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- 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 Intense marine volcanism was superceded at different times in different parts of tile district by a stage of reduced volcanism, uplift resulting in subregional disconformities, and then differential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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- Billström, Kjell, et al.
(författare)
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Age and provenance of host rocks and ores in the Paleoproterozoic Skellefte District, northern 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. 1054-1072
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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- Björklund, Lennart, et al.
(författare)
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Geochemistry and tectonic setting of the Orvar Hill mafic volcanic rocks of the Tiveden area, south-central Sweden
- 1997
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Ingår i: GFF. - : Informa UK Limited. - 1103-5897 .- 2000-0863. ; 119:2, s. 127-134
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Tidskriftsartikel (refereegranskat)abstract
- The Orvar Hill formation in Tiveden, south-central Sweden, constitutes a unique low-strain window of well preserved Svecofennian mafic volcanic rocks on the southwestern border of the Svecokarelian orogen. The area can be considered as the southwestern border of the Bergslagen region of the Svecokarelian orogen. The Orvar Hill formation consists of coherent pillowed and non-pillowed basalts alternating with mafic volcaniclastic racks in the lower part of the Lindberga supracrustal succession. Only minor felsic volcanic rocks occur in the upper part. Quartz-bearing metagreywackes comprise the top part of the Lindberga supracrustal succession. Geochemistry of lavas and volcaniclastic rocks suggests that the Orvar Hill mafic volcanic rocks were emplaced in a volcanic-are setting. This demonstrates that the Tiveden supracrustal units probably formed in response to volcanism related to subduction. The Tiveden area may thus represent a 1.89 Ga primitive, sediment-starved volcanic are at the margin of the continental volcanic are of the Bergslagen district. The relationship between Tiveden and Bergslagen at the time of formation is not clear and Tiveden may represent a remnant of an are that accreted to a continent at c. 1.88-1.86 Ga.
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