Underground mining of natural iron ore deposits occurred on the Mesabi Range in northeastern Minnesota from 1892 to 1961. The Mesabi Range is the ledge rock exposure of the Biwabik Iron Formation, a geological feature that stretches in length for just over 100 miles, from west of Grand Rapids in Itasca County to east of Babbitt near the St. Louis / Lake County line. Mining activity, past and current, along the Mesabi Range averages 1.5 miles in width, while the Biwabik Iron Formation can reach thicknesses as great as 500 feet.
Sixty-nine years of underground mining activity have resulted in an extensive network of surface subsidence and potential subsurface voids along much of the Mesabi Range. Surface subsidence is the result of caving done intentionally at the time of mining or caving that occurred naturally as underground structures and overlying rock weakened with the passage of time. These structures range in age from 47 to 116 years old. Subsidence is manifested today as sinkhole depressions, pools, ponds, lakes and, most notably in recent years, road and structure damage. Voids remaining from uncollapsed or incompletely collapsed underground mine workings impact area hydrology and pose unknown risks to existing and potential surface developments, infrastructure, present-day open-pit mining operations, and public safety.
There were three methods of mining natural iron ore used on the Mesabi Range: open pit, underground, and milling. One to all three methods could be utilized at a given mine. Often a mine would start out as an underground operation and later become open pit. Open pit mines may host scramming operations via adits into pit walls, thus employing underground methods.
Milling was a combination of open pit and underground mining. It was of particular use for a thick, narrow ore body lying at the surface that had restricted access for rail and truck haulage out of a deep pit. The overburden was removed by steam or electric shovel. A shaft was then sunk just off the ore body and drifts were cut below the ore body. From these drifts, raises were put up to the surface, typically every 50 to 100 feet. Ore was milled from the surface into chutes in the raises. The chutes were emptied into tramcars that transported the ore to the hoisting shaft, where it was brought to the surface.
Initially, underground mining on the Mesabi Range involved bottom-up ore removal on a given level. Rooms were opened (stoped) by means of square sets, a framework of timbers that could be stacked multiple sets high. This made for dangerous working conditions in the soft ores of the Mesabi Range. Square setting was soon replaced, for the most part, by top-slice drifting, a top-down mining method whereby the ore was removed in slices from the top of a given level.
Top-slicing the Mesabi ores involved extensive use of timber to frame the slice drifts and support the roof. Following removal of ore in three to four slices, smaller timber, rough-cut lumber, and / or chicken wire was used to line the floor and the unmined wall. Charges were set into the vertical support timbers and these were blasted down, caving the roof and surface above. As mining continued downward, the surface was successively caved with it, leading to subsidence features seen today as water-filled depressions.
For a complete description of these mining methods please consult the literature below, particularly van Barneveld, 1913. These selections will provide the reader with a real-time look at mining and mining methods from the early years on the Mesabi Range.
(1) Listing was obtained from Gregory, Winifred, 1915, Bibliography of Minnesota Mining and Geology: University of Minnesota, Minnesota School of Mines Experiment Station, Bulletin No. 4
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