R. S. Cannon, A. P. Pierce, 1967. "Isotopic Varieties of Lead in Stratiform Deposits", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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A review of known lead isotope variations indicates high probability that the class of stratiform lead-zinc-barite-fluorite deposits includes two distinct types, presumably in consequence of some fundamental difference in genesis. Most deposits in Europe and adjacent areas of the Eastern Hemisphere contain ordinary ore-lead, almost invariant in isotopic composition, that evidently originated during a relatively short span of geologic time in a uniform geologic source, probably the earth's mantle. Most deposits in the central lowland of North America, on the other hand, contain J-lead of variable isotopic composition, part or all of which evidently originated by mobilization of rock-lead from crustal rocks, such as Pre-cambrian basement or enclosing marine sedimentary rocks. Exceptions to the geographic pattern of distribution of the two types—Pine Point in Canada and deposits of Laisvall-type in Sweden and Norway—are likely to prove of special interest.
Future lead isotope studies, if appropriately integrated with other means of field and laboratory study, can help establish whether a real difference exists between two types, whether sources of ore-lead were in the mantle or in the crust, and whether episodes of ore deposition were synchronous with, or distinctly later than, sedimentation and diagenesis of enclosing rocks.
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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