Bedded-ore structures, Jefferson City Mine, Jefferson City, Tennessee

Abstract

The major ore structures of most of the Mascot-Jefferson City and Copper Ridge zinc districts of Tennessee are large breccia masses that cut vertically through as much as 200 feet of strata and extend as much as several thousand feet along the trend of the breccia zone. Breccias of this type are important ore producers at the Jefferson City mine, but the bulk of production has been from much smaller, bedded-ore structures in which the vertical extent is only 5 to 15 feet and which have a rather sinuous plan wherein the structure width is from 10 to 20 feet. These structures are characteristically developed in limestone in most places, just below a primary dolomite bed. Of principal interest are the reef-like structures, which in themselves are not necessarily important ore producers, but which seem to be the connecting links between larger ore zones; channel-like structures which have provided excellent plumbing for ore solutions; scour-and-fill structures along apparent disconformities between limestone and overlying primary dolomite; and tight folds in the bedding with subsequent brecciation, a feature most common in the uppermost (R bed) part of the limestone section.As mining progresses in the bedded-type ore zones, two major trends become evident, especially in areas where the channel-like and tight-fold structures predominate. A conjugate joint system may account for the orientation of these structures. The reef-like and scour-and-fill structures show less tendency toward any preferred orientation.