Origin of the Stratiform Fluorite Deposits of Southern Illinois
Robert M. Grogan, J. C. Bradbury, 1967. "Origin of the Stratiform Fluorite Deposits of Southern Illinois", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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The stratiform fluorite deposits of Southern Illinois constitute an important aspect of the widespread Cretaceous mineralization which also produced the numerous veins in the Illinois-Kentucky region. Although fluorite is the dominant ore mineral, some deposits contain abundant sphalerite and generally minor galena. Calcite, pyrite, quartz and barite are important gangue minerals.
The bedded ores occur in a limited section of sedimentary carbonate rocks of Mississippian age, closely associated with faults which have very minor displacement but are always clearly related to major faults that evidently served as master conduits. Ore emplacement, clearly epigenetic, involved extensive replacement as well as the filling of spaces resulting from solution of the host strata by the ore fluid. Fluid inclusions identify the ore fluid as a concentrated hydrothermal brine basically of connate type but somewhat modified by successive additions of magmatic and meteoric waters. The occurrence of numerous alkalic igneous dikes and intrusive breccias supports the assumption of a magmatic source for most or all of the fluorine and metals, and of thermal convection of the ore fluid. Isotopic data identify the lead components as of J-type, similar to that of other Mississippi Valley deposits, and as having a close kinship to the igneous materials.
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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