The European Scene, Section B La Vue Française
After the end of Hercynian orogeny, most of the territory of continental France had been consolidated into a rigid platform ("môle des Gaules"). The base of the trasgressive secondary layers, deposited during Triassic or Jurassic eras on this platform, consists of neritic or lagunal deposits.
All around the exposures of ancient massifs (Massif Armoricain, Massif Central), very near from the old shore lines, are located different deposits of the strata-bound Pb. Zn, Ba, F type, as shown in Figure 1.
The concept of regional metallotect (cf. reference) gives a way of understanding the geological regularities and the geographical location of these deposits which seems a much better one than a controversy on genetic theories. This is, in fact, connected with the author's conception as follows: this type of deposit results from the action of different genetic processes which took place simultaneously or successively (Fig. 2).
The regional metallotect which is connected with the deposits shown on Figure 1 (neritic layers of shallow water sediments transgressive after orogeny near the shore lines), gives a good guide to ore exploration from a strategic point of view.
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