IGNEOUS INTRUSIONS ASSOCIATED WITH THE COTTAGE GROVE FAULT SYSTEM IN SOUTHERN ILLINOIS: THEIR EFFECTS ON MINE PLANNING

The Cottage Grove Fault System in Southern Illinois is a major structural feature (Nelson and Krausse, 1981) that has affected the mineability of the Springfield (No. 5) Coal (Pennsylvanian) of Saline County, Illinois. Associated with some of the Cottage Grove System faults are igneous dikes and sills that are almost always accompanied by parallel faults or fractures. Where dikes intersect the Springfield (No. 5) Coal, the coal is coked and sometimes shows successive zones of alteration and mineralization (Clegg, 1955). The Big Ridge No. 2 Mine in Saline County is located just north of the main Cottage Grove Fault System and along with subsidiary faults; two igneous dikes have been encountered during the underground mining process.

The most extensive dike encountered was on the order of 30 feet wide striking N30degW with a dip angle of 87 degrees (nearly vertical). The dike was defined by surrounding old works, exploration drilling, and magnetic ground survey equipment. Compositionally, the dike is described as an alkaline ultramafic dike and more commonly a peridotite. The igneous dike was of sufficient hardness that the mine hired a drilling and blasting contractor to drive east through the dike. Five entries were driven in a little more than three weeks; with few problems encountered. Once across the dike, unaltered coal was found within half the width of the dike.