Internal sediments within are bodies of the Mascot-Jefferson City zinc district, East Tennessee

The Mascot-Jefferson City zinc district lies within the Valley and Ridge Province of the Southern Appalachians, approximately 30 kilometers northeast of Knoxville, Tennessee. The zinc ore is restricted to brecciated zones within the Lower Ordovician Kingsport and Mascot formations, the upper units of the Upper Cambrian-Lower Ordovician Knox Group. The mineralized solution-collapse structures contain irregularly distributed "sand bodies"- pockets of variable dimensions (up to 50 cm in height and extending 2-3 meters laterally) filled with detrital fine-grained carbonate sand locally enriched in silica and sphalerite. Extensive sand development occurred in well-defined channelways with previously mineralized margins as well as in basal parts of collapse structures where the extent of the sand is gradational and unclear. The sand is detrital as evidenced by the abrasion of coarse clasts, textural and compositional difference between sediment grains and surrounding host rock, and sedimentary structures in the finer material, including laminations and graded layering (1mm to 2cm thick). Dropstones and scour and fill structures are minor.

The internal sediment records a period of cyclic deposition, with 40 or more coarsening to fining sequences within a cavity fill. The hydrologic system during sedimentation must have had a fluctuating flow velocity. It is interpreted to have been of meteoric origin and climatically controlled, based on the similarity between these deposits and recent cave sediments (Bull, 1981). Bed shear velocities along the bed surface on the order of 8 cm/sec would have been needed to initiate movement of 5mm sphalerite grains in the coarse fraction of well-graded sediment, while settling of 10μ grains in the fine fraction would require water flow essentially at a standstill. The minimum depth of sediment formation was the depth below the post-Knox unconformity, or approximately 200 meters; the maximum depth is constrained by penetration of surface processes to control groundwater flow oscillations.

The sand formation and deposition followed all diagenetic and brecciation events. Main-stage sphalerite preceded sedimentation as well, and is present as brecciated rinds on dolomite clasts and as spotty replacement of limestone clasts within the sand pockets. Veins filled with white gangue dolomite and minor sphalerite cross-cut the sand locally. These veins were distorted by later bedding-parallel pressure solution, which also caused widespread removal of material at host rock/sand boundaries, along margins of sphalerite clasts within the sediment, and within layers of fine grained graded sediment. Fracture systems (presumably tectonic in origin) filled with calcite and minor sphalerite post-date stylolitization. Sphalerite is present in the fracture systems only in well-mineralized areas and may be related to tectonic remobilization. The large amount of bedding-parallel pressure solution post-dating the sand formation suggests that sedimentation occurred prior to a large amount of porosity decrease from burial compaction.

If the initial solution collapse development was a result of dissolution of interbedded limestone in the upper Knox Group during the post-Knox unconformity (Hoagland et al., 1965; Harris, 1971; Hill and Wedow, 1971), then solution collapse, mineralization, and internal sedimentation all must have occurred prior to deposition of the overlying Middle Ordovician carbonates. Influx of a relatively hot brine (110-180°C, from fluid inclusion data of Roedder, 1971; Taylor et al., 1983; and this study) must have been responsible for sphalerite and associated dolomite precipitation following and in part contemporaneous with solution brecciation of the host rocks. Flushing of the system by a penetrating meteoric water preceded development of the sand bodies as evidenced by the cessation of solution brecciation and the establishment of a hydrologic system with a fluctuating flow velocity prior to internal sedimentation. The sphalerite deposition is clearly pre-Alleghenian in age. An early Middle Ordovician age precludes the dewatering of the mid to late Middle Ordovician Sevier Basin as the source for the ore fluid as well.