François Foglierini Et André Bernard, 1967. "L’histoire Geologique D’un Gisement Stratiforme Plombo-Zincifere: Les Malines (Gard-France)", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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Recalling the idea of the type of a deposit as put forward by F. Blondel and developed by P. Routhier, the authors propose to explain the typical geologic history of deposits of the "Tri-State," "Silesian" or "Alpine" type according to the specifications of the sponsors of this Symposium. It is thus intended to encompass the entire chain of processes of metalliferous concentration.
For this purpose, the history of Les Malines district is examined in general terms, and the probable, or at least, the most plausible sequence of the processes of metalliferous concentration is deduced therefrom. Comparing the chain of geological events which appears in Les Malines district with the similar histories known in other metalliferous provinces, the authors attempt to outline a general common sequence of processes of concentration which may be considered as the genetic pattern of the type. An indecision remains between the "exhalative-sedimentary" and the "pedological-sedimentary" patterns.
To shed light on this dilemma, it is interesting to examine the nature of the basements underlying or adjacent to the mineralized cover. It appears, on one hand, that the various lithologic zones of the basement do not possess an equal capacity for enriching the cover, and, on the other hand, the variable depths of erosion of the eugeosyncline—the most favorable (zone) with respect to stratiform mineralization in the cover—have a significant effect. The weak mineralization of the cover (strata) situated above or near the areas of eugeanticlines where late orogenic vulcanism is characteristic leads the authors to reject the "exhalative-sedimentary" hypothesis, at least with respect to stratiform deposits of lead-zinc, barite and fluorite, in a carbonate environment lacking in evidences of volcanic activity contemporaneous with the sedimentation.
It is no doubt still necessary to further evaluate the "pedological-sedimentary" possibilities, but the importance of the phenomena of enrichment due to meteoric alteration (residual deposits, karsts,…) is a certainty, though it is sometimes forgotten, compared to the possible recognition of tuffs, a dubious sign of metallizing activity, since it is plain that, properly understood, not all vulcanism is metallogenic.
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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