Edwin Roedder, 1967. "Environment of Deposition of Stratiform (Mississippi Valley-Type) Ore Deposits, from Studies of Fluid Inclusions", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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Fluid inclusions in ore and gangue minerals provide a wealth of data on the conditions of origin of these deposits with respect to density, temperature, rate of movement; salinity, and composition of the fluids that deposited the ores, at the site and time of deposition. Although some assumptions must be made that certainly cannot be strictly valid in every case, the available evidence indicates that, if care is used in inclusion sample selection and experimental technique, the exceptions to these assumptions will be few and of minor significance, and the accuracy of the results adequate for use in discussions of genesis.
From inclusion evidence alone, it is apparent that the ore fluids forming a number of these deposits were slow-moving, hot, dense, Na-Ca-Cl brines, containing abundant organic matter. The most striking feature is the uniformity of temperature, density, chemical composition, and salinity, over a wide range of stratiform-type deposits, including those in the Joplin, Southern Illinois, Southeast Missouri, East Tennessee, Derbyshire, North Pennine, and Santander districts, and in other smaller occurrences.
Although these data do not determine the origin or origins of these deposits, they place severe limitations on the possible mechanisms of ore transport and deposition. Origin by sedimentary-syngenetic, volcanic exhalative, simple magmatic-hydrothermal, and meteoric circulation processes seem to be precluded by the inclusion data; deposition from modified, deep-circulating, heated connate brines is compatible with these data, and is considered to be a satisfactory working hypothesis.
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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