Underground Mining Trends In The Great Swedish Export Iron Ore Mines

THE great Swedish export iron ore mines are Kiruna and Malmberget in the north above the Arctic circle and Grängesberg in Central Sweden, see Figs. 1 to 3. These mines exported in 1951 about 13 million metric tons of high grade iron ore and concentrates. The origin of these three ore fields seems to be the same. They are of pre-Cambrian age and are regarded as typical instances of primary eruptive ore that has thrust up as glowing masses of lava flow between two somewhat older beds of porphyry. Apparently the ore and the porphyry became segregated from common parent magmas by a process of differentiation. Despite the fact that the Kiruna orebody belongs to the earliest pre-Cambrian formation, it has been influenced to an unusually small extent by later metamorphosisms, apart from its having been set on edge through folding and wrenching off by a number of later faults. The Kiruna ore occurs along a stretch of more than 4 miles as a series of slabs or lenses sloping steeply toward the east and sandwiched between two beds of porphyry. The main orebody has a length of about 3 miles and an average width of 300 ft with a maximum of 600 ft. The dip varies from 65° to 45° toward the east and the known depth is 3000 ft, although magnetic surveys indicate it may be at least 6000 ft. Malmberget has undergone a more thorough transformation, having been pressed and crumpled into a meandering band of disconnected layers and lenses. The largest of the more connected bodies of ore, the Great Ore Layer, outcrops on a low mountain ridge and has a total length of about 4 miles and a maximum width of about 300 ft. The dip varies from 30° to 70° towards the north. The main orebody in Grängesberg, the so-called