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  • 1.
    Allen, Rodney
    et al.
    Volcanic Resources Ltd, Stavanger.
    Weihed, Pär
    Geological Survey of Sweden.
    Svensson, S. Å.
    Boliden AB.
    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, Sweden1996Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 91, nr 6, s. 1022-1053Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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 Intense marine volcanism was superceded at different times in different parts of tile district by a stage of reduced volcanism, uplift resulting in subregional disconformities, and then differential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.

  • 2.
    Bauer, Tobias
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Andersson, Joel
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik. LKAB, Malmberget.
    Sarlus, Zimer
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Lund, Cecilia
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Mineralteknik och metallurgi.
    Kearney, Thomas
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Structural controls on the setting, shape and hydrothermal alteration of the Malmberget IOA deposit, northern Sweden2018Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 113, nr 2, s. 377-395Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Malmberget iron oxide-apatite (IOA) deposit in northern Sweden is one of the largest underground iron ore mine operations in the world with estimated ore reserves in 2015 of 346 million metric tons (Mt) at 42.5% Fe. The underground operation is concentrated in 10 orebodies of 5 to 245 Mt each, which currently produce 17.4 Mt of apatite iron ore per year. Structural investigations were combined with data on hydrothermal mineral assemblages in order to reconstruct the relative timing of ore-forming, deformation, and overprinting hydrothermal events. The results improve the understanding of structural geometries, relationships, and control on orebody transposition in the deposit. A first compressional event (D1) around 1.88 Ga represents the main metamorphic event (M1) in the area and was responsible for a strong transposition of potential primary layering and the orebodies and led to the formation of a composite S0/1 fabric. A subsequent F2 folding event around 1.80 Ga resulted in the formation of an open, slightly asymmetric synform with a steeper southeast limb and a roughly SW-plunging fold axis. The result of structural modeling implies that the ore formed at two separate horizons. The folding was accompanied by stretching, resulting in boudinage of the iron orebodies. D2-related high-strain zones and syntectonic granites triggered the remobilization of amphibole, biotite, magnetite, and hematite and controlled the formation of iron oxide-copper-gold (IOCG)-type hydrothermal alteration, including an extensive K-feldspar alteration accompanied with sulfides, scapolite, and epidote. This shows a distinct time gap of at least 80 m.y. between the formation of iron oxides and sulfides. Brittle structures and the lack of an axial planar parallel fabric in conjunction with previous results suggest upper crustal, low-pressure, and high-temperature conditions during this D2 deformation phase, indicating a hydrothermal event rather than a purely regional metamorphic compression. It is proposed in the present study that the Malmberget IOA deposit was deformed and metamorphosed during a 1.88 Ga crustal shortening event. Moreover, the Malmberget IOA deposit was affected by a 1.8 Ga folding and hydrothermal event that is related to a regional IOCG overprint.

  • 3.
    Billström, Kjell
    et al.
    Swedish Museum of Natural History.
    Weihed, Pär
    Age and provenance of host rocks and ores in the Paleoproterozoic Skellefte District, northern Sweden1996Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 91, nr 6, s. 1054-1072Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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 turbiditic sedimentary rocks and coarser clastic rocks, as well as younger, mainly mafic, volcanic rocks (Vargfors Group). To the north, subaerial volcanic rocks of the Arvidsjaur Group are probably coeval with the Vargfors Group. The sedimentation in the Bothnian basin, south of the Skellefte district, appears to have started at ca. 2.0 Ga and continued until ca. 1.86 Ga, as indicated by the presence of granitoids spanning this time interval. The first main magmatic episode in the Skellefte district was a felsic stage at around 1.89 Ga as confirmed by two new U-Pb zircon ages from volcanic rocks situated in the central and eastern part of the district (Bjurvattnet, 1884 + or - 6 Ma; Melestj rn, 1889 + or - 4 Ma). No basement is known to the felsic magmatism, but granitoids occurring to the south of the district, which have been dated at 2.0 to 1.9 Ga, could constitute remnants of a basement which was destroyed by 1.89 Ga arc volcanism within the Skellefte district. The Vargfors Group overlies the Skellefte Group with no major unconformity, and one new age from an ignimbrite in the Vargfors Group (1875 + or - 4 Ma) confirms the temporal relationship with the deposition of subaerial volcanic rocks of the Arvidsjaur Group.An evaluation of age data for the early, synvolcanic (ca. 1890 Ma) Joern-type granitoids suggests that these should be further subdivided. Three different generations of Joern-type granitoids may exist. The GI phase has an age of about 1.89 Ga, the GII and GIII phases within the major Joern batholith probably formed at around 1.87 Ga, and the Siktr sk intrusion in the southern part of the district, has a crystallization age of ca. 1.86 Ga.A number of distinctive isotopic characteristics have been observed, e.g., significant data scatter for Sr whole-rock data, reversely discordant zircon data, and unusually young lower intercept ages for zircon discordia. These features seem to relate preferentially to volcanic rocks, and it is suggested that this behavior is due to Phanerozoic hydrothermal processes that have mobilized elements at different scales. Upper intercepts for zircon discordia, however, are with one exception thought to represent true crystallization ages. The 1847 + or - 3 Ma age for a mass flow at Petiktr sk, as defined by a three-point discordia, is for geologic reasons too young, but a considerably higher (super 207) Pb/ (super 206) Pb age at 1890 Ma for one zircon fraction is more consistent with the field relationships.Volcanic-hosted massive sulfide ores occur in the upper part of the volcanic sequence of the Skellefte Group and, in some cases, also in the lower part of the Vargfors Group. A good approximation of the age of massive ore formation is provided by the age of the host rocks. It is suggested that two main depositional stages of massive ore occurred at ca. 1885 to 1880 Ma and at ca. 1875 Ma. Gold occurs in two principal settings, as a constituent in the volcanic-hosted massive sulfide ores, and related to quartz veins found both in intrusive and supracrustal rocks. In the massive ores, gold was probably emplaced in connection with the hydrothermal processes which concentrated the base metals. Gold in some major intrusive-related Au deposits (e.g., Bjoerkdal) is likely to have concentrated at a premetamorphic stage, tentatively at 1.87 Ga, and still other Au ores (e.g., Boliden) may be epithermal in origin and were possibly formed at a relatively late stage at ca. 1.85 Ga. Later, during peak metamorphic conditions, some mesothermal Au-As vein deposits (e.g., Grundfors) formed at ca. 1.84 to 1.82 Ga.

  • 4.
    Frank, Katherine
    et al.
    Department of Geological and Atmospheric Sciences, Iowa State University.
    Spry, Paul
    Department of Geological and Atmospheric Sciences, Iowa State University.
    Raat, Hein
    Raat Geoservices.
    Allen, Rodney
    Volcanic Resources AB.
    Jansson, Nils
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Ripa, Magnus
    Geological Survey of Sweden.
    Variability in the Geological, Mineralogical, and Geochemical Characteristics of Base Metal Sulfide Deposits in the Stollberg Ore Field, Bergslagen District, Sweden2019Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 114, nr 3, s. 473-512Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 5.
    Frietsch, Rudyard
    Luleå tekniska universitet.
    On the magmatic origin of iron ores of the Kiruna type: reply1984Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 79, nr 8, s. 1949-1951Artikel i tidskrift (Övrigt vetenskapligt)
  • 6.
    Jansson, Nils
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Erismann, Fabian
    Boliden Mineral AB, Exploration Department, 776 98 Garpenberg.
    Lundstam, Erik
    Boliden Mineral AB, Exploration Department, 776 98 Garpenberg.
    Allen, Rodney
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Evolution of the paleoproterozoic volcanic-limestone-hydrothermal sediment succession and Zn-Pb-Ag and iron oxide deposits at Stollberg, Bergslagen region, Sweden2013Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 108, nr 2, s. 309-335Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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

  • 7.
    Jansson, Nils
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Sädbom, Stefan
    Lovisagruvan AB.
    Allen, Rodney
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Billström, Kjell
    Swedish Museum of Natural History.
    Spry, Paul G
    Iowa State University.
    The Lovisa stratiform Zn-Pb deposit, Bergslagen, Sweden: Structure, stratigraphy, and ore genesis2018Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 113, nr 3, s. 699-739Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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.

  • 8.
    Kampmann, Tobias Christoph
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Jansson, Nils
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Stephens, Michael
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Majka, Jarosław
    Uppsala universitet, AGH University of Science and Technology, Mickiewicza.
    Lasskogen, Jonas
    Boliden Mineral AB.
    Systematics of Hydrothermal Alteration at the Falun Base Metal Sulfide Deposit and Implications for Ore Genesis and Exploration, Bergslagen ore district, Fennoscandian Shield, Sweden2017Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 112, nr 5, s. 1111-1152Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Paleoproterozoic Falun Zn-Pb-Cu-(Au-Ag) pyritic sulfide deposit in the Bergslagen ore district, Sweden, is enveloped by hydrothermally altered rocks metamorphosed to the lower amphibolite facies. Immobile-element ratios suggest that the alteration precursors were volcanic rocks of mainly rhyolitic to dacitic composition. Least altered examples of these rocks plot along magmatic fractionation trends outlined by late- to post-ore feldspar-phyric metadacite dikes and post-ore granitoid plutons, consistent with a comagmatic relationship between these calc-alkaline, coeval (<10-m.y.) suites. Dolomite or calcite marble, as well as diopside-hedenbergite or tremolite skarn, form subordinate but important lithologic components in the hydrothermally altered zone. Marble occurs as fragments in the massive pyritic sulfide mineralization, suggesting that at least some mineralization formed by carbonate replacement.

    Mass-change calculations suggest that the hydrothermally altered volcanic rocks gained Mg and Fe and generally lost Ca, K, and Na. Proximal, quartz-anthophyllite-rich altered rocks additionally gained Si, whereas several types of biotite-rich altered rocks lost this element. These mass changes along with mineral chemical data for anthophyllite, biotite, cordierite, and garnet, and the common occurrence of quartz indicate that chloritization, sericitization, and silicification were the dominant premetamorphic alteration styles. A zonation from distal sericitized and silicified volcanic rocks to intermediate sericitized rocks, partly overprinted by chloritization (Mg-rich chlorite), and proximal siliceous and intensely chloritized (Fe-rich chlorite) rocks has been identified. Furthermore, mass changes in more peripheral parts of the altered zone toward the southeast of the deposit suggest that the alteration weakens gradationally toward the volcanic and subvolcanic rocks surrounding the deposit. These patterns represent vectors toward mineralization.

    Intensely chloritized rocks, largely represented by a single, rhyolitic precursor, envelop the central pyritic massive sulfide bodies to the east, south, and west, supporting a structural model in which the massive sulfide mineralization formed the stratigraphically highest preserved unit in the center, surrounded in a tubular manner by stratigraphic footwall rocks. The northern side represents a portion of the footwall, which was separated by a major shear zone. These spatial relationships also have implications for near-mine exploration, since quartz-rich footwall rocks locally host disseminated to semimassive stockwork Cu-Au mineralization.

    Cooling of a hot (300°–400°C), acidic (pH ≤4) and reducing fluid carrying metals and sulfur is suggested for formation of stockwork Cu-Au vein mineralization and hydrothermal alteration in the stratigraphic footwall. The Zn-Pb-Cu-rich massive sulfide mineralization is inferred to have formed by fluid neutralization upon interaction with carbonates and mixing with cooler seawater upon fluid entry into porous pumice breccia in a subseafloor setting. Dissolution processes, primary porosity in the pumice breccia, and secondary porosity produced during synvolcanic faulting are all suggested to have contributed to the creation of space necessary for the formation of the massive sulfide mineralization. Falun differs from other deposits of the same type in Bergslagen mainly in the high pyrite content of the massive sulfide mineralization, the absence of related Fe oxide deposits, as well as the dominant replacement of volcaniclastic sediments compared to carbonates. The types of host rocks, the inferred premetamorphic feldspar-destructive alteration types, and the style of mineralization and alteration zonation at the deposit are reminiscent of pyritic volcanogenic massive sulfide (VMS) deposits. However, the importance of chemical trapping by fluid-limestone interaction, as well as the spatial association with subordinate skarn alteration constitute important differences to a classic VMS model.

  • 9.
    Romer, Rolf L.
    et al.
    Luleå tekniska universitet.
    Lehmann, Bernd
    Institut für Mineralogie und Mineralische Brukstoffe.
    U-Pb columbite age of Neoproterozoic Ta-Nb mineralization in Burundi1995Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 90, nr 8, s. 2303-2309Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Tin granites and associated Ta-Nb pegmatites in the Kibran belt of central Africa have been dated earlier only by highly scattered Rb-Sr data that were interpreted to reflect events at 1000 to 900 and 650 to 550 Ma. New U-Pb dating of columbite from the Kuvuvu and Ruhembe pegmatites in Burundi precisely determines the age of the mineralization and indirectly also the age of the associated granite magmatism. Columbites from the Kuvuvu and Ruhumbe pegmatites have variably discordant U-Pb data that define emplacement ages at 962 ± 2 Ma (2σ) and 968-29+33 Ma, respectively. The lower intercept ages at 628 ± 110 Ma and 622 ± 56 Ma are interpreted to reflect the pan-African brittle reworking of the pegmatite system when Au and Bi were redistributed on fractures and secondary sericite formed. The U-Pb age data from columbite thus confirm earlier interpretations of Rb-Sr data

  • 10.
    Romer, Rolf L.
    et al.
    Luleå tekniska universitet.
    Martinsson, Olof
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Perdahl, Jan-Anders
    Luleå tekniska universitet.
    Geochronology of the Kiruna iron ores and hydrothermal alterations1994Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 89, nr 6, s. 1249-1261Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The magnetite-apatite iron ores at Kiruna are hosted in a suite of alkaline volcanic and subvolcanic rocks that was formed in a short time interval of about 20 m.y. between 1900 and 1880 Ma. U-Pb dating of titanite from magnetite-titanite dikes in the footwall of the Luossavaara orebody indicates that the age of the magnetite-apatite ores is 1888 + or - 6 Ma. Titanite-actinolite-calcite assemblages in amygdules are related to the hydrothermal and metasomatic alteration of the wall rocks and the ore. The U-Pb titanite age of the amygdule fill is 1876 + or - 9 Ma. The geographic distribution of the alteration suggests that it is not related to the formation of the iron ores. The youngest major magmatic units in the Kiruna area are 1792 + or - 4 Ma syenites that intruded the supracrustal rocks. The Kiruna area underwent a weak metamorphic overprint during the Caledonian orogeny. Our data are inconsistent with a hydrothermal event at ca. 1.5 Ga as has been suggested earlier.

  • 11.
    Saintilan, Nicolas J. D
    et al.
    University of Geneva, Department of Earth and Environmental Sciences, Section of Earth and Environmental Sciences, University of Geneva.
    Schneider, Jens C.
    Department of Mineralogy, Technische Universität Bergakademie Freiberg.
    Stephens, Michael
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Chiaradia, Massimo
    Section of Earth and Environmental Sciences, University of Geneva.
    Kouzmanov, Kalin
    Section of Earth and Environmental Sciences, University of Geneva.
    Wälle, Marküs
    Institute of Geochemistry and Petrology, ETH Zürich.
    Fontboté, Lluís
    Section of Earth and Environmental Sciences, University of Geneva.
    A middle ordovician age for the laisvall sandstone-hosted Pb-Zn deposit, Sweden: A response to early caledonian orogenic activity2015Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 110, nr 7, s. 1779-1801Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Ten sphalerite separates isolated from mineralized samples in proximal and distal positions relative to the proposed main feeder fault systems at the Laisvall deposit were used to obtain an absolute age determination of this world-class Pb-Zn deposit hosted by autochthonous Ediacaran to Lower Cambrian sandstone and located currently along the erosional front of the Scandinavian Caledonides in northern Sweden. Residue and leachate fractions of each separate were obtained using the crush-leaching technique. All samples correspond to sphalerite formed using reduced sulfur derived from thermochemical sulfate reduction, three of them from disseminated ore in the Lower Sandstone, two from the disseminated ore in the Upper Sandstone, and five from steeply dipping galena-sphalerite-calcite veinlets interpreted in previous works as remobilization of disseminated ores. The isotope dilution-thermal ionization mass spectrometry (ID-TIMS) data yield an overall complex Rb-Sr isotope pattern with two distinct trends in the 87Sr/86Sr vs. 87Rb/86Sr isochron diagram. The three sphalerite residues of disseminated mineralization from the Lower Sandstone orebody show Rb-Sr isotope systematics indicative of undisturbed primary precipitates, and yield an isochron model age of 467 ± 5 Ma (mean square weighted deviation, MSWD, 1.4). Since the isochron is based on three points, the obtained age is to be considered as preliminary. Yet, the obtained age is fully consistent with geologic evidence reported by previous authors and pointing to Middle Ordovician timing of ore formation. The ID-TIMS data were complemented by laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses on the same sphalerite samples. The data support the hypothesis that the measured ID-TIMS Rb and Sr contents in these sphalerite residues are held in the sphalerite structure itself and are not related to micro-inclusions. The most viable hypothesis, in agreement with published work, is that during rapid growth, sphalerite may incorporate Rb and Sr ions from the hydrothermal fluids in its structure, most probably in octahedral voids. By contrast, the second trend in the 87Sr/86Sr vs. 87Rb/86Sr space defined by most other sphalerite residues and corresponding inclusion fluid leachates from the Upper Sandstone orebody and the veinlet samples is too steep to account for a realistic isochron age determination. This steep linear trend is interpreted to represent a postmineralization disturbance involving fluids rich in Sr. This disturbance of the Rb-Sr isotope system is consistent with the presence of the steeply dipping galena-sphalerite-calcite veinlets and the fact that the Upper Sandstone is, in places, tectonically disrupted because of its proximity to the basal Caledonian décollement. The attempt to date the Granberget deposit, located in tectonically disrupted allochthonous units inside the Caledonian orogen, failed because the Rb-Sr isotope systematics of the three analyzed sphalerite samples are also disturbed. The obtained Middle Ordovician (467 ± 5 Ma) mineralization age at Laisvall can be interpreted as a far-field foreland response to an early Caledonian arc-continent collision and the subsequent development of a foreland basin. Basinal brines formed in the foredeep of the orogen could be conveyed cratonward, interact with permeable Baltica crystalline basement rocks, and resurge as metal-bearing fluids in sandstone at Laisvall along reactivated Paleoproterozoic crystalline basement faults. Mixing of metal-bearing brines with hydrocarbon and H2S-rich fluids in Ediacaran to Lower Cambrian sandstone may explain the initial Sr isotope signature (87Sr/86Sr = 0.715900 ± 60) of the isochron intersect

  • 12.
    Saintilan, Nicolas J. D
    et al.
    University of Geneva, Department of Earth and Environmental Sciences.
    Stephens, Michael
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Lundstam, Erik
    Boliden AB.
    Fontboté, Lluís
    University of Geneva, Department of Earth and Environmental Sciences.
    Control of Reactivated Proterozoic Basement Structures on Sandstone-Hosted Pb-Zn Deposits along the Caledonian Front, Sweden: Evidence from Airborne Magnetic Data, Structural Analysis, and Ore-Grade Modeling2015Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 110, nr 1, s. 91-117Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Strata-bound, nonstratiform, epigenetic galena-sphalerite-cement mineralization in Ediacaran-Cambrian sandstone, including the previously mined deposits at Laisvall and Vassbo, occurs along the eastern erosional front of the Caledonian orogen in Sweden. The sandstone is part of an autochthonous siliciclastic sedimentary sequence that rests unconformably on top of Proterozoic crystalline basement beneath the Caledonian thrust nappes. Linear anomalies have been identified in high-resolution airborne magnetic data that correspond to geologic features in the Proterozoic basement. Furthermore, the Laisvall and Vassbo strata-bound Pb-Zn deposits are both spatially associated with areas of change in the trend of the magnetic lineaments. Magnetic anomalies, trending either N-S to NE-SW and WNW-ESE to NW-SE in the Laisvall area, and NNE-SSW to NNW-SSE and NW-SE to W-E in the Vassbo area, were identified. In the Laisvall area, some magnetic minima and edges along magnetic gradients can be correlated with faults in the Proterozoic basement. The reactivation of these basement structures is expressed in the Ediacaran-Cambrian sedimentary cover rocks as newly formed faults with Phanerozoic displacement. Along individual faults belonging to two sets (NE-SW to N-S and WNW-ESE to NW-SE), synsedimentary block movement has been recognized. The highest Pb and Zn grades in Laisvall delineate orebodies and orebody trends that follow these faults. Areas where the faults change strike contain some of the largest and richest orebodies. In the Vassbo area, the orebody footprint reflects a folded dolerite dike in the underlying Proterozoic basement. The dike, modeled on the basis of borehole data, is recognized by a magnetic maximum and an edge along a magnetic gradient. No faults have been mapped at the ground surface as being related to the location of dolerite dikes in the basement. However, it is considered that the basement dikes illustrate a structural control, emplacement either producing a local fracture network or being driven by preexisting basement structures. The main orebodies in both deposits display funnel-shape geometry, fault-rooted in Laisvall and located close to the hinges of the folded dolerite dike in the basement at Vassbo. Metal distribution patterns are similar in both deposits and are characterized by Pb-rich cores proximal to the basement-steered structures while Zn-rich shells are distal from these structures. The funnel-shaped ore geometry is interpreted to reflect a fault-rooted migration path and the metal precipitation mechanism. In both deposits, the highest Pb and Zn grades occur at the top of sandstone paleoaquifers. Similar mineralization footprints, variation in grades, and paleoaquifer settings were recognized in several carbonate-hosted Mississippi Valley-type (MVT) Zn-Pb deposits (e.g., San Vicente deposit, Peru; Topla-Mežica deposits, Slovenia). This geometry is suggestive of a sour gas trap that accumulated by density at the top of paleoaquifers. This gas could have provided H2S by thermogenic sulfate reduction to the metal-bearing fluids and triggered precipitation of Pb-Zn sulfides. The combined evidence from the airborne magnetic data, the structural analysis and the geometry of the orebodies, and metal distribution suggests that the basement faults reactivated during the Ediacaran-Cambrian sedimentation, acted at a later time as feeders for the metal-bearing fluids to fertile horizons for mineralization, and localized deformation during postsedimentary and postmineralization tectonics.

  • 13.
    Stensgaard, Bo Møller
    et al.
    Geological Survey of Denmark and Greenland.
    Chung, Chang-Jo
    Geological Survey of Canada.
    Rasmussen, Thorkild Maack
    Geological Survey of Denmark and Greenland.
    Stendal, Henrik
    Geological Survey of Denmark and Greenland.
    Assessment of mineral potential using cross-validation techniques and statistical analysis: A case study from the paleoproterozoic of West Greenland2006Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 101, nr 7, s. 1397-1413Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    An ideal mineral exploration program characterizes all types of data by describing its relationship to an integrated mineral deposit and exploration model. The model is used to interpret the mineralization and plays a role in the decision making for exploration activities and investments. Successful application of the model depends on validated techniques that quantify the signatures of geologic features, combine data, and lead to parameters that can be expressed in economic terms. A step-by-step procedure for assessment of mineral potential is proposed here based on probabilistic models, on empirical analysis, and on location and quantitative characterization of known mineral occurrences. Mineral potential maps are constructed as a part of the procedure. The reliability of the results is evaluated mathematically by cross validations and prediction rate curves. Estimations of the probability for new discoveries are made and the results are discussed in economic terms. To illustrate the procedure we apply the method to exploration f or volcanogenic massive sulfide deposits in a poorly to moderately explored area of about 25,800 km2 within the Paleoproterozoic Nagssugtoqidian orogen of West Greenland. The input data for the analysis include 67 noneconomic occurrences and 15 different types of geophysical and geochemical data. Based on statistical characterizations, 36 of the occurrences were divided into three groups (the Naternaq, Arfersiorfik, and Ataneq groups). The remaining 31 occurrences did not have consistent characteristics in terms of the 15 geophysical and geochemical parameters, and these were excluded from further statistical study. A consistent set of geophysical and geochemical characteristics was established for each of the three groups and used to construct mineral potential maps of the exploration area. Each potential map was divided into 200 equal-size classes of 129 km2 each (0.5% of the entire study area). The probability that the most favorable 129 km2 will host an occurrence of the type assigned to a particular group is estimated as 71, 32, and 100 percent for the three groups (Naternaq, Arfersiorfik, and Ataneq). The probabilities of new discoveries within a specific minimum prospective area targeted for exploration are also discussed. The probabilities of new discoveries within 1 km2 of the 129 km2 most favorable area were estimated as 1.0, 0.3, and 23 percent, respectively for the three types of occurrences

  • 14.
    Stephens, Michael B.
    et al.
    Geological Survey of Sweden.
    Halenius, Ulf
    Swedish Museum of Natural History.
    Widenfalk, Lennart
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser.
    A group of papers devoted to the geology of two paleoproterozoic base metal sulfide and gold mining districts in the Baltic shield, Sweden: preface1996Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 91, nr 6, s. 977-978Artikel i tidskrift (Övrigt vetenskapligt)
  • 15.
    Sundblad, K.
    et al.
    Geological Survey of Sweden.
    Zachrisson, E.
    Geological Survey of Sweden.
    Smeds, S-A
    Institute of Geology, University of Uppsala.
    Berglund, S.
    Luleå tekniska universitet.
    Ålinder, C.
    Geological Survey of Sweden.
    Sphalerite geobarometry and arsenopyrite geothermometry applied to metamorphosed sulfide ores in the Swedish Caledonides1984Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 79, nr 7, s. 1660-1668Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Metamorphosed massive sulfide deposits in the Swedish Caledonides were investigated using sphalerite geobarometry and arsenopyrite geothermometry. The sphalerite investigations yield pressure estimates of 3 to 5 kb for six deposits from host rocks of greenschist-lower amphibolite facies. A bimodal FeS distribution is apparent for the chlorite-grade deposits whereas the FeS distribution in biotite-grade deposits is unimodal. The arsenopyrite geothermometry is apparently sensitive to high contents (>0.2 wt %) of either of the trace elements Co and Ni, and possibly also of Sb. Temperature estimates for six deposits from the chlorite zone of greenschist facies yield 371 degrees + or - 45 degrees C, whereas one deposit from the biotite zone gives a temperature of 422 degrees + or - 25 degrees C. The pressure and temperature data obtained for the deposits correspond excellently with the metamorphic grade of the host rocks. Thus, sphalerite geobarometry and arsenopyrite geothermometry can be applied successfully to metamorphosed sulfide orebodies if attention is paid to textural relationships and minor element distribution in the minerals investigated

  • 16.
    Trygvason, Ari
    et al.
    Department of Earth Sciences, Uppsala University.
    Malehmir, Alireza
    Department of Earth Sciences, Uppsala University.
    Rodriguez-Tablante, Johiris
    Department of Earth Sciences, Uppsala University.
    Juhlin, Christopher
    Department of Earth Sciences, Uppsala University.
    Weihed, Pär
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Reflection seismic investigations in the western part of the paleoproterozoic VHMS-bearing Skellefte district, northern Sweden2006Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 101, nr 5, s. 1039-1054Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    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 Rävliden 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.

  • 17.
    Weihed, Jeanette Bergman
    et al.
    Institute of Earth Sciences, Mineralogy-Petrology, Uppsala University.
    Bergström, Ulf
    Billström, Kjell
    Swedish Museum of Natural History.
    Weihed, Pär
    Geology, tectonic setting, and origin of the Paleoproterozoic Boliden Au-Cu-As deposit, Skellefte District, northern Sweden1996Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 91, nr 6, s. 1073-1097Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Skellefte district in northern Sweden comprises more than 85 pyritic volcanic-hosted massive sulfide deposits which mainly occur within, and at the top of, a felsic-dominated volcanic unit overlain by a sedimentary sequence. The Boliden Au-Cu-As deposit was one of the first discovered in the district, and it has attracted a continuous interest since then due to its significant size and high gold grade (avg 15 ppm). The Boliden ore can be divided into massive ore, with arsenopyrite- and pyrite-dominated lenses, and vein ore which comprises a quartz-chalcopyrite-sulfosalt-dominated assemblage, occurring in brecciated parts of the arsenopyrite bodies, and quartz-tourmaline veins mainly in host rocks below the massive ore. As a rule, the gold is found in deformational structures in vein ore. Most gold is present as an Au-Ag-Hg alloy with variable compositions, from Au (sub 0.17) Ag (sub 0.68) Hg (sub 0.16) to Au (sub 0.93) Ag (sub 0.07) (in atomic proportions).For the last two decades, the approximately 1.88 Ga massive sulfide ores in the Skellefte district have collectively been interpreted as volcanic exhalative formations resembling the Miocene kuroko ores of Japan. However, this view has recently been challenged and a subsurface replacement origin has been proposed for some of the ores in the district.The Boliden ore is not bound to one particular host rock but occurs in feldspar porphyritic dacite, quartz porphyry, and basalt-andesite. Textural observations suggest that these rocks represent intrusions or lavas. Geochemically, they are typical calc-alkaline volcanic rocks, enriched in large ion lithophile elements, depleted in heavy rare earth elements, and with troughs for Th, Nb, Hf, and Ti. The ore zone, in its present setting, is in a more or less vertical position and oblique to lithological contacts. Ore-related hydrothermal and regional metamorphic processes (lower amphibolite facies) have created a complex alteration system around the ore. This forms a symmetric pattern with an inner sericite-rich zone, locally containing abundant andalusite, and an outer chlorite-dominated zone. The nature of the alteration is consistent with leaching of elements and a silica-alumina-rich residue--features which are often found in epithermal environments.Structural observations suggest that three ductile foliation-forming events have affected the rocks near the ore. These include a regional S (sub 1) foliation, formed during isoclinal folding, which was subsequently sheared causing formation of a strong cleavage S (sub s) and extensive deformation of the ore itself. A late S (sub 2) cleavage crenulated earlier fabrics.The available data and observations are not consistent with a volcanic exhalative model for the ore and the following scenario is favored. Shallow intrusions of dacite and andesite into unlithified sediments occurred around 1.87 Ga. At this time, the earlier marine environment had been lifted up to a shallow-marine or possibly subaerial position. Shortly thereafter, fluids which generated the massive ore at Boliden were focused along a fault, and arsenopyrite and pyrite lenses were precipitated in more than one host rock discordantly to lithological contacts. Regional deformation with folding and shearing, possibly at around 1.85 Ga, led to brecciation of previously formed ores and stretching of orebodies. In relation to this shearing event, Au was introduced and/or remobilized and concentrated in brecciated portions of the ore zone. Thereafter, ores and host rocks recrystallized during peak metamorphism at around 1.82 Ga, and a second deformation at around 1.80 Ga caused crenulation of early fabrics.The crosscutting nature of the ore with respect to the host rocks, the hydrothermal alteration pattern with strongly leached host rocks, and the ore association with early massive sulfides followed by gold, chalcopyrite, and sulfosalts in brittle structures all indicate that a modern analogue for ore formation may be a high-sulfidation epithermal environment. The epigenetic nature of the Boliden deposit has significant implications for exploration of gold deposits elsewhere in the region.

  • 18.
    Weihed, Pär
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Weihed, Janette Bergman
    Uppsala University.
    Sorjonen-Ward, Peter
    Geological Survey of Finland.
    Structural Evolution of the Björkdal Gold Deposit, Skellefte District, Northern Sweden: Implications for Early Proterozoic Mesothermal Gold in the Late Stage of the Svecokarelian Orogen2003Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 98, nr 7, s. 1291-1309Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Björkdal gold deposit is situated in the eastern part of the Paleoproterozoic Skellefte district in northern Sweden. The Skellefte district constitutes a 1.89 to 1.88 Ga volcanic arc with numerous volcanic massive sulfide deposits and lode gold deposits of which the Björkdal deposit is the largest, at ca. 20 Mt with 2.5 g/t Au. The gold at Björkdal is associated with centimeter- to meter-wide, subvertical quartz veins at the northwestern contact between a quartz-monzodioritic to tonalitic intrusion and the surrounding supracrustal rocks. The main quartz veins strike north-northeast, and a minor set of veins strike east-northeast. The quartz veins terminate against a major thrust duplex at the contact between the intrusion and the structurally overlying supracrustal rocks. The mylonitic thrust zone has a 20° to 40° dip toward north and trends approximately east-west. A few kinematic observations indicate reverse to obliquely reverse slip on the thrust. Deformed quartz veins exist in lithons between thrusts within the duplex. In the mine, the quartz veins in the footwall to the thrust are spatially and temporally associated with moderately to steeply west dipping reverse shear zones with a northeast strike. It is suggested here that the quartz veins and the steep reverse shear zones are related to the thrust duplex and formed more or less simultaneously. Fluid inclusion and isotopic results from previous studies indicate that juvenile magmatic fluids were responsible for the precipitation of quartz and sulfides at moderate temperatures and pressures. Furthermore, titanites from the quartz veins give ages of ca. 1.78 to 1.79 Ga, whereas the host pluton is dated at ca. 1.90 Ga, indicating a time gap of over 100 m.y. between the emplacement of the host rock and titanite growth in quartz veins. The regional deformation and metamorphism are poorly constrained in the area to some time between 1.87 and 1.80 Ga. As the quartz veins are virtually undeformed and do not exhibit metamorphic fluid inclusions or other evidence of premetamorphic origin, we interpret the titanite ages in the quartz veins as the age of emplacement of the veins. The ca. 1.78 to 1.79 Ga age is also constrained for the crustal-scale, north-south–striking shear zones in the area, and it is suggested here that the thrust duplex and steep reverse shear zones in the mine are third-order structures related to east-west shortening at ca. 1.80 Ga. Gravity data from the Björkdal area indicate the presence of a less dense body at depth beneath the Björkdal pluton. The geophysical signature is best explained by the presence of a 1.80 Ga Skellefte-type intrusion at depth. Magmatic fluids from this S-type granite may have interacted with the host pluton and precipitated gold in the more competent pluton during the east-west shortening. The common occurrence of scheelite in the quartz veins is further evidence for magmatic fluids derived from a younger pluton at depth.

  • 19.
    Westhues, Anne
    et al.
    Department of Earth Sciences, Memorial University of Newfoundland.
    Hanchar, John M.
    Department of Earth Sciences, Memorial University of Newfoundland.
    Whitehouse, Martin J.
    Department of Geosciences, Swedish Museum of Natural History, Stockholm.
    Martinsson, Olof
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    New constraints on the timing of host-rock emplacement, hydrothermal alteration, and iron oxide-apatite mineralization in the Kiruna District, Norrbotten, Sweden2016Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 111, nr 7, s. 1595-1618Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High spatial resolution zircon U-Pb geochronological data obtained directly on the Kiirunavaara iron oxide-apatite (IOA) deposit, related orebodies, and host rocks provide new constraints on the timing of mineralization in these deposits. These data raise new arguments in the debate of a magmatic versus hydrothermal/metasomatic genesis of these major (2,500 Mt, 30-70 wt % Fe) Paleoproterozoic deposits. The main orebody at Kiirunavaara contains Ti-poor magnetite and minor (0.05-5 wt % P) apatite, located between a trachyandesite footwall and a rhyodacite hanging wall, which also hosts smaller orebodies (Nukutus, Rektorn, and Tuolluvaara). The pervasive Na and K metasomatism in the host rock is documented by whole-rock geochemical data and cathodoluminescence (CL) microscopy. Zircon U-Pb data for the metavolcanic rocks in the footwall and hanging wall cluster between 1884 ± 4 and 1880 ± 3 Ma. In the footwall, a syenite-aplite system yields ages of 1880 ± 7 and 1881 ± 4 Ma; a granite pluton exposed underground has an age of 1874 ± 4 Ma. Zircons in two ore samples, never directly dated before this study, yield ages of 1877 ± 4 and 1874 ± 7 Ma. Brecciation at the contacts between the ore and host rocks, the tight age at ca. 1880 Ma for most volcanic and plutonic rocks in the footwall and hanging wall, and the marginally younger age for ore at ca. 1877 to 74 Ma, matching the age of the spatially related granite pluton, suggest a magmatic-hydrothermal emplacement model for the Kiruna area IOA ores

  • 20.
    Yesares, Lola
    et al.
    Department of Geology, University of Huelva, Avenida de las Fuerzas Armadas.
    Aiglsperger, Thomas
    Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona.
    Sáez, Reinaldo
    Department of Geology, University of Huelva, Avenida de las Fuerzas Armadas.
    Almodóvar, Gabriel R.
    Department of Geology, University of Huelva, Avenida de las Fuerzas Armadas.
    Nieto, José Miguel
    Department of Geology, University of Huelva, Avenida de las Fuerzas Armadas.
    Proenza, Joaquín A.
    Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona.
    Gómez, Carmelo
    Geological Area, Mining Department of Cobre Las Cruces S.A., Gerena, Seville.
    Escobar, Juan Manuel
    Geological Area, Mining Department of Cobre Las Cruces S.A., Gerena, Seville.
    Gold behavior in supergene profiles under changing redox conditions: The example of the Las Cruces Deposit, Iberian Pyrite Belt2015Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 110, nr 8, s. 2109-2126Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Las Cruces deposit is in the eastern end of the Iberian Pyrite Belt (SW Spain). It is currently being mined by Cobre Las Cruces S.A. The main operation is focused on the supergene Cu-enriched zone (initial reserves of 17.6 Mt @ 6.2% Cu). An Au-Ag-Pb–rich gossan resource (3.6 Mt @ 3.3% Pb, 2.5 g/t Au, and 56.3 g/t Ag) occurs in the upper part of the deposit. The Au grade ranges from 0.01 ppm to >100 ppm, and occurs as three different Au ore types: (1) Au mineralization in the upper part of the gossan linked to Fe-oxides lithofacies, (2) Au concentration in the lower part of the gossan associated with leached black shales, and (3) Au ore in the cementation zone related to subvertical fractures.

    A hydroseparation device has been used to obtain heavy mineral concentrates from selected samples of different ore types. Reflected-light microscopy, scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and electron probe microanalysis (EPMA) were used to study the separated Au particles. Significant differences between the defined ore types include the Au-bearing lithologies, mineral associations, textural features, particle sizes, morphologies, and fineness. Au-rich minerals include native Au, Au-Ag electrum, and Au-Ag-Hg amalgams. Gold-bearing mineral associations include Pb-oxihalides, Fe-oxides, galena, pyrite, cinnabar, and Ag-sulfosalts.

    The Au enrichment mechanism in the supergene profile involves (1) dissolution of Au from the primary sulfides as chloride-rich ionic complexes during the weathering of the deposit under subaerial exposure; dissolved Au is transported downward through the supergene profile under acidic and oxidized conditions; (2) destabilization of the Au complexes by Fe-controlled redox reactions; as a consequence, coarse-grained, high-fineness Au particles precipitated in association with Fe-oxyhydroxides. This resulted in secondary concentration in the upper gossan; and (3) after deposition of cover sediments took place a progressive change in the system conditions resulting in a later Au remobilization as hydroxidehalide, hydroxide, thiosulfate, and bisulfide complexes in the lowermost gossan and cementation zone. The main pathways for migration of enriched fluids to the cementation zone are secondary permeability zones linked to Alpine reactivated faults. Deposition of Au seems to be related to fluid interaction with reductant lithologies, including black shales and the primary sulfides.

  • 21.
    Öhlander, Björn
    et al.
    Luleå tekniska universitet, Institutionen för samhällsbyggnad och naturresurser, Geovetenskap och miljöteknik.
    Nisca, Dan
    Swedish Geological Co..
    Tectonic control of Precambrian molybdenite mineralization in Northern Sweden1985Ingår i: Economic geology and the bulletin of the Society of Economic Geologists, ISSN 0361-0128, E-ISSN 1554-0774, Vol. 80, nr 2, s. 505-512Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The major molybdenite occurrences of N Sweden are mostly confined to the Proterozoic continental domain. The molybdenite occurrences are in aplites, pegmatites and metamorphosed volcanics, located in narrow supracrustal belts surrounding granites. The Mo deposits are associated with faults and dome structures (apical parts of cupolas), as recognized by geophysical methods

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