Edwin T. McKnight, 1967. "Bearing of Isotopic Composition of Contained Lead on the Genesis of Mississippi Valley Ore Deposits", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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The lead of Mississippi Valley deposits shows anomalous isotope ratios and great variability in these ratios within the separate districts. The anomaly consists of an excess of both Pb206 and Pb208 in comparison to ordinary lead. These facts suggest that the isotopic evolution of lead in this mineral province has been modified from that prevailing in most districts by some unusual variation of the geologic history that allowed evolution for a considerable period in an environment richer in uranium and thorium than that in which ordinary lead has evolved. The hypothesis is here presented that this special environment was the Precambrian granites of the Central craton; and that the remobilization was an igneous phenomenon closely tied to the evolution of alkaline and, particularly, potassic igneous rocks.
Post-Precambrian igneous rocks of the Mississippi Valley are widespread, though not closely related in space to the ore deposits. They are predominantly alkaline, rich in potash. Such rocks are believed to have evolved in continental areas from a hybrid magma produced through partial assimilation of crustal granitic rocks by primary olivine basalt from depth. Basalt contains uranium, thorium, and lead approximately in ratios suitable for evolution of ordinary lead. Granites, however, contain higher ratios of uranium and of thorium to lead. From the time of emplacement of the granite in Precambrian time, its contained lead evolved in an environment exceptionally rich in radiogenic lead. Absorption of this granite into a basaltic magma should produce a hybrid alkaline magma whose lead is isotopically anomalous.
Alkaline magmas are evolved typically in tectonically stable platform areas of the earth's crust, characterized by simple fracturing without pronounced orogenic deformation. These magmas contain up to several times as much radioactive material as the subalkaline rocks, and this may have contributed much of their magmatic heat. Individual magmas could be isolated below a thick competent crust where they could undergo a long period of quiet assimilation of country rock and of differentiation. Their structural setting lacks the orogenic pressures that would tend to propel them toward the surface, but ore deposits given off from such bodies could occur in overyling crustal blocks with little evidence of igneous association.
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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