Geological and petrographical investigations in the Arendal district
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The rocks of the Arendal district in Sørlandet, Norway, are all of pre-Cambrian age and belong to the Kongsberg- Bamble formation. This paper chiefly deals with the south-western part, which is shown in the geologocal map, flg. 13 and marked by the hatched triangle on flg. 1.
Evidence is cited to show that the oldest rocks are of supra-crustal origin, including quartzites, limestones, effusives, para-gneisses etc. The banded gneisses, so common in pre-Cambrian formations, are also supposed to belong here.
During an orogenic period the rocks were strongly folded and invaded by igneous and migmatic rocks, ranging in composition from hyperites, norites and hypersthene diorites to charnockites and granites. During this time the rocks of the old complex were to a high degree changed to migmatites and granites.
The skarn-iron ores of Arendal all belong to the old supra-crustal system and are of metamorphic origin.
They have been mined for more than 250 years and were in the 18th century among the most important in Norway.
Some of the mines are about 250 m deep, but the total production is only about 2 3/4 mill. tons. After 1870 there has been work only at Klode borg and Bråstad, which were abandoned in 1920. Tectonics and mineral parageneses of the deposits are briefly discussed.