Pierre Nicolini, 1967. "Les Gisements Stratiformes De Plomb-Zinc-Barytine-Fluorine De Tunisie", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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In Tunisia, as in most of the world's metalliferous provinces, the problem of the origin of lead-zinc-barite-fluorite deposits is far from being resolved, and only hypotheses can be formulated.
The one certainty is the existence, in the Miocene which overlies the stratiform mineralizations of the Cretaceous, of pebbles of mineralized Cretaceous (Djebel Chambi). However, in the north this same Miocene, as at Sidi Bou Aouane, Djebel Semene, etc., contains stratiform lead deposits. Examples of the redistribution of mineralization, even within historic time, also are known. It is impossible, therefore, to determine age, and consequently an origin, for lead-zinc mineralization.
Observed facts, nevertheless, do permit the rigorous determination of the relations between lead-zinc mineralizations and their geological setting as follows:
a. All deposits (100%) are at well-defined stratigraphic horizons (stages or formations). All are along axes of diapirism. All are in fault breaks or folds. Almost all (90%) are in zones of subsidence during Upper Cretaceous time, even if the mineralized formation does not belong to the Upper Cretaceous. Nearly all (80%) are near the borders of Pliocene-Pleistocene plains. The majority (62%) are near outcrops of Triassic.
b. On the scale of a stage or a formation: All are in biostromal facies either of the Lutetian, the Jurassic or the Aptian. They correlate with formation isopachs. They are near ancient shore lines, etc. Mineralizations of fluorite or barite follow rules of distribution either similar to those for lead and zinc, or in some cases different.
Finally, one must realize that the natural empirical facts in the knowledge of ore deposits involve many more elements than the stated theories.
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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