Allex V. Heyl, 1967. "Some Aspects of Genesis of Stratiform Zinc-Lead-Barite-Fluorite Deposits in the United States", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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Stratiform ore deposits may comprise geologically similar end members formed by different complex paths of genesis. Low-temperature stratiform deposits of the Mississippi Valley type are found in three regions in the United States: (1) Appalachian Valley and Ridge province, (2) greater Mississippi Valley, and (3) scattered districts in the mountains of the West. The deposits in the three major regions show similar geologic features, but in each region variations in lead-isotope composition, mineralogy, trace elements, geochemistry of the ore-forming fluids, and other geologic features indicate diverse origins.
The Appalachian Valley deposits of Tennessee may have been deposited by heated early Paleozoic metamorphic hydrothermal solutions formed in the volcanic and tectonically active belt that is now the Piedmont. Possibly the heated metamorphic solutions migrated northwestward into sediments filled with connate brines, mingled with them, and deposited sphalerite and dolomite.
In the Mississippi Valley, the available geologic and geochemical data suggest that the main solutions depositing the ores were connate brines heated to temperatures of about 100º C and enriched by smaller potassium-rich fractions carrying a distinctive assemblage of minor elements; probably the heat, potassium, and minor elements were derived from deep-seated alkalic batholiths and stocks. In addition to the main deposits, beds of siderite nodules containing lead and zinc sulfides probably were deposited during diagenesis, and sulfide-bearing geodes may be of meteoric origin. Syngenetic pyrite occurs in black shale beds in microspherules.
Most stratiform deposits of the Mississippi Valley type in the West are probably low-temperature hydrothermal deposits in which the ore-forming fluids were mainly circulated ground water, magmatically heated in a granitic province, plus a small magmatic fraction of solutions.
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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