R. D. Russell, W. F. Slawson, 1967. "Constraints Imposed by Lead Isotope Abundances on the Origin of Ores", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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A review is given of the physical concepts involved in the interpretation of lead isotope evolutionary patterns. A new approach is suggested based on the observation that each of the integral decay equations can be considered to be a weighted average. Thus, variations from sample to sample in the Pb206/Pb204 abundance represent precisely variations in the weighted average of the U/Pb ratio in the rocks with which the leads have been associated throughout their lifetimes. A second weighted average of the same geochemical ratio may be obtained from the Pb207/Pb204 ratio, and a weighted average of the Th/Pb ratio, from the Pb208/Pb204 ratio.
The pre-depositional history of a lead sample can be described in terms of the U/Pb and Th/Pb ratios of the environment in which it has existed. One lead ratio gives comparatively little information about this environment, since many histories will give the same mean U/Pb. However, the requirement that two different weighted means be mutually consistent greatly restricts the possible choice. This provides an important constraint that must be satisfied by theories for the pre-depositional history of ore samples.
To illustrate the use of this isotopic constraint, published analyses by Bate et al (1) for Mississippi-valley-type leads which are thought to occur over a single Precambrian province, assumed to be 1,350 m.y. old, are examined. It is shown why the observed abundances require a multi-stage history to satisfy the conditions outlined above. Possible histories are described which involve the evolution of lead in subcrustal, as well as in crustal environments.
It is emphasized that the studies of lead isotope abundances give information about time intervals and not events, so that ages are only an indirect product of such studies.
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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