WHAT IS THE MODERN METEORIC CONTRIBUTION TO MINNESOTA'S SOUDAN IRON MINE'S DEEP CRUSTAL BRINES?

Anoxic, Fe²⁺ rich CaCl₂ brines, which seep from old drill holes in the lowest 27^th (700 m) level of the Soudan Iron Mine, are the focus of ongoing chemical, isotopic and microbiological studies. These old brines are being diluted by variable amounts of surface infiltration through the overlying mine workings. Infiltration occurs from the surface through fractures in the abandoned open pit and subsurface mine works. The purpose of this study is to examine climate records and mine pumping records to see if any correlation between the two variables exists. Such information could then be used to quantify the ratio of recent meteoric water versus deep crustal brines.

In 1892 the mine began extracting high grade hematite using open pit methods. The ore body is dipping ~ 80 degrees to the north in the 2.7 Ga cratonic rocks of the Superior Province. When the surface pits became unsafe, the mine transitioned to underground mining. Before the mine’s closure in 1962 an estimated 15.5 million long tons of high grade iron ore were produced. The mine consists of 18 levels with two horizontal drifts on each level that radiate in the east and west along strike from the shaft. The closed mine was transferred to the State of Minnesota and is currently operated as the Soudan Underground Mine State Park.

A 119-year precipitation record for the Soudan area has been assembled from weather records. Reasonably complete records of the number hours that dewatering pumps have run exist back to 1995. Fragmentary records exist for earlier pumping which may be relevant. The records consist of the number of operating hours of pumps on 12^th, 22^nd, and 27^th levels. We have assembled the data for the 27^th level. Analogous records for the 12^th and 22^nd levels are being processed. These records are a (complex) proxy for the volumes of water pumped. If statistically robust lags between recharge events and increased mine drainage can be extracted, such lags can provide information on the hydrodynamics of the mine’s recharge. The ultimate goal of the effort we have begun is to provide a more complete, quantitative estimate of the quantities of water routinely entering the mine – both from the surface and from the subsurface. Such estimates will provide boundary conditions for biogeochemical models of the mine.