SPHALERITE-BEARING DETRITAL SAND BODIES IN MISSISSIPPI VALLEY-TYPE ZINC DEPOSITS MASCOT JEFFERSON-CITY DISTRICT, TENNESSEE - IMPLICATIONS FOR THE AGE OF MINERALIZATION

The Mississippi Valley-type sphalerite mineralization in the Mascot-Je fferson City zinc district of East Tennessee occurs as open-space fill ings in breccia bodies within the upper part of the Knox Group (Lower Ordovician) which is truncated by a regional unconformity. A lower age limit of mineralization is constrained by the formation of solution-c ollapse breccia bodies, which are believed to be related to the post-K nox unconformity. The breccias contain irregularly distributed ''sand' ' bodies that represent cavities filled with well-laminated and size-g raded, sphalerite-bearing, detrital, internal sediments. The texture, composition, and fluid inclusion characteristics of the sphalerite, ar e consistent with its local derivation from the wallrocks as detrital grains. The conformability between the laminations in the sediments an d the bedding planes of the host carbonate rocks suggests that the san d bodies formed prior to the regional deformation event (Alleghenian o rogeny). The stylolitization of carbonate and sphalerite clasts in the internal sediments as well as the deformation of the sphalerite are a lso consistent with a pre-Alleghenian age for the emplacement of the m ain-stage sphalerite mineralization in the Mascot-Jefferson City distr ict and, by analogy, in other Lower Ordovician-hosted Mississippi Vall ey-type districts of the southern Appalachians.