John S. Brown, 1967. "Isotopic Zoning of Lead and Sulfur in Southeast Missouri", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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The Upper Cambrian Bonneterre formation (400 feet) is widely mineralized, predominantly with galena. The Bonneterre, dolomite or limestone, is underlain generally by highly porous Lamotte sandstone (0–500+ feet) resting on a Precambrian basement that rises in many places into the carbonate series. Above the Bonneterre is the Davis shaly limestone (150 feet), succeeded, where not eroded, by several hundred feet of dolomites which contain extensive barite deposits with accessory galena.
Isotopic studies show conclusively that all ore lead in the Paleozoic beds is anomalous, of J-type, varying considerably within deposits. In general, any vertical section of Bonneterre ore lead, as in a drill hole, is distinctly more radiogenic at the base of the formation, less so upward. Furthermore, any large mineralized area is generally more radiogenic centrally, along guiding faults or controlling basement highs (knobs), and less toward perimeters. Finally, in all clear-cut cases noted, the earlier deposited galena is least radiogenic.
Note, however, that the trace galena in overlying barite deposits is notably more radiogenic than in the Bonneterre ore bodies, apparently due to association with large deep-seated faults.
Traces of post-Precambrian lead in the basement are excessively radiogenic. However, minor veinlets of typically Precambrian galena also are known.
Sulfur isotopes are virtually identical with sea water sulfate (21.80) in the less radiogenic lead but in more radiogenic galena are lighter (22.00). Sulfur variations also show definite stratigraphic relations to horizons of high fossil content (reef rock) in the Bonneterre.
The conclusion is that radiogenic lead from the Precambrian basement mingled with normal lead in the connate fluids of the sediments to yield the anomalous hybrids. The relative proportion of the basement contribution possibly was on the order of one-third of total lead.
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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