The Evolution of the Lower Archæan Rocks in Central Sweden and their Iron, Manganese, and Sulphide Ores

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Abstract
The high-grade iron and manganese ores of central Sweden are well known to geologists, many of whom, however, are not aware of modern views of the relationship of the ores to the enclosing rocks and the origin of the latter. The ores are found in an ancient complex of highly altered rocks, including an extensive development of gneisses which differ so widely from their parent material that their genesis can only be elucidated by carefully tracing the sequence of metamorphic changes through a series of transitional types. In recent years much work has been done on these lower Archæan rocks, and the following account, which is mainly for non-Scandinavian readers, has been written to provide a summary of the results and to record the present state of opinion concerning the many problems encountered. Nearly all iron ores of central Sweden belong to the leptite formation, which is the oldest formation known in Fennoscandia. It consists of volcanic rocks (lavas and tuffs) and of sediments such as slates, greywackes, and conglomerates. The volcanic rocks comprise leptites and hälleflintas, the difference between them consisting in the grade of their metamorphism, the hälleflintas having dense groundmasses, the leptites fine-grained, recrystallized groundmasses. The leptites are, in effect, more or less metamorphosed hälleflintas. The volcanic rocks are best preserved in the westernmost part of the ore-bearing region. In the Grythytte field, for instance, N. Sundius has found extremely well-preserved hälleflint-lavas, with euhedral phenocrysts of quartz and felspar, spherulites, micro-poikilitic quartz, and fluidal and granophyric structures.

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