Petrology and mineralogy of certain portions of the Fredericktown deposits
"The Fredericktown area is part of the 'Lead Belt' district of southeastern Missouri, the major lead-producing region in the world for fifty years. The district is a typical example of the Mississippi Valley Type deposits. The ores occur in Upper Cambrian sediments overlying Precambrian rocks. The Fe-Cu-Pb-Zn (-Co-Ni) - sulphide deposits of Fredericktown occur over a few square kilometers in sediments overlying two 'granite knobs'. The sedimentary units lap upon and pinch-out against these buried basement hills. The basal sedimentary unit, the Lamotte Formation, is a time-transgressive, marine, submature orthoquartzite. It contains a Boulder Bed at the base or at intermediate levels. Occasional shale and arkosic sandstone bands are interlayered with the sandstone. A Transition Zone separates the Lamotte from the Overlying Bonneterre lithofacies and shows a slight unconformity. This zone is essentially a polymict greywacke sandstone with some lamination of glauconitic shale and dolomitic limestone. It contains most of the sulphide ore body. The sulphides occur in solid layered patterns, as disseminated agglomerations, concretions, individual grains, and as fragments of grains. They display distinct polar features that serve as top-bottom indicators, connoting depositional and/or diagenetic accumulation. In the Bonneterre dolomitic limestone, massive sulphide layers predominate. However, some sulphides, especially galena, occur in form of veinlets and other cross-cutting structures. Disruption features often contain galena, and transitions from these to complete brecciation of the monogenic type exist. Galena also exhibits diagenetic recrystallization as well as corrosion features. The geometric relations between the sulphides are used for: l) The classification of 4575 bravoite-pyrite grains into eight types. Abrupt or nongradational zoning and unindirectional-gradational or solid solution zoning are present. Both can form under the same conditions; 2) The statistical counting of homogenous reflectivity areas in the same grains, which may indicate the absence of a gap in the pyrite-bravoite solid solution series; 3) The application of a statistical approach to the intergrowth indices to derive a paragenetic sequence for the sulphides, based on quantitative geometric observations of ore samples and concentration products. The paragenetic sequence of the sulphides is as follows: marcasite and pyrite-bravoite; siegenite and chalcopyrite I; chalcopyrite II and pyrite-bravoite; sphalerite and galena. Marcasite is the earliest and galena the latest mineral to form. In view of the geometric features exhibited by about 90 per cent of the sulphides and their relations to the enclosing sediments, the geometric relationships between the sulphides themselves and geochemical conditions, it is concluded that the sulphides are of syngenetic origin, formed contemporaneously in and with the associated sediments. The source of the base metals and of at least part of the sulphur is most probably volcanic exhalative. Diagenetic processes and adjustments may have acted as collectors and/or recrystallizers of the ore matter. Late diagenetic readjustments appear to be responsible for the various disruption features exhibited by the remaining 10 per cent of the sulphides"--Abstract p. xi-xiii
Missouri University of Science and Technology
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