I. De Magnee, 1967. "Contribution A L’etude Genetique Des Gisements Belges De Plomb, Zinc Et Barytine", Genesis of Stratiform Lead-Zinc-Barite-Fluorite Deposits (Mississippi Valley Type Deposits), J.S. Brown
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Lead-zinc deposits of Belgium all are of telethermal type chiefly fillings along transverse, nearly vertical faults. Mineralization affects only the folded Devonian and Carboniferous of the "Massif des Ardennes," western part of the "Rheinische Schiefergebirge." Cambrian and Silurian anticlinal structures are not mineralized, nor are the transgressive Mesozoic and Cenozoic sediments except in the lowermost Triassic conglomerates.
Economic mineralization is limited to the intersection of faults with limestones and dolomites of the middle and upper Devonian and of the Carboniferous limestone.
Irregular "replacement deposits" develop where cross-cutting veins intersect contact planes between the Carboniferous limestone and the upper (Coal Measures) and lower (upper Devonian) shales. Despite their strata-bound extension, these are not true replacements but are fillings of solution cavities. Comparable stratiform lead-zinc disseminations exist where veins cut the dolomitized reefs of the lowermost Upper Devonian: galena and sphalerite fill pores and vugs in coarse-grained secondary dolomite.
The northern, and richest, mineralized belt follows the east-west Namur syncline, from Charleroi to Liège, then the Vesdre syncline. crossing into Germany at the famous Moresnet mine and reaching the border faults of the lower Rhine rift-valley. There the basal conglomerates and porous sandstones of the Triassic are mineralized forming the important stratiform lead deposits of Maubach and Mechernich.
The Belgian lead-zinc mines, closed since 1945. afford little opportunity for study along modern lines, but their geological situation, well known, limits rather strictly the possible theories of origin. Lindgren. in 1933, cited the "Moresnet type" among the lead-zinc deposits independent of igneous activity. Clearly, their origin is connected with the deep circulation of connate and/or meteoric water through a "plumbing system" whose main pipes were the cross-cutting, living faults. After obstruction by precipitated sulfides they frequently were re-brecciated with additional cavity formation in the wall rocks by acid marcasite-forming solutions.
The Carboniferous Limestone bears lead-zinc deposits only where its lower part is dolomitized. Anhydrite beds in the transgressive Middle Devonian were revealed by two deep boreholes in 1960. Deep waters of the Carboniferous Limestone in Northern Belgium presently are Na-Ca-Cl brines.
Isostatic uplift, with erosion, during the arid Permo-Triassic, constitutes a paleo-hydrogeological evolution which, in our opinion, is essential. The marine formation waters could move only when erosion had removed parts of the thick, impervious cover of Coal-Measures. These waters then were replaced progressively by meteoric waters, modified by the residual salinity of the permeable horizons. They must have been able to dissolve geochemical lead and zinc, reprecipitating some in the ascending parts of their deep channelways, i.e., the fault breccias and still porous rocks.
Mineralization by this mechanism may have taken many millions of years. It ceased when the Ardennes Mountains became a peneplain, before invasion by the Upper Cretaceous sea.
Dissolved CO2 certainly played a significant role by retarding the reprecipitation of the heavy cations until the lowered pressure near the surface permitted its escape.
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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