Exceptionally well preserved Precambrian ignimbrites and basic lavas in N. Sweden

Lilljequist, Robert; Svenson, Sven-Åke

doi:10.1080/11035897409454948

Cited by 7 publications

(2 citation statements)

References 3 publications

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“…The Arvidsjaur Group is characterized by brown to red, subaerial, felsic to intermediate volcanic rocks including welded ignimbrite, ash-fall tuff, and volcaniclastic sedimentary rocks (Lilljequist and Svenson, 1974…”

Section: Arvidsjaur Groupmentioning

confidence: 99%

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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The 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 arc volcanism. From facies analysis we interpret the palcogeography 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 belowwave 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 Intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism, uplift resulting in subregional disconformities, and then differential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 palcogeography. Uplift of the arc is attributed to the relaxation of crustal extension and the eraplacement 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 arc 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 turbidires. Facies associations define seven main volcano types, which range from basaltic shields to andesitc cones and rhyolite calderas. Despite this diversity of volcano types, most massive sulfide ores 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 eraplaced volcaniclastic facies and are interpreted to have formed by infiltration a...

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Section: Arvidsjaur Groupmentioning

confidence: 99%

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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“…More commonly, however, these features have been almost obscured by varying degrees of welding and devitrification (e.g. Ross and Smith, 1961;Lilljequist and Svensson, 1974 Feldspar and quartz phenocrysts are common throughout the ignimbrites and eutaxitic porphyry structures are abundant ( fig. 5c).…”

Section: Petrology Of the Ignimbritesmentioning

confidence: 99%

The mineralogy and genesis of uranium in rhyolitic ignimbrites of precambrian age from Duobblon, Sweden

Smellie

1982

Mineral. mag.

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ABSTRACT. The Duobblon rhyolitic ignimbrites, of middle Precambrian age are 60 m thick. They consist of at least three flows with varying degrees of welding and have undergone devitrification, producing lithophysal and spherulitic textures. They are overlain by thick fluviatile red-bed-type conglomerates and sandstones, which in turn are capped by terrestrial acid volcanics.Uranium enrichments of up to 3000 ppm U occur within two or three peneconcordant tabular horizons which are mostly lithophysae-bearing. Fission-track investigations of the ignimbrites and overlying conglomerates and sandstones, which supplement earlier mineralogical studies, show that U occurs as fine pitchblende disseminations; as complex uranotitanates associated with Fe-Ti-Mn oxides; and as coatings associated with matrix sericite. Small amounts of O are present in such primary accessory minerals as sphene, apatite, and zircon.It is suggested that oxidizing U-bearing solutions, generated partly during devitrification of the ignimbrites and partly from the overlying volcano-sedimentary pile, produced the U enrichments with later sulphide deposition, along the more permeable lithophysal horizons in the ignimbrites.

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Spatial and temporal diversity of early Proterozoic volcanic sequences — comparisons between the Baltic and Laurentian shields

Pharaoh

Brewer

1990

Precambrian Research

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Exceptionally well preserved Precambrian ignimbrites and basic lavas in N. Sweden

Cited by 7 publications

References 3 publications

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

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

The mineralogy and genesis of uranium in rhyolitic ignimbrites of precambrian age from Duobblon, Sweden

Spatial and temporal diversity of early Proterozoic volcanic sequences — comparisons between the Baltic and Laurentian shields

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