En övergripande beskrivning av malmbildande processer med detaljstudier av en bandad järnmalm från södra Dalarna, Bergslagen

Detta är en Magister-uppsats från Lunds universitet/Geologiska institutionen

Sammanfattning: The Bergslagen region in south central Sweden is known for its diversity and quantity of ores. This study focuses on a tiny BIF (Banded Iron Formation) and its surrounding rocks in the southern part of Dalarna. The fieldwork was carried out in an area close to the town of Ludvika. The BIF occurs from an area approximately one kilometre south of the lake Väsman trending for about 17 km in a NNE direction. In this area, supracrustal volcanic rocks and associated metasediments dominate. The BIF forms part of these metasediments, which also include dolomitic limestone, quartzite and meta-argillites. The BIF was deposited in a local basin, where layers of iron oxides, silica oxides and carbonates accumulated. It seems logical that layers of volcanic material were added interrupting the BIF sedimentation during local eruptions. The present occurrence of BIF is classified to be of the Algoma type. Its deposition differs from other known localities in Sweden especially in its close relation to carbonate sedimentation. The BIF and associated rocks show signs of strong hydrothermal overprinting. This is indicated i.e. by the splitting of originally alkali-intermediate volcanic rocks into Na- and K-enriched ones. Furthermore, elements like fluorine, phosphorus, sulphur and magnesium have been added to the ore and its country rocks. This is suggested by the occurrence of minerals like fluorite, apatite, pyrite and bornite as well as minerals rich in magnesium such as chlorite and cordierite. Today these minerals occur interspersed in the BIF and its country rocks.

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