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- Romer, Rolf L., et al.
(författare)
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Geochronology of the Kiruna iron ores and hydrothermal alterations
- 1994
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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. ; 89:6, s. 1249-1261
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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- Romer, Rolf L., et al.
(författare)
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Scapolite: a tracer for the initial lead isotopic composition in sulfide deposits with later additions of radiogenic lead
- 1996
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Ingår i: Mineralium Deposita. - 0026-4598 .- 1432-1866. ; 31:1-2, s. 134-139
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Tidskriftsartikel (refereegranskat)abstract
- The initial lead isotopic composition of metamorphosed and tectonically reworked sulfide deposits is not always preserved, as sulfides easily change their lead isotopic composition through incorporation of lead derived from external fluids or redistribution and recrystallization of the deposit. Sulfide trace-lead and in cases even galena-lead from such deposits may show exceedingly radiogenic lead isotopic compositions. Thus, the initial lead isotopic composition has to be estimated from other minerals. Scapolite, which is a common phase in alteration haloes associated with epigenetic sulfide deposits in northern Sweden, has very low uranium-contents. Therefore, its trace-lead contents could preserve the initial isotopic composition of the ore-forming fluids. As scapolite is more resistant to recrystallization, it is more likely to reflect the original lead isotope signature of the deposit. This is illustrated using scapolite and sulfides from the Pahtohavare Cu-Au deposit in northern Sweden, which is hosted by Palaeoproterozoic mafic tuffites and graphitic schists and was affected by a mild thermal metamorphism during the Caledonian orogeny.
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