Alan D. Hoagland, 1967. "Interpretations Relating to the Genesis of East Tennessee Zinc Deposits", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
Download citation file:
The most important zinc deposits in Tennessee occur in a narrow stratigraphic zone about 200 feet thick in Lower Ordovician dolomitized limestone and dolomite. The ore-bearing zone constitutes less than 4% of the carbonate section of which it is a part. The remarkable selectivity of the mineralization with respect to this very restricted horizon appears to be related to the great permeability which was established initially by post-Lower Ordovician pre-Middle Ordovician weathering that extended to depths of 800 feet below the surface of the Mascot Formation. The limestone horizons of the lower Kingsport formation probably were a significant factor in the development and localization of the zones of permeability and in the genesis of the zinc deposits. The release of connate water from the limestones is suggested as a possible mechanism or factor in the preparation of the host rock for ore deposition and this process may have played an active role in the mineralization. It appears to be significant that the greatest concentration of sphalerite deposition was at depths below the surface of the Lower Ordovician Mascot formation of between 600 feet and 800 feet. The mineralization was accomplished either before or during an early stage in the Middle Ordovician marine sedimentation. Although there are no known igneous rocks which might have been a source of metal or energy, there appears to be a mineral zoning associated with these Ordovician zinc deposits. It is suggested that genetic theory would be advanced significantly by regional geochemical study.
Figures & Tables
Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits)
Proponents of syngenetic theory base their interpretation largely on widespread uniform mineralization within a restricted stratigraphic interval and a consistent relationship of mineralization to sedimentary features. Proponents of epigenetic theory base their interpretation on mineralization of post-depositional structures, changes in extent and grade of ore, open space filling, district-wide lack of close control by sedimentary features, and relation of ore to tectonic structures. These and other criteria are evaluated in an attempt to define diagnostic criteria.
On the basis of the criteria defined the major lead-zinc deposits of Mid-continent United States must be considered as epigenetic.
Features of the Southeast Missouri lead district are listed. The deposits are epigenetic. The metals are believed to have been derived from nearby sedimentary basins and carried out of basins onto shelf areas in a concentrated brine. Movement of solutions was controlled by basement topography and deposition of metals occurred when solutions entered the Bonneterre formation on the flanks of and over buried knobs.
Objective.—The problem of origin of stratiform ore bodies cannot be resolved until we define, and agree upon, what constitutes diagnostic evidence for each type of deposit. This paper is an attempt to review the nature of geologic evidence; to define those features that must be regarded as unique and necessary criteria in classifying any deposit or district; and to apply the criteria to a major district, the Southeast Missouri lead deposits.
Theories of Origin.—The major elements of theories on origin of stratiform ore bodies are summarized in Table 1. A deposit is Syngcnetic if formed by processes similar to and simultaneously with the enclosing rock; epigenetic if introduced into a pre-existing rock (3). A diagenetic origin implies deposition of metals with the host sediments but with recrystallization, rearrangement, and limited migration.
The search for an acceptable theory of origin must be separated into its two component parts: (1) definition of whether the deposit has syngenetic, diagenetic, or epigenetic features and. (2) history of mineralization to explain source, transport, and deposition of metals. A statement of preferred hypothesis is meaningless until the first is answered and accounts for all geologic facts. The answer must be based solely on observed megascopic and microscopic features and on geochemical and isotopic data; it should not be biased by lack of knowledge to answer all phases of the second. In evaluating the evidence to determine type of deposit one cannot be concerned