CHARACTERIZATION OF VEINS AND ASSOCIATED ALTERATION OBSERVED IN THE CHATTAHOCHEE TUNNEL, COBB COUNTY, GA

Mineral veins that fill brittle fractures along the Brevard Zone near Atlanta offer an opportunity to obtain information about the post orogenic and exhumation history of the Southern Appalachian Mountains. We have been studying vein mineralogy, fracture orientation, host rock alteration and fluid inclusions within vein minerals from deep rock cores taken for tunneling projects in the Atlanta area. The rocks belong to the Sandy Springs Group, which consists of amphibolite grade gneiss and schist. Exposure of pristine unweathered rock in the walls of the recently constructed Chattahochee Tunnel as well as in the cores taken for site characterization, presents an unprecedented opportunity to study the geologic history of this area.

The veins fill high-angle fractures that occur in a least four different sets. These fractures crosscut regional the foliation. Two vein mineral assemblages and episodes of alteration have been identified: 1) an earlier quartz-calcite-chlorite assemblage that is associated with replacement of biotite in the host rock by muscovite and 2) a later laumontite, prehnite, calcite assemblage which is associated with pervasive orange staining of albite and replacement of biotite by chlorite and in some instances replacement of the host rock by epidote. Muscovite separated from an alteration halo associated with a large vein is currently being age dated. We will also present results of microthermometric measurements of inclusions hosted in quartz associated with the first episode of alteration.