Hubert Pelissonnier, 1967. "Analyse Paleohydrogeologique Des Gisements Stratiformes De Plomb, Zinc, Baryte, Fluorite Du Type “Mississippi Valley”", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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Most genetic theories concerning the Mississippi Valley type appeal to circulations of aqueous solutions, not only the classical hydrothermal theories but also the sedimentary theories which often call upon "re-mobilizations" of syngénetic deposits. Under these conditions it appears logical, in order to discriminate, to consider the possibilities of circulation during the formation of the deposits, i.e., to carry out a paleohydro-geological analysis of the mineralized zone and its surroundings.
Within the local setting of the deposit itself circulations are put forward by the analysis; they are possible owing to the permeability of the host rocks and they are evidenced by dissolution figures and by the epigenetic texture of the crystallizations. These circulations are posterior to the consolidation of the host rocks.
Then. taking into account the regional setting of the deposits the circuits liable to be involved are examined: circulations of meteoric and connate waters within one and the same horizon; circulations affecting the basement and locally ascending toward the emplacements of the deposits. This last hypothesis appears the most satisfactory from many points of view, especially as it demands a "constriction" of the circulation where the latter goes through certain structures such as faults and "paleinsules" obviously related to the deposits. which explains well the anomalous concentrations of the metals.
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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