Redox Conditions in Overburden at Gibbons Creek Lignite Mine, Texas

Jan K. Horbaczewski

Overburden geochemistry, and especially redox (oxidation-reduction) conditions, play a key role in the design of reclamation strategies at Gibbons Creek Lignite Mine. The mine, which is located in east-central Texas, approximately 75 miles northwest of Houston, recovers coal from five seams in the uppermost Eocene of the Tertiary Period. The coal is estimated to be 38 million years old.

The procedures used to determine redox conditions rely on analysis of pre-mining cores obtained from drilling programs and stratigraphic correlations based on lignite exploration drilling and geophysical logging. Direct sampling of the highwall in the open pit has also been performed.

A critical geochemical feature of the overburden is the redox boundary which separates pyritic, reduced material from the overlying non-pyritic, oxidized material. The boundary is clearly marked by a color change in the field, and is remarkably sharp. Its depth is generally uniform at approximately 27 feet below the surface, but has been observed to vary from less than 15 feet to more than 40 feet. It transgresses geological strata which dip at 1 to 2 degrees to the southeast.

Current reclamation strategies involve the separate handling of the oxidized overburden for placement at the surface of the regraded spoil after mining. This eliminates the risk of potential acid formation and adverse environmental impacts from the chemical weathering of pyrite.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995