Reconnaissance of the geology and ground-water resources in the Aurora area, St. Louis county, Minnesota

Abstract

The Aurora area is a glaciated upland of drift-mantled slopes, channels, swamps, and glacial-lake plains. It covers about 24 square miles of the eastern part of the Mesabi Iron Range in northeastern Minnesota. A deep narrow channel along the Embarrass River, the principal outlet of a former large glacial lake north of the Embarrass Mountains, lies partly within the area. The deposits in the report area consist of bedrock and unconsolidated glacial drift. The 'bedrock belongs to the Animikie Group of late Preeambrian age and consists of taconite (an iron-rich silicate rock) in the northern part and slightly metamorphosed argillite in the middle and southern parts. Bedrock is exposed only in the open-pit iron mines. Large quantities of ground water are pumped from porous and permeable ore zones in the St. James Mine. Small quantities of ground water are obtainable from openings along bedding planes and fractures in the argillite. Unconsolidated deposits consisting of till and water-laid glacial and alluvial materials mantle the bedrock to depths ranging from about 20 feet in the north-central part of the Aurora area to more than 300 feet near the Embarrass River. Thick deposits of sand and gravel in the Embarrass channel are capable of yielding large quantites of water. At places along the Partridge River glaeiofluvial deposits (glacial sediments deposited in running water) could yield moderate to large quantities of water. Sandy to bouldery till yields small quantities of water to domestic wells. Well yields in the Aurora area range from less than 5 gpm (gallons per minute) to about 250 gpm from a well tapping an ore body. The specific capacity of wells penetrating ore zones ranges from about 7 gpm per foot of drawdown to 25 gpm per foot of drawdown. Although no attempt has been made to develop a high-yield well in the sand and gravel deposits of the Embarrass channel, more than 5,000 gpm is pumped from sumps which collect water from these deposits in the Embarrass mine. Most domestic wells yield about 5 gpm and are drilled and finished in sand or gravel in either the bouldery till or glaciofluvial deposits. Ground water from the unconsolidated deposits is hard and commonly contains large, undesirable amounts of iron and manganese. Water from the 'bedrock aquifers contains less iron and manganese than does water from the unconsolidated deposits.