mining district occurs in an Early Proterozoic, mainly 1.90-1.87 Ga (Svecofennian) magmatic province of low to medium metamorphic grade in the Baltic Shield in northern Sweden. The district contains over 85 pyritic Zn-Cu-Au-Ag massive sulfide deposits and a few vein Au deposits and subeconomic porphyry Cu-Au-Mo deposits, The massive sulfide deposits mainly occur within, and especially along the top of: a regional felsic-dominant volcanic unit attributed to a stage of intense, extensional, continental margin are volcanism. From facies analysis we interpret the paleogeography of this stage to have comprised many scattered islands and shallow-water areas. surrounded by deeper seas. All the major massive sulfide ores occur in below-wave base facies associations: however, some ores occur close to stratigraphic intervals of above-wave base facies associations, and the summits of some volcanoes that host massive sulfides emerged above sea level. Intense marine volcanism was superceded at different times in different parts of tile district by a stage of reduced volcanism, uplift resulting in subregional disconformities, and then differential uplift and subsidence resulting in a complex horst and graben paleogeography. Uplift of the are is attributed to the relaxation of crustal extension and the emplacement of granitoids to shallow crustal levels. A few massive sulfide ores formed within the basal strata of this second stage. The horst and graben system was filled by prograding fluvial-deltaic sediments and mainly mafic lavas, and during this stage the Skellefte district was a transitional area between renewed are volcanism of more continental character to the north, and subsidence and basinal mudstone-turbidite sedimentation to the south. This whole volcanotectonic cycle occurred within 10 to 15 m.y. We define 26 main volcanic, sedimentary, and intrusive facies in the Skellefte district. The most abundant facies are (1) normal-graded pumiceous breccias, which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris, (2) porphyritic intrusions, and (3) mudstone and sandstone turbidites. Facies associations define seven main volcano types, which range from basaltic shields to andesite cones and rhyolite calderas. Despite this diversity of volcano types, most massive sulfide ol es are associated with one volcano type: subaqueous rhyolite cryptodome-tuff volcanoes. These rhyolite volcanoes are 2 to 10 km in diameter, 250 to 1,200 m thick at the center, and are characterized by a small to moderate volume rhyolitic pyroclastic unit, intruded by rhyolite cryptodomes, sills, and dikes. Massive sulfide ores occur near the top of the proximal (near vent) facies association The remarkable coincidence in space and time between the ores and this volcano type indicates an intimate, genetic relationship between the ores and the magmatic evolution of the volcanoes.Many of the massive sulfide ores occur within rapidly emplaced volcaniclastic facies and are interpreted to have formed by infiltration and replacement of these facies. Some of the ore deposits have characteristics of both marine massive sulfides and subaerial epithelial deposits. We suggest that massive sulfides in the Skellefte district span a range in ore deposit style from deep-water sea floor ores, to subsea-floor replacements, to shallow-water and possible subaerial synvolcanic replacements. Facies models are provided for the mineralized rhyolite volcanoes and volcanological guides are provided for exploration for blind ores within these volcanoes.
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| 16. |
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| 17. |
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| 18. |
- Bauer, Tobias, et al.
(författare)
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3D-modelling of the Central Skellefte District, Sweden
- 2009
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Ingår i: Smart science for exploration and mining. - : James Cook University of North Queensland. - 9780980558685
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Konferensbidrag (refereegranskat)abstract
- The central part of the Palaeoproterozoic Skellefte District in northern Sweden is host to several VMS deposits. This area is dominated by upright folds with axial surfaces trending WNW - ESE. Northeast - SW trending faults crosscut WNW - ESE trending faults and impart a distinct fault pattern. Subvertical stretching as expressed by subvertical mineral lineations as well as gently W-plunging mineral lineations parallel to the F2 fold axes indicate not only significant vertical movement, but also pronounced lateral movement. The faults formed in an extensional stage and were reactivated during a compressional stage oblique to the earlier phase. This crustal shortening caused folding and development of the main foliation. Overturned, tight to isoclinal folds within the Vargfors meta-sediments coincide with 1st and 2nd order faults and are considered to be related to reactivation of the early normal and transfer faults. A three dimensional model taking into account the structures was constructed using the GoCAD 3D-modelling software.
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| 19. |
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| 20. |
- Bauer, Tobias, et al.
(författare)
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Correlation between distribution and shape of VMS deposits, and regional deformation patterns, Skellefte district, northern Sweden
- 2014
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Ingår i: Mineralium Deposita. - : Springer Science and Business Media LLC. - 0026-4598 .- 1432-1866. ; 49:5, s. 555-573
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Tidskriftsartikel (refereegranskat)abstract
- The Skellefte district in northern Sweden is host to abundant volcanogenic massive sulphide (VMS) deposits comprising pyritic, massive, semi-massive and disseminated Zn–Cu–Au ± Pb ores surrounded by disseminated pyrite and with or without stockwork mineralisation. The VMS deposits are associated with Palaeoproterozoic upper crustal extension (D1) that resulted in the development of normal faults and related transfer faults. The VMS ores formed as sub-seafloor replacement in both felsic volcaniclastic and sedimentary rocks and partly as exhalative deposits within the uppermost part of the volcanic stratigraphy. Subsequently, the district was subjected to deformation (D2) during crustal shortening. Comparing the distribution of VMS deposits with the regional fault pattern reveals a close spatial relationship of VMS deposits to the faults that formed during crustal extension (D1) utilising the syn-extensional faults as fluid conduits. Analysing the shape and orientation of VMS ore bodies shows how their deformation pattern mimics those of the hosting structures and results from the overprinting D2 deformation. Furthermore, regional structural transitions are imitated in the deformation patterns of the ore bodies. Plotting the aspect ratios of VMS ore bodies and the comparison with undeformed equivalents in the Hokuroko district, Japan allow an estimation of apparent strain and show correlation with the D2 deformation intensity of the certain structural domains. A comparison of the size of VMS deposits with their location shows that the smallest deposits are not related to known high-strain zones and the largest deposits are associated with regional-scale high-strain zones. The comparison of distribution and size with the pattern of high-strain zones provides an important tool for regional-scale mineral exploration in the Skellefte district, whereas the analysis of ore body shape and orientation can aid near-mine exploration activities.
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| 21. |
- Bauer, Tobias, et al.
(författare)
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Fault-controlled sedimentation in a progressively opening extensional basin : the Palaeoproterozoic Vargfors basin, Skellefte mining district, Sweden.
- 2013
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Ingår i: International journal of earth sciences. - : Springer Science and Business Media LLC. - 1437-3254 .- 1437-3262. ; 102:2, s. 385-400
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Tidskriftsartikel (refereegranskat)abstract
- The Vargfors basin in the central part of the Skellefte mining district is an inverted sedimentary basin within a Palaeoproterozoic (1. 89 Ga) marine volcanic arc. The fault-segmented basin formed from upper-crustal extension and subsequent compression, following a period of intense sub-marine volcanism and VMS ore formation. New detailed mapping reveals variations in stratigraphy attributed to syn-extensional sedimentation, as well as provenance of conglomerate clasts associated with tectonic activity at the transition from extension to compression. The onset of fan delta to alluvial fan sedimentation associated with basin subsidence indicates that significant dip-slip displacement accommodating rapid uplift of the intrusive complex and/or subsidence of the adjacent volcano-sedimentary domain took place along a major fault zone at the southern margin of the intrusive complex. Subsidence of the Jörn intrusive complex and/or its burial by sedimentary units caused a break in erosion of the intrusion and favoured the deposition of a tonalite clast-barren conglomerate. Clast compositions of conglomerates show that the syn-extensional deposits become younger in the south-eastern parts of the basin, indicating that opening of the basin progressed from north-west to south-east. Subsequent basin inversion, associated with the accretion to the Karelian margin, involved reverse activation of the normal faults and development of related upright synclines. Progressive crustal shortening caused the formation of break-back faults accompanied by mafic volcanic activity that particularly affected the southern contact of the Jörn intrusive complex and the northern contact of the Vargfors basin
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| 22. |
- Bauer, Tobias, et al.
(författare)
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Syn-extensional faulting controlling structural inversion : Insights from the Palaeoproterozoic Vargfors syncline, Skellefte mining district, Sweden
- 2011
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Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 191:3-4, s. 166-183
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Tidskriftsartikel (refereegranskat)abstract
- The Vargfors basin in the central Skellefte district, Sweden, is an inverted sedimentary sub-basin within a Palaeoproterozoic (1.89 Ga) marine volcanic arc. The sub-basin formed from upper-crustal extension and subsequent compression, following a period of intense marine volcanism and VMS ore formation. Detailed mapping and structural analysis reveals a pattern of SE–NW-striking normal faults and interlinked NE–SW-striking transfer faults, which define distinct fault-bound compartments, each with an individual structural geometry and stratigraphy. Constraints on the deformation style and mechanisms achieved by 2D forward modelling are in agreement with the previously inferred inversion of the early normal faults during a regional crustal shortening event. A rheologically weak carbonate-rich layer at the base of the sedimentary sequence favoured the fault inversion over more distributed shortening as the controlling deformation mechanism. Transposition of sedimentary strata into the approximately SE–NW faults led to formation of asymmetric synclines that were tightened during progressive shortening. Structural analysis infers a progressive opening of the basin towards SE and NW with time. Furthermore, it is inferred that a displacement gradient was developed along the main structural grain, with decreasing dip-slip displacements towards SE and NW, both during the extension and the structural inversion.VMS deposits in the vicinity of the contact between the volcanic and the overlying sedimentary rocks were formed along early normal faults, which reacted as fluid conduits. Subsequently, the deposits were transposed into the inverted faults during crustal shortening. Consequently, the inverted faults provide a useful tool for mineral exploration in the district.
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| 23. |
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| 24. |
- Dahlin, Peter, 1974-, et al.
(författare)
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Palaeoproterozoic metavolcanic and metasedimentary succession hosting the Dannemora iron ore deposits, Bergslagen region, Sweden
- 2012
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 134:2, s. 71-85
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Tidskriftsartikel (refereegranskat)abstract
- The Dannemora supracrustal inlier is located in the north-eastern part of the Bergslagen region in south-central Sweden and hosts the second largest iron ore deposit in the region. The metasupracrustal succession of the inlier consists of c. 1.9 Ga Palaeoproterozoic rocks that are mainly sub-alkaline, rhyolitic to dacitic, pyroclastic deposits, reworked pyroclastic deposits and metalimestone. It is c. 700-800-m thick and termed the Dannemora Formation. The formation is divided into lower and upper members and the former is in turn subdivided into subunits 1 and 2. The great thickness of individual pyroclastic deposits indicates deposition within a caldera. The rocks show characteristics of a pyroclastic origin by containing abundant pumice, cuspate and Y-shaped former glass shards, and fragmented crystals of quartz and subordinate feldspars. Scattered spherulites and lack of welding-compacted fiamme suggest that the lower member was slightly welded, where as the upper member contains sericite-replaced glass shards with preserved primary shapes indicating no welding. Undisturbed layers of ash-siltstone with normal grading and fluid-escape structures are attributed to subaqueous deposition below storm wave base in the eastern part of the inlier, where as erosion channels and cross-bedding in some of the volcaniclastic deposits imply deposition and reworking above wave base in the central part of the inlier. Epidote spots, previously interpreted as altered limestone fragments and an indicator for subaquatic deposition, are here reinterpreted as the result of selective alteration related to the intrusion of mafic dykes and to Ca release during dolomitisation of limestone.
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| 25. |
- Frank, Katherine S., et al.
(författare)
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Magnetite as a provenance and exploration tool to metamorphosed base-metal sulfide deposits in the Stollberg ore field, Bergslagen, Sweden
- 2022
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Ingår i: Mineralogical magazine. - : Cambridge University Press. - 0026-461X .- 1471-8022. ; 86:3, s. 373-396
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Tidskriftsartikel (refereegranskat)abstract
- Magnetite is a common mineral in the Paleoproterozoic Stollberg Zn–Pb–Ag plus magnetite ore field (~6.6 Mt of production), which occurs in 1.9 Ga metamorphosed felsic and mafic rocks. Mineralisation at Stollberg consists of magnetite bodies and massive to semi-massive sphalerite–galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite and magnetite) hosted by metavolcanic rocks and skarn. Magnetite occurs in sulfides, skarn, amphibolite and altered metamorphosed rhyolitic ash–siltstone that consists of garnet–biotite, quartz–garnet–pyroxene, gedrite–albite, and sericitic rocks. Magnetite probably formed from hydrothermal ore-bearing fluids (~250–400°C) that replaced limestone and rhyolitic ash–siltstone, and subsequently recrystallised during metamorphism. The composition of magnetite from these rock types was measured using electron microprobe analysis and LA–ICP–MS. Utilisation of discrimination plots (Ca+Al+Mn vs. Ti+V, Ni/(Cr+Mn) vs. Ti+V, and trace-element variation diagrams (median concentration of Mg, Al, Ti, V, Co, Mn, Zn and Ga) suggest that the composition of magnetite in sulfides from the Stollberg ore field more closely resembles that from skarns found elsewhere rather than previously published compositions of magnetite in metamorphosed volcanogenic massive sulfide deposits. Although the variation diagrams show that magnetite compositions from various rock types have similar patterns, principal component analyses and element–element variation diagrams indicate that its composition from the same rock type in different sulfide deposits can be distinguished. This suggests that bulk-rock composition also has a strong influence on magnetite composition. Principal component analyses also show that magnetite in sulfides has a distinctive compositional signature which allows it to be a prospective pathfinder mineral for sulfide deposits in the Stollberg ore field.
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| 26. |
- Frank, Katherine S, et al.
(författare)
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Whole Rock-Rare Earth Element and Magnetite Chemistry as Guides to Exploration for Metamorphosed Base Metal Sulfide Deposits in the Stollberg Ore Field, Bergslagen, Sweden
- 2014
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- The Stollberg ore field (~12 Mt), 50 km W of the giant Garpenberg Zn-Pb-Ag-(Cu-Au) district (>100 Mt) occurs in the regional Stollberg F2 syncline within 1.9 Ga bimodal felsic and mafic rocks metamorphosed to the amphibolite facies. Sulfide mineralization is hosted by volcanic rocks and skarn and consists of massive to semi-massive sphalerite-galena and pyrrhotite (with subordinate pyrite, chalcopyrite, arsenopyrite, and magnetite). The trace element composition of magnetite, which locally forms ore-grade masses and occurs as a common accessory in most rocks types at Stollberg, has previously proven to be a pathfinder in the exploration for ore deposits elsewhere and is evaluated here along with the rare earth element (REE) chemistry of altered rocks. At Stollberg, the dominant country rocks are metamorphosed rhyolitic pumice breccia and rhyolitic ash-silt-sandstone with minor amphibolite sills. On the eastern side of the Stollberg syncline, mineralization at Stollberg and Dammberget occurs as stratabound replacement of limestone/skarn that grades into iron formation spatially related to garnet-biotite and gedrite-albite alteration. At Gränsgruvan on the western side of the syncline, sulfides occur in a silicified zone along with garnet-biotite and quartz-garnet-pyroxene alteration. Although the Tvistbo and Norrgruvan deposits along the north end of the syncline are small, they show geological characteristics that are transitional to deposits found on the western and eastern side of the syncline in that the ore is hosted by skarn rock and associated with quartz-garnet-pyroxene alteration. The Gränsgruvan deposit more closely resembles deposits found at Garpenberg than those located on the eastern limb of the Stollberg syncline. Whole-rock analyses of altered and unaltered host rocks suggest that most components were derived from a felsic volcaniclastic component and that elements were immobile during alteration. These rocks (including altered rocks in the stratigraphic footwall) are light REE enriched, heavy REE depleted, and show negative Eu anomalies, whereas mineralized rocks (Fe- and base metal-rich) and altered rocks in the ore zone show the same REE pattern but with positive Eu anomalies. Trace element compositions (using LA-ICP-MS techniques) of magnetite in high-grade ore, limestone/skarn, massive magnetite, and garnet-biotite, gedrite-albite, garnet-pyroxene alteration show a range of compositions. Such ranges in composition are inconsistent with previous studies in other ore fields that suggest the composition of magnetite can be used to define compositional fields characteristic of ore deposit type (e.g., Al+Mn vs. Ti+V wt. %) or approximate temperature of the ore-forming fluid. Magnetite in garnet-biotite and gedrite-albite alteration spatially associated with Dammberget typically contains > 200 ppm Ga, > 10 ppm Sn, and Ti/V ratios of >10 whereas magnetite in garnet-biotite alteration associated the smaller Cederkreutz deposit contains < 25 ppm Ga, < 2 ppm Sn, and Ti/V ratios < 0.1. Magnetite in garnet-biotite alteration associated with the Gränsgruvan deposit contains > 10 ppm Sn, 20 to 180 ppm Ga, and Ti/V ratios of 0.1 to 2. These and other trace element compositions of magnetite as well as REE patterns of altered host rocks show potential as exploration guides to ore in the Stollberg district.
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| 27. |
- Frank, Katherine, et al.
(författare)
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Variability in the Geological, Mineralogical, and Geochemical Characteristics of Base Metal Sulfide Deposits in the Stollberg Ore Field, Bergslagen District, Sweden
- 2019
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - Littleton, Colorado : Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 114:3, s. 473-512
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Tidskriftsartikel (refereegranskat)abstract
- The Stollberg ore field occurs in the Bergslagen region of south-central Sweden, a polydeformed ca. 1.9 Ga igneous province dominated by bimodal felsic and mafic rocks. Sulfide mineralization is hosted by metavolcanic rocks, marble, and skarn and consists of massive to semimassive polymetallic sulfides and iron oxide in a semiregional F2 syncline termed the Stollberg syncline. The dominant country rocks are rhyolitic pumice breccia and rhyolitic ash-siltstone with minor mafic sills metamorphosed to the amphibolite facies. On the eastern limb of the Stollberg syncline, sulfide mineralization occurs as stratabound premetamorphic replacement of volcaniclastic rocks and limestone that grades into iron formation. The development of skarn assemblages is the result of low-temperature replacement of limestone and volcaniclastic rocks rather than formation by high-temperature metasomatism or synmetamorphic or late hydrothermal replacement of marble. Metamorphosed, hydrothermally altered rocks on the eastern limb are dominated by the assemblages garnet-biotite and gedrite-albite. Silica-altered rocks are generally subordinate in the Stollberg ore field; however, sulfides at Gränsgruvan, on the western limb of the syncline, are located in a silicified zone along with metamorphosed, altered rocks dominated by sericite and the assemblage quartz-garnet-pyroxene. Although the Tvistbo and Norrgruvan prospects along the northern end of the syncline are small, they show geologic characteristics that are transitional to deposits found on the western and eastern limbs of the syncline. Ore at Tvistbo is hosted by skarn and is spatially associated with quartz-garnet-pyroxene rocks, whereas sulfides at Norrgruvan are hosted by quartz-fluorite rocks that are similar to those hosting the Brusgruvan deposit on the eastern limb of the syncline.Whole-rock analyses of variably altered host rocks in the Stollberg ore field suggest that most components were sourced from felsic volcaniclastic rocks and that Zr, Ti, Al, Hf, Nb, Sc, Th, Ga, U, and rare-earth elements (REEs) were immobile during alteration. These rocks are enriched in light REEs, depleted in heavy REEs, and have negative Eu anomalies, whereas sulfide-bearing rocks (Fe- and base metal-rich) and altered rocks in the ore zone show the same REE pattern but with positive Eu anomalies. Indicators of proximity to sulfides in altered rocks in the Stollberg ore field include positive Eu anomalies, an increase in the concentration of Pb, Sb, As, Tl, Ba, Ba/Sr, and K2O, as well as an increase in a modified version of the Ishikawa alteration index, which accounts for the presence of primary Ca in an original limestone component. Garnet and pyroxene enriched in either Ca or Mn are also considered to be pathfinders to ore. Cooling of an acidic, reduced hydrothermal fluid that carried sulfur and metals, which became neutralized as it reacted with limestone, is likely responsible for the formation of sulfides in the Stollberg ore field. The nature of the host rock types, the style of the alteration spatially associated with sulfide mineralization, and the spatial association with iron formation bear some resemblance to volcanogenic massive sulfide and Broken Hill-type deposits. However, the stratabound replacement of limestone by sulfides distinguishes it from these deposit types and is a so-called SVALS-type ore system, which is a class of stratabound, volcanic-hosted, limestone-skarn deposits restricted to the Bergslagen district.
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| 28. |
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| 29. |
- Imana, Marcello, et al.
(författare)
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Volcanic sequence, chemostratigraphy and alteration at the Storliden Zn-Cu deposit, Skellefte District, northern Sweden
- 2005
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Ingår i: GAC-MAC-CSPG-CSSS [joint Meeting], Halifax, 2005 : building bridges - across science, through time, around the world. - St. John's : Geological and Mineralogical Association of Canada. ; , s. 92-
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)
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| 30. |
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| 31. |
- Jansson, Nils, et al.
(författare)
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A comment on the occurrence of gallium and germanium in the Zinkgruvan Zn-Pb-Ag-(Cu) deposit, Bergslagen, Sweden
- 2016
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 138:4, s. 533-535
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Tidskriftsartikel (refereegranskat)abstract
- The Zinkgruvan deposit has been included in compilations of exceptionally Ga- and Ge-endowed deposits in Sweden. Available published data sets do however not support a substantial enrichment in Ga and Ge. In this contribution, we investigate the Ga- and Ge-endowment based on a whole-rock lithogeochemical data and ore grade analyses from the deposit. Based on our results, we find it highly unlikely that a Ga-endowment exists in the ore. A Ge-endowment may exist, but we find no evidence of Ge grades at the 1,000 ppm level that have been reported previously.
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| 32. |
- Jansson, Nils, et al.
(författare)
-
Bergslagen & Broken Hill
- 2017
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Ingår i: Geologiskt Forum. - : Geologiska föreningen. - 1104-4721. ; :94, s. 24-28
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Tidskriftsartikel (populärvet., debatt m.m.)
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| 33. |
- Jansson, Nils, et al.
(författare)
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Evolution of the paleoproterozoic volcanic-limestone-hydrothermal sediment succession and Zn-Pb-Ag and iron oxide deposits at Stollberg, Bergslagen region, Sweden
- 2013
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - : Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 108:2, s. 309-335
-
Tidskriftsartikel (refereegranskat)abstract
- The Stollberg Zn-Pb-Ag and magnetite mining field is located in the Bergslagen region of the Fenno -scandian Shield. The main Stollberg ore deposits comprise a chain of orebodies that occur discontinuously for5 km along a prominent marble and skarn horizon. Orebodies mainly contain magnetite and combinations ofsphalerite, galena, pyrrhotite, and lesser pyrite and chalcopyrite within marble and skarn. Previously, the twomain limestone (marble) units in the Stollberg area were regarded as structural repetitions of one single horizon.Based on sedimentary and volcanic facies and structural analysis, the mineralized Stollberg limestone isnow shown to be the uppermost of two different limestone units within a ca. 3-km-thick Paleoproterozoic (∼1.9Ga) volcanosedimentary succession. Approximately 2 km of preserved footwall stratigraphy is recognized belowthe Stollberg limestone, as opposed to ca. 500 m in previous structural models. This new interpretation hasallowed the stratigraphic evolution prior to the mineralizing event and extent of the Stollberg hydrothermal systemto be investigated in detail.After formation of the Staren limestone ca. 1 km below Stollberg, the depositional basin subsided to belowwave base, while adjacent areas were uplifted and eroded. This led to the deposition of a ca. 600-m-thick, shallowing-upward sedimentary sequence in which normal-graded subaqueous mass flow deposits pass upward topolymict limestone-volcanic breccia-conglomerates. This sequence is attributed to progradation of a fan deltadepositional system. The breccia-conglomerates are overlain by ca. 500 m of juvenile rhyolitic pumice brecciathat is interpreted as a major pyroclastic deposit. Conformably above is the Stollberg ore host, which comprisesplanar-stratified, rhyolitic ash-siltstone interbedded with Fe-Mn-rich hydrothermal sedimentary rocks andlimestone, all deposited below wave base. This ore host package is extensively altered to skarn and mica schist.The thickness, extent, and homogeneous composition of the rhyolitic pumice breccia below the ore host suggestthat volcanism was accompanied by caldera subsidence and that the Stollberg ore deposits formed withinthe caldera structure. The ore host is overlain by planar-stratified, rhyolitic ash-siltstone and subordinate sedimentarybreccias deposited below wave base from turbidity currents and suspension.Skarns in the Stollberg ore host unit are interpreted as metamorphosed mixtures of variably altered rhyolite,limestone, and hydrothermal sediments. Whole-rock contents of Al, Ti, Zr, Hf, Nb, Sc, Th, Ta, U, and heavyrare-earth elements are highly correlated in skarns, limestone, magnetite mineralization, and variably alteredrhyolites in the Stollberg succession, suggesting that these elements were supplied by a felsic volcaniclasticcomponent and were immobile during alteration. The felsic volcaniclastic component is calc-alkaline and characterizedby negative Eu anomalies and light rare-earth element enrichment. Strong positive Eu anomalies areonly observed in limestone, skarn, and iron ore in the Stollberg ore host, i.e., in samples rich in Mn, Ca, andFe.The Stollberg ore deposits are interpreted as metamorphosed, hydrothermal-exhalative and carbonate replacement-type mineralization. The hydrothermal-exhalative component formed first by accumulation of sedimentsrich in Mn and Fe, coeval with limestone formation during waning volcanism. Burial of the hydrothermal systemby sediments of the stratigraphic hanging wall led to a gradual shift to more reducing conditions. At thisstage, the Stollberg limestone interacted with more sulfur rich hydrothermal fluids below the sea floor, producingstrata-bound, replacement-type Zn-Pb-Ag sulfide and additional iron oxide mineralization
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| 34. |
- Jansson, Nils F., et al.
(författare)
-
Age constraints on c. 1.9 Ga volcanism, basin evolution and mineralization at the world-class Zinkgruvan Zn-Pb-Ag(-Cu) deposit, Bergslagen, Sweden
- 2023
-
Ingår i: Precambrian Research. - : Elsevier. - 0301-9268 .- 1872-7433. ; 395
-
Tidskriftsartikel (refereegranskat)abstract
- We present improved age constraints for the world-class Zinkgruvan Zn-Pb-Ag and Cu deposit: one of the largest Zn deposits of the Fennoscandian shield, and one of the earliest large, basin-hosted Zn deposits that formed from oxidized saline brines. Secondary Ion Mass Spectrometry (SIMS) U-Pb dating on zircon is used to constrain at least two phases of c. 1.9 Ga volcanism in the Zinkgruvan area, separated by a period of fluvial sedimentation, all of which predated formation of the stratiform Zn-Pb-Ag mineralization. A 1908 ± 4 Ma age for a rhyolitic rock of the first volcanic phase is the oldest recorded U-Pb zircon age of a volcanic rock in the Bergslagen lithotectonic unit (BLU) where Zinkgruvan is located. Similarly, two identical ages of 1898 ± 5 Ma for volcanic rocks belonging to the second volcanic phase indicate that the local volcanic activity, which formed the stratigraphic footwall, ended earlier in the Zinkgruvan area than in other parts of the BLU, where intense explosive felsic volcanic and intrusive activity until c. 1891 Ma has been demonstrated. This, along with a hybrid siliciclastic-volcaniclastic (tuffitic) character of the Zinkgruvan ore host, confirms earlier interpretations that the Zinkgruvan deposit formed in an actively subsiding basin, distal to active volcanic centers in the BLU in the time range 1.90–1.89 Ga. Our results support models suggesting that basinal brine-driven hydrothermal systems in sedimentary basins distal to volcanic centers could form world-class Zn deposits as early as c. 1.90 Ga.
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| 35. |
- Jansson, Nils F, et al.
(författare)
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An updated genetic model for metamorphosed and deformed, c. 1.89 Ga magnesian Zn-Pb-Ag skarn deposits, Sala area, Bergslagen, Sweden
- 2019
-
Ingår i: Proceedings of the 15th SGA Biennial Meeting, 27-30 August. - : University of Glasgow Publicity Services. ; , s. 166-169
-
Konferensbidrag (refereegranskat)abstract
- This contribution presents an updated view on the genesis of stratabound Zn-Pb-Ag mineralization in the Sala area, Bergslagen, Sweden. Integrated legacy and new geological, geochemical and geophysical data reveal that the deposits are hosted by a complex array of magnesian skarn-altered zones in dolomitic marble. These mineralized zones parallel early faults and metavolcanic interbeds in the host marble, and converge downwards in the stratigraphy adjacent to a 1.89 Ga calc-alkaline granite-granodiorite batholith. Prograde alteration involved formation of early barren ferroan diopside- and forsterite-bearing skarns. Mineralization is mainly associated with subsequent alteration to tremolite, chlorite, serpentine, magnetite and calcite. The hydrous associations overlap mineralogically with assemblages formed during subsequent greenschist facies regional metamorphism between 1.87 Ga and 1.8 Ga. However, ferroan diopside and forsterite are unique to the alteration system, and indicate mineralization in conjunction with an early, high T, metasomatic alteration event at 1.89 Ga. The Sala deposits can be classified as Zn skarn deposits, albeit atypical in the magnesian nature of the skarns and the lack of minerals with essential Mn. The Fe and Mn content in magnesian silicates and carbonates is however sufficient to induce clear enrichment haloes of these elements around the deposits. The magnesian nature of the skarns probably reflect formation in a shallow marine continental backarc tectonic setting, and an importance of seawater in early pre-skarn alteration stages, such as dolomitization.
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| 36. |
- Jansson, Nils F., et al.
(författare)
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Multistage ore formation at the Ryllshyttan marble and skarn-hosted Zn-Pb-Ag-(Cu) + magnetite deposit, Bergslagen, Sweden
- 2015
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 69, s. 217-242
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Tidskriftsartikel (refereegranskat)abstract
- Numerous magnetite skarn deposits and marble- and skarn-hosted base metal sulphide deposits occur in polydeformed and metamorphosed, felsic-dominated metavolcanic inliers in the Palaeoproterozoic Bergslagen region of south-central Sweden, including the Ryllshyttan magnetite and Zn-Pb-Ag-(Cu) sulphide deposit, approximately 2.5 km SW of the large Garpenberg Zn-Pb-Ag-(Cu-Au) deposit. The Ryllshyttan deposit, from which approximately 1 Mt of Zn-rich massive sulphide ore and 0.2 Mt of semi-massive magnetite were extracted, is located near a transition between magnesian skarn and dolomitic marble. The host unit consists of a 10-20 m-thick former calcitic limestone of likely stromatolitic origin that is commonly pervasively altered to skarn, locally hosting magnetite skarn deposits. The ore-bearing unit is one of several mineralised marble units within a more than 1 km-thick, felsic-dominated metavolcanic succession that includes a metamorphosed, large caldera-fill pyroclastic deposit, 800 m stratigraphically above the Ryllshyttan host succession. The Garpenberg stratabound Zn-Pb-Ag-(Cu)-(Au) deposit is located higher in the stratigraphy, just below the caldera fill deposits. The metavolcanic succession is bounded to the NW by a large granitoid batholith and intruded by a microgranodiorite pluton less than a 100 m from the Ryllshyttan deposit. Magnetite laminae in bedded skarns and metavolcanic rocks in the hanging wall of Ryllshyttan indicate an early (syngenetic) accumulation of Fe-rich exhalites. In contrast, the sulphide mineralisation consists of stratabound replacement-style ore associated with dolomitisation of the host and with discordant K-Mg-Fe±Si alteration of volcanic rocks and early porphyritic intrusions in the footwall and hanging wall. The microgranodiorite that intrudes the host succession crosscuts the K-Mg-Fe±Si alteration envelope and is overprinted by Na-Ca alteration (diopside and plagioclase-bearing mineral associations) that also overprints K-Mg-Fe±Si-altered rocks. The Na-Ca alteration is interpreted to be associated with the formation of calcic and magnesian iron skarn deposits semi-regionally at a similar stratigraphic position. Despite superimposed amphibolite facies regional metamorphism and substantial syn-D2-D3 remobilisation of sulphides concurrent with retrograde alteration of skarn assemblages, cross-cutting field relationships indicate that the Ryllshyttan magnetite and Zn-Pb-Ag-(Cu) sulphide deposit results from protracted VMS-style hydrothermal activity including early seafloor mineralisation (Fe-rich exhalites), closely followed by sub-seafloor carbonate-replacement-style mineralisation (base metal-bearing massive sulphides). Both mineralisation styles were overprinted by contact metasomatism associated with the formation of abundant magnetite skarn deposits during the emplacement of granitoid intrusions. As for other deposits in the Bergslagen region, the ore-forming system at Ryllshyttan thus has similarities to both metamorphosed VMS deposits and metasomatic Fe and Zn skarn deposits. Our results suggest that the sequence of volcanic, intrusive and hydrothermal events in this region is compatible with prograde heating of a long-lived hydrothermal system, wherein a shift from a convective seawater-dominated system to a contact metamorphic and/or metasomatic environment occurred during the early stage of the 1.9-1.8 Ga Svecokarelian orogeny. This model partly resolves the controversy regarding genesis of the iron oxide and base metal sulphide deposits in Bergslagen, as we recognise that these deposits have a complex history of alteration, metamorphism, deformation and (re)mobilisation, and no unique established genetic model can account for all their features.
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| 37. |
- Jansson, Nils F., et al.
(författare)
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Origin of Palaeoproterozoic, sub-seafloor Zn-Pb-Ag skarn deposits, Sala area, Bergslagen, Sweden
- 2022
-
Ingår i: Mineralium Deposita. - : Springer. - 0026-4598 .- 1432-1866. ; 57:3, s. 455-480
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Tidskriftsartikel (refereegranskat)abstract
- Unravelling the genesis of metamorphosed mineral deposits can be complicated due to difficulties in separating between primary features and features that formed during the metamorphic overprint. Such uncertainty exists for stratabound and dolomite- and skarn-hosted Zn-Pb-Ag sulfide deposits in 1.89 Ga rocks in the Bergslagen lithotectonic unit (BLU) of Sweden, where a metasomatic vs. regional metamorphic origin for skarns has long been discussed. By integrating geological mapping with new lithogeochemical, mineralogical, and stable isotope data (C, O, S), we show that complexly zoned garnet and clinopyroxene skarns in the Sala area of the central BLU predate mineralization. Sphalerite-galena mineralization formed after the deposition of a younger, more Mn-rich ferroan diopside and andradite-grossular garnet, and is associated with phlogopite, tremolite-actinolite, chlorite, serpentine, and calcite. Mineralization in conjunction with a transition from high-T metasomatism to hydrolytic alteration is inferred. An average δ34SV-CDT of 1.6 ± 1.9‰ in sulfides is consistent with a primordial sulfur source. Trends defined by negative shifts in δ18OV-SMOW and δ13CV-PDB in dolomite and calcite are consistent with fluid infiltration at 300–500 °C. The alteration system is sharply truncated by unaltered, c. 1.89 Ga calc-alkaline granite and porphyritic intrusions, which along with F1 folding of the alteration zones and mineralization suggest that mineralization predate regional metamorphism. The Sala deposits are interpreted as Zn skarn deposits formed in conjunction with the emplacement of intrusions into penecontemporaneous marine volcanic and dolomitized limestone strata. The unusually Mg-rich mineralogy in relation to Zn skarns worldwide most likely reflects the dolomitic precursor.
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| 38. |
- Jansson, Nils F., et al.
(författare)
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Principal component analysis and K-means clustering as tools during exploration for Zn skarn deposits and industrial carbonates, Sala area, Sweden
- 2022
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Ingår i: Journal of Geochemical Exploration. - : Elsevier. - 0375-6742 .- 1879-1689. ; 233
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Tidskriftsartikel (refereegranskat)abstract
- This contribution presents an application of principal component analysis (PCA) and K-means clustering as tools for data dimension reduction and grouping of multivariate, whole-rock lithogeochemical data. The study dataset consists of 64 geochemical variables and measurements of spectrophotometric brightness determined from 181 dolomite marble samples, collected at various distance from two contrasting types of mineral deposits, 1) stratabound, dolomite marble- and skarn-hosted Zn-Pb-Ag sulphide deposits and 2) industrial dolomite deposits. Clustering and PCA outputs are assessed based on spatial distribution relative to known mineral deposits and interpretability using geological domain knowledge, to test if the methods can provide a non-biased classification of dolomite samples which is useful for exploration vectoring. The PCA illustrate that three principle components derived from centered log-ratio transformed data can account for 79.69% of the dataset variance. K-means clustering provide unsupervised division of samples into different groups reflecting relative contents of detrital (siliciclastic-volcaniclastic), biogenic and hydrothermal components in the marble protoliths. Spatial analysis of principal components and K-means clusters reveal systematic distribution patterns relative to known deposits, thus providing an exploration guide. The samples most prospective for Zn-Pb-Ag deposits are divided into groups of ‘halo dolomite’ exhibiting elevated Fe and Mn, and an ‘ore dolomite’ also showing elevated Zn, Pb, Ag, Sb, Hg. This can be reconciled with magnetite and Mn-bearing Mg-silicates and carbonates in hydrothermal alteration haloes, and proximal enrichment in hydrothermal sulphides (galena, pyrrhotite, pyrite, sphalerite). Samples in these groups returned low spectrophotometric brightness, resulting from sulphides and Fe oxides grinding to dark powders during sample preparation, significantly lowering the brightness of powdered dolomite marble, even when occurring in low concentrations. Conversely, a ‘clean dolomite’ group is characterized by low contents of the elements above, high contents of Ca, Mg, Sr and total carbon, low magnetic susceptibility and high spectrophotometric brightness, and spatially coincide with known industrial dolomite deposits. An additional group of ‘detrital-rich dolomite’ is distinct from the other groups in an elevated content of high field strength elements and Al, and intermediate spectrophotometric brightness. This variety represent samples containing a higher content of co-settled volcaniclastic-siliciclastic material in the marble precursor. Assessment of the clustered data in relation to magnetic susceptibility measurements from the same samples show that Halo and Ore dolomite can be differentiated from other dolomite types by geomagnetic methods, hence providing a proxy for their indirect detection during geophysical surveys.
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| 39. |
- Jansson, Nils F., et al.
(författare)
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Timing of volcanism, hydrothermal alteration and ore formation at Garpenberg, Bergslagen, Sweden
- 2011
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 133:1-2, s. 3-18
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Tidskriftsartikel (refereegranskat)abstract
- The timing of Palaeoproterozoic magmatism in the Garpenberg area in the Bergslagen region of the Fennoscandian shield has been constrained by secondary ion mass spectrometry (SIMS) U-Pb zircon dating of metamorphosed igneous rocks. Volcanism is constrained by igneous crystallisation ages of 1895 ± 4Ma for a syn-volcanic rhyolite porphyry intrusion and 1893 ± 3Ma for a rhyolitic pumice breccia. Granite and microgranodiorite, which intruded into the stratigraphy, are dated at 1895 ± 3 and 1894 ± 4 Ma, respectively. The identical U-Pb ages suggest rapid geological evolution from the emplacement of volcanics, their burial and subsidence to 2-5 km depths and intrusion by granitoids. The timing of metamorphism and the extent of metamorphic resetting of titanite have been evaluated. SIMS titanite 207Pb-206Pb ages from the same samples as the zircon yield younger ages. Although errors are large in individual analyses and fractions, a weighted average of 59 analyses from four samples yields a 207Pb-206Pb age of 1858 ± 14 Ma, interpreted as the age of regional metamorphism. The results add constraints to the timing of sulphide and iron oxide mineralisation at Garpenberg. The rhyolite porphyry is intruded into a syngenetic iron formation. Its crystallisation age provides a minimum age for syngenetic iron oxide deposits at Garpenberg. The major Zn-Pb sulphide deposits are accompanied by alteration envelopes. Units formed before alteration yield similar igneous crystallisation ages as intrusions post-dating alteration. It is concluded that both iron oxide and sulphide mineralisation formed within the same age-span as the dated units.
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| 40. |
- Jansson, Nils, et al.
(författare)
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Genesis of the Zinkgruvan stratiform Zn-Pb-Ag deposit and associated dolomite-hosted Cu ore, Bergslagen, Sweden
- 2016
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Ingår i: Ore Geology Reviews. - : Elsevier BV. - 0169-1368 .- 1872-7360. ; 82, s. 285-308
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Tidskriftsartikel (refereegranskat)abstract
- Zinkgruvan, a major stratiform Zn-Pb-Ag deposit in the Paleoproterozoic Bergslagen region, south-central Sweden, was overprinted by polyphase ductile deformation and high-grade metamorphism (including partial melting of the host succession) during the 1.9-1.8 Ga Svecokarelian orogeny. This complex history of post-ore modification has made classification of the deposit difficult. General consensus exists on a syngenetic-exhalative origin, yet the deposit has been variably classified as a volcanogenic massive sulfide (VMS) deposit, a sediment-hosted Zn (SEDEX) deposit, and a Broken Hill-type (BHT) deposit. Since 2010, stratabound, cobaltiferous and nickeliferous Cu ore, comprising schlieren and impregnations of Cu, Co and Ni sulfide minerals in dolomitic marble, is mined from the stratigraphic footwall to the stratiform Zn-Pb-Ag ore. This ore type has not been fully integrated into any of the existing genetic models. Based on a combination of 1) widespread hematite-staining and oxidizing conditions (Fe2O3>FeO) in the stratigraphic footwall, 2) presence of graphite and reducing conditions (Fe2O3 5 km) Zn-Pb-Ag ore was precipitated.Both ore types are characterized by significant spread in δ34S, with the sulphur in the Cu ore and associate marble-hosted Zn mineralization on average being somewhat heavier (δ34S = -4.7 to +10.5 ‰, average 3.9 ‰) than that in the stratiform Zn-Pb-Ag ore (δ34S = -6 to +17 ‰, average 2.0 ‰). The ranges in δ34S are significantly larger than those observed in syn-volcanic massive sulphide deposits in Bergslagen, for which simple magmatic/volcanic sulphur sources have been invoked. Mixing of magmatic-volcanic sulfur leached from underlying volcanic rocks and sulfur sourced from abiotic or bacterial sulfate reduction in a mixing zone at the seafloor could explain the range observed at Zinkgruvan.A distinct discontinuity in the stratigraphy, at which key stratigraphic units stop abruptly, is interpreted as a syn-sedimentary fault. Metal zonation in the stratiform ore (decreasing Zn/Pb from distal to proximal) and the spatial distribution of Cu mineralization in underlying dolomitic marble suggest that this fault was a major feeder to the mineralization. Our interpretation of ore-forming fluid composition and a dominant redox trap rather than a pH and/or temperature trap differs from most VMS models, with Selwyn-type SEDEX models, and most BHT models. Zinkgruvan has similarities to both McArthur-type SEDEX deposits and sediment-hosted Cu deposits in terms of the inferred ore fluid chemistry, yet the basinal setting has more similarities to BHT and felsic-bimodal VMS districts. We speculate that besides an oxidized footwall stratigraphy, regionally extensive banded iron formations and limestone horizons in the Bergslagen stratigraphy may have aided in buffering ore-forming brines to oxidized, near-neutral conditions. In terms of fluid chemistry, Zinkgruvan could comprise one of the oldest known manifestations of Zn and Cu ore-forming systems involving oxidized near-neutral brines following oxygenation of the Earth’s atmosphere.
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| 41. |
- Jansson, Nils, et al.
(författare)
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Geochemical vectors for stratiform Zn-Pb-Ag sulfide and associated dolomite-hosted Cu mineralization at Zinkgruvan, Bergslagen, Sweden
- 2018
-
Ingår i: Journal of Geochemical Exploration. - : Elsevier. - 0375-6742 .- 1879-1689. ; 190, s. 207-228
-
Tidskriftsartikel (refereegranskat)abstract
- The Zinkgruvan deposit is the largest stratiform Zn-Pb-Ag mineralization in Sweden. The most recent genetic model attributes ore formation to the discharge of oxidized, near-neutral pH, metalliferous brines into a reduced basin, forming laterally extensive, stratiform sulfide mineralization on the seafloor. It has a known strike extent of 5 km and is underlain by a regionally extensive zone of K-altered metavolcanic rock and dolomitic marble, the latter hosting Cu-(Co-Ni) replacement mineralization near the inferred hydrothermal vent to the stratiform sulfides. The deposit is stratigraphically overlain by migmatized, pyrrhotite- and graphite-rich pelite that is in turn overlain by a banded almandine-biotite-quartz-ferrosilite-bearing unit at the base of an regionally extensive metasedimentary succession. These laterally continuous units are interpreted as metamorphosed organic-rich sulphidic mudstone and silicate-dominated Fe formation, respectively.The favorable stratigraphic interval contains anomalously high Zn, Pb, Ag, Cu, K2O/(K2O+Na2O), Mn, Co, Tl, Ba and B relative to adjacent metatuffite. However, only Zn, Pb, Ag, K2O/(K2O+Na2O) and Mn are significantly enriched relative to adjacent strata beyond the known lateral extent of the ore. Elevated copper, Co and Tl only occur in the vent-proximal part of the deposit, whereas anomalous enrichments of Ba and B are sporadic and occur mainly in the stratigraphic footwall. Many elements such as Si, Fe, Mg, Ca and Cs are of limited use in vectoring due to low enrichment factors relative to inferred background compositions and/or strong lithological controls on their distribution.Although ore metal (Zn, Pb and Ag) enrichments are the best quantitative and qualitative guides to ore, K, Mn and Co enrichments also provide corroborative support. The most useful elements for vectoring have been synthesized into exploration indices. The Modified Sedex Metal Index (MSMI; Zn+3Pb+100Ag) is a vector towards stratiform Zn-Pb-Ag mineralization, whereas MSMI2 [Zn+3Pb+10(Cu+Co)] alsoallows targeting of proximal Cu mineralization.The banded iron formation and the pyrrhotite- and graphite-rich pelite of the stratigraphic hangingwall are consistently enriched in base metals (e.g. 500-1000 ppm Zn), total S and Mn throughout the entire Zinkgruvan area. However, these units are not known to grade laterally along strata into economic base metal sulfide mineralization, and they are not obviously products of the same hydrothermal system which formed the stratiform Zn-Pb-Ag deposit.In a vent-distal setting, the somewhat spurious metal anomalies of the hangingwall units can be difficult to distinguish from those of the favorable interval. The favorable stratigraphic interval can, however, be recognized by also taking into account that positive Zn anomalies are mainly coincident with positive anomalies in both K and Mn only in the favorable interval. Furthermore, samples from the favorable interval generally have Co/Ni > 1 and displays a positive Co/Ni vs. Zn trend, whereas samples of the pyrrhotite- and graphite-rich pelite have Co/Ni < 1 and define a negative Co/Ni vs. Zn trend. Thus, the index (Co/Ni)*Zn allows easy detection of weak Zn anomalies associated with the stratiform Zn-Pb-Ag mineralization.
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| 42. |
- Jansson, Nils, et al.
(författare)
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Geological controls on light dolomite deposits related to polymetallic sulphide deposits, Sala area, Bergslagen, Sweden : insights from whole-rock lithogeochemistry, spectrophotometry and magnetic susceptibility
- 2019
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Ingår i: EGU General Assembly 2019. - : Copernicus Publications.
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Regionally extensive, dolomitic and calcitic marble units comprise a subordinate but important component of a c. 1.91-1.89 Ga supracrustal succession in the Bergslagen lithotectonic unit, southern Sweden. These units occur as interbeds in predominantly felsic metavolcanic successions, and originated as shallow marine, stromatolitic limestone. Upon burial, the limestones were subjected to dolomitization and hydrothermal alteration, and at c. 1.87-1.80 Ga during the Svecokarelian orogeny, greenschist-amphibolite facies metamorphism and ductile deformation led to further modification.The marble units host abundant replacement-type, syn-volcanic sulphide and iron oxide deposits and locally industrial carbonate deposits. In the Sala area, current mining targets exceptionally light dolomite marble proximal to formerly mined Zn-Pb-Ag mineralization. The geological controls on marble lightness are poorly understood, e.g. the relative importance of primary impurity caused by co-settled volcaniclastic or siliciclastic material as opposed to secondary modifications caused by hydrothermal alteration. The following study addresses these controls based on 179 geospatial dolomitic marble samples analyzed for whole-rock lithogeochemistry, spectrophotometric lightness and magnetic susceptibility.Centered log-ratio transformed major element data suggest that lightness is positively correlated with Mg, Ca and Tot C, negatively correlated with Fe and Mn, and lack a clear relationship to Al and Si. Robust principal component analysis of major and trace element data allows representation of 81.1% of the cumulative variance by three principal components. PC1 (46.7%) mainly differentiates ‘hydrothermal’ elements such as Zn, Pb and As from elements such as Si, Mg, Ca, Tot C, Al, Zr, Ce, La and Eu. PC2 (21%) differentiates the latter into a ‘volcaniclastic-siliciclastic’ group dominated by Zr, Al, Ce, La and Eu and ‘calcite-dolomite’ group dominated by Ca, Mg and Tot C. PC3 (13.4%) is primarily controlled by Fe and Mn, with Fe+Mn-enriched samples outlining a c. 200 m wide halo around known Zn-Pb-Ag deposits. Optic microscopy and normative mineralogy suggest a negative correlation between lightness and the opaque minerals magnetite, pyrrhotite, pyrite, sphalerite and galena, which are variably enriched in this alteration halo.Collectively, the results imply that whereas a volcaniclastic component is detrimental to lightness, this effect is negligible relative to the effect of even weak Fe-enrichments (> 1 wt.%) at SiO2 < 10 wt.% and Al2O3 < 2 wt.%. This likely reflect that in contrast to the opaque minerals, the main silicates tremolite, serpentine, chlorite and phlogopite grind into light powders during processing. Light dolomite and sulphide-mineralized samples cluster in opposite corners in RQ1-RQ3 space and form coherent geospatial clusters corresponding to known deposits. Magnetic susceptibility data effectively differentiate these clusters and is hence useful for outlining prospective marble volumes. It is speculated that light marble constitutes a peripheral member of the sulphide-associated alteration halo. Thus, light marble is in itself a vector to polymetallic sulphides, and a potential byproduct in operations targeting the latter. High lightness is tentatively attributed to leaching and recrystallization of dolomite under elevated temperatures, either driven by ‘spent’ ore-forming fluids exiting the mineralizing system, or by marine fluids in peripheral, secondary hydrothermal aleration cells.
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| 43. |
- Jansson, Nils, et al.
(författare)
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Oxidized Brines Inferred in the Formation of c. 1.9 Ga Stratiform Zn-Pb-Ag and Dolomite-Hosted Cu Ores, Zinkgruvan, Bergslagen, Sweden
- 2015
-
Ingår i: Mineral Resources in a Sustainable World. - 9782855550664 ; , s. 1925-1928
-
Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- Zinkgruvan is an elusive Palaeoproterozoic stratiform Zn-Pb-Ag deposit which has been discussed in the context of sediment-hosted Zn-Pb (SEDEX), volcanic-hosted massive sulphide (VHMS) and Broken Hill-type (BHT) deposits. In this contribution, we address the chemistry of the ore-forming fluid, the nature of the depositional environment and the controls on ore formation based on a review of previous work complemented with new geological data from a stratigraphically underlying dolomite-hosted, zinciferous, cobaltiferous and nickeliferous Cu ore. We conclude that both deposit types can be explained as the product of a saline, oxidizing metalliferous brine which formed Cu mineralization by interaction with reduced pore waters, prior to exhalation into an anoxic brine pool, forming the stratiform Zn-Pb-Ag deposit. Our inference of fluid composition differs from many inferences on the chemistry of hydrothermal fluids involved in the formation of typical VHMS and BHT deposits, but is similar to that inferred for Proterozoic sediment-hosted Zn-Pb deposits in the McArthur basin, Australia
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| 44. |
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| 45. |
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| 46. |
- Jansson, Nils, et al.
(författare)
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The Lovisa stratiform Zn-Pb deposit, Bergslagen, Sweden : Structure, stratigraphy, and ore genesis
- 2018
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Ingår i: Economic geology and the bulletin of the Society of Economic Geologists. - : Society of Economic Geologists. - 0361-0128 .- 1554-0774. ; 113:3, s. 699-739
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Tidskriftsartikel (refereegranskat)abstract
- Medium- to high-grade metamorphosed, 1.9 Ga, stratiform, syngenetic Zn-Pb±Ag sulfide deposits comprise an economically important type of ore deposit in the Bergslagen lithotectonic unit of the Fennoscandian shield. The Lovisa Zn-Pb deposit occurs in a metamorphosed succession of rhyolitic ash-siltstone, rhyolitic mass flow deposits, limestone and iron formation, deposited at a stage of waning volcanism in Bergslagen.Accessory graphite, absence of Ce anomalies in shale-normalized rare-earth element (REE) data, and absence of hematite in Mn-rich iron formations stratigraphically below the Lovisa Zn-Pb deposit indicate a suboxic-anoxic depositional environment. The uppermost Mn-rich iron formation contains disseminated, inferred syngenetic Pb-Ag mineralization with mainly negative δ34S values in sphalerite and galena (-6.1 to -1.9‰).Deposition of this iron formation terminated during a pulse of explosive felsic volcanism. The Lovisa Zn-Pb deposit is interpreted to have formed in an alkali-rich brine pool developed immediately after this volcanic event, based on lithogeochemical and stratigraphic evidence. The first stage of mineralization deposited stratiform sphalerite mineralization with mainly positive δ34S values (-0.9 to +4.7‰). This was succeeded by deposition of more sphalerite-galena stratiform mineralization with δ34S values close to 0‰ (-2.1 to +1.5‰). The more galena-rich mineralization partitioned strain and was partly remobilized during later ductile deformation.The stratigraphic context, sulfide mineralogy, sulfur isotopes and alteration geochemistry suggest that the metalliferous fluids and the depositional environment were H2S-deficient (S-poor or SO42--dominant). The source of sulfur is interpreted to have been a mixture of H2S derived from bacterial and thermochemical seawater sulfate reduction, and sulfur derived from leaching of volcanic rocks, with the latter becoming more important over time.Lovisa formed in a setting where basin subsidence was periodically punctuated by the deposition of thick, syn-eruptive felsic volcaniclastic massflow deposits. Coeval volcanism was likely important for driving hydrothermal activity and supplying a reservoir of metals and sulfur. However, the high rate of deposition of volcaniclastic sediment in Bergslagen also precluded the establishment of long-lived, deep and anoxic environments favorable for accumulation of organic matter and H2S. This stratigraphic pattern is common in Bergslagen and may explain why large stratiform Zn-Pb deposits are uncommon in the region and restricted to the uppermost part of the metavolcanic succession, directly stratigraphically beneath post-volcanic pelitic rocks.
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| 49. |
- Jansson, Nils, et al.
(författare)
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The origin of skarn beds, Ryllshyttan Zn–Pb–Ag + magnetite deposit, Bergslagen, Sweden
- 2011
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Ingår i: Mineralogy and Petrology. - : Springer Science and Business Media LLC. - 0930-0708 .- 1438-1168. ; 103:1-4, s. 49-78
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Tidskriftsartikel (refereegranskat)abstract
- Thin- to medium-bedded, stratiform calc-silicate deposits (banded skarns) are a peculiar, but important, component of the supracrustal successions in the Palaeoproterozoic Bergslagen mining district of central Sweden. They are referred to as “skarn-banded leptites” in the literature and are common in areas and at stratigraphic levels that contain iron oxide and base metal sulphide deposits. The stratigraphic hanging wall of the stratabound Ryllshyttan Zn–Pb–Ag + magnetite deposit at Garpenberg, contains approximately 100–150 m of interbedded aluminous skarn beds and rhyolitic ash-siltstones. The skarn beds are mineralogically variable and dominantly composed of grandite, spessartine, epidote, actinolite, quartz, clinopyroxene, and locally magnetite. Integrated field-mapping, and whole-rock lithogeochemical, microscopic and mineral chemical analyses suggest that the stratiform skarn beds are the products of at least two discrete hydrothermal events and subsequent metamorphism. The first event comprised accumulation in a quiescent subaqueous environment, below wave base, of calcareous and ferruginous sediments rich in Fe, Mn, Ca, and Mg. These chemical sediments were deposited concurrently with rhyolitic ash-silt sedimentation, thus forming a (now metamorphosed) laminated calcareous Fe formation with both a detrital rhyolitic component and rhyolitic siltstone interbeds. Positive Eu-anomalies and negative Ce-anomalies for normalized rare earth element analyses of skarn beds suggest that the iron may have been derived from exhalation of hot and reduced hydrothermal fluids, which upon mixing with more oxidized seawater, precipitated Fe oxides and/or carbonates that settled from suspension to the seafloor. The size of the positive Eu-anomalies of the chemical sediments are modified by the content of rhyolitic volcaniclastic material, which has a negative Eu anomaly, such that positive Eu-anomalies are only observed in skarn beds that possess a minor volcaniclastic component. Subsequently, the calcareous Fe formations were subjected to post-depositional alteration by hydrothermal fluids, locally yielding more manganoan and magnesian assemblages. The Mn-alteration is manifested by lateral gradations from epidote-grandite-clinopyroxene±magnetite rocks into significantly more Mn-rich quartz-spessartine rocks and massive andradite rocks over distances of less than 10 cm within individual skarn beds. Magnesian alteration is manifested by the development of discordant zones of pargasite para-amphibolites and formation of stratiform pargasite rocks texturally similar to the interlaminated grandite-epidote-ferroan diopside rocks. The latter increase in abundance towards the Ryllshyttan deposit and are associated with pre-metamorphic/pre-tectonic K–Mg–Fe±Si alteration (now biotite-phlogopite-garnet-cordierite-pargasite rocks) that is related to base metal mineralization. The zone of Mn- and Mg-altered skarn beds extends beyond the zone of pervasive K–Mg–Fe±Si alteration around Ryllshyttan. This suggests that the skarn bed progenitors, or their sedimentary contacts against rhyolitic ash-siltstones, acted as conduits to outflowing hydrothermal fluids. The chemical and mineralogical imprint, imposed on affected beds by alteration, may serve as indicators of proximity to intense K–Mg–Fe±Si alteration envelopes around other base metal sulphide deposits in Bergslagen. The last recorded event comprised syn-tectonic veining of competent massive andradite skarn beds. The veins contain quartz-albite-epidote-ferroan diopside-actinolite assemblages.
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| 50. |
- Jansson, Nils, et al.
(författare)
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Timing and setting of skarn and iron oxide formation at the Smältarmossen calcic iron skarn deposit, Bergslagen, Sweden
- 2013
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Ingår i: Mineralium Deposita. - : Springer Science and Business Media LLC. - 0026-4598 .- 1432-1866. ; 48:3, s. 313-339
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Tidskriftsartikel (refereegranskat)abstract
- Abundant iron oxide deposits including banded iron formations, apatite iron oxide ores, and enigmatic marble/skarn-hosted magnetite deposits occur in the Palaeoproterozoic Bergslagen region, southern Sweden. During the last 100 years, the latter deposit class has been interpreted as contact metasomatic skarn deposits, metamorphosed iron formations, or metamorphosed carbonate replacement deposits. Their origin is still incompletely understood. At the Smältarmossen mine, magnetite was mined from a ca. 50-m-thick calcic skarn zone at the contact between rhyolite and stratigraphically overlying limestone. A syn-volcanic dacite porphyry which intruded the footwall has numerous apophyses that extend into the mineralized zone. Whole-rock lithogeochemical and mineral chemical analyses combined with textural analysis suggests that the skarns formed by veining and replacement of the dacite porphyry and rhyolite. These rocks were added substantial Ca and Fe, minor Mg, Mn, and LREE, as well as trace Co, Sn, U, As, and Sr. In contrast, massive magnetite formed by pervasive replacement of limestone. Tectonic fabrics in magnetite and skarn are consistent with ore formation before or early during Svecokarelian ductile deformation. Whereas a syngenetic-exhalative model has previously been suggested, our results are more compatible with magnetite formation at ca. 1.89 Ga in a contact metasomatic skarn setting associated with the dacite porphyry.
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