J. Minčeva-Stefanova, 1967. "The Genesis of the Stratiform Lead-Zinc Ore Deposits of the “Sedmochislenitsi” Type in Bulgaria", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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All ideas, generally known in the literature, on the origin of the stratiform lead-zinc ore deposits, have already been applied to the genesis of those discussed in this paper.
A characteristic trait of mineralizations of the Sedmochislenitsi type is the presence, together with the lead-zinc mineralization, of a lead-copper one which has been superimposed on the former. Whereas the first ore deposition is encountered almost exclusively in the calcareous sediments of the Anisian, the lead-copper mineralization is found both in the same calcareous rocks and in the calcareous sandstones of Lower Triassic and Lower Jurassic age, as well as in the Paleozoic phyllites.
The features that do not support the conception of a synsedimentary origin of the mineralizations are the following:
Their relationship to the main fault structures in the Western Balkan Mountains and to the crossings of these structures by transverse faults. Although the localization is related also to synclines (of a very low order), the mineralization at many places is restricted (tectonically) by impermeable rocks of other ages.
The lack of facies alterations in the mineralized and nonmineralized Anisian sediments.
The occurrence in the mineralized areas of metasomatic secondary dolomites which have been developed in connection with synchronous dolomitization accompanying ore deposition.
The peculiarities of the mineralizations and the complex geochemical development of the total ore formation of the Sedmochislenitsi type may be most properly explained as due to the penetration of solutions into the already-formed carbonate complex. The metasomatic replacement of the rocks has preferred the limestones, they being the rocks most active in a chemical respect. The sheet-like shape of the ore bodies has been influenced by the bedding of the country rocks and by faults and fractures parallel to the beds.
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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