Intergrowth and Crystallization Features in the Cambrian Mud Volcanoe of Decaturville, Missouri, U.S.A.

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The presence of sulfides and/or clay matter may considerably retard diagenetic crystallization. Semi-consolidated sediments of this kind may intrude adjacent and higher, more consolidated beds; movements or directed stresses resulting from unbalanced distribution of overlying sediment may cause joints and fissures to open up through which this soft sediment can intrude. The Decaturville mud volcanoe appears to have formed in this manner and it is probably the first such example with sulfides; the authors present here observations on the intergrowths and the paragenetic sequence of the various minerals which, in their order of decreasing abundance, are as follows: pyrite/marcasite, sphalerite, galena and chalcopyrite. The identity with the Mississippi Valley type of deposits is obvious.

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The fossil sulfide and carbonate breccia of Decaturville, Missouri, is described and the structural position of the breccia mass discussed. The large and the small carbonate rock fragments show remnant textures typical of marine carbonates. The isotopic compositions of the carbonate fragments and the “flow” carbonates of the breccia compare well with those of sedimentary carbonates from the host sequence of Mississippi Valley-type stratiform lead-zinc deposits of southeastern Missouri. Therefore, an involvement of deep-seated (mantle) carbon in the process of breccia formation is not indicated.
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The Devonian rocks at Pugh Quarry have three distinct types of sphalerite (banded massive, spheroidal, and tiny euhedral). Occurrence of the banded massive sphalerite is restricted to the mineral zone, predominantly as blebs in marcasite. The color of banded sphalerite ranged from nearly colorless to various hues of yellow. The replacement of banded sphalerite by marcasite was observed. The spheroidal sphalerite occurred in association with marcasite of euhedral habit. The spherules were small, the largest no greater than 1 mm across. Where present in great profusion, the sphalerite spherules merged together forming botryoidal surfaces. The euhedral sphalerite occurred in the voids of the sponge-like and stromatolitic dolostone below the mineral zone, and in a layer of soft laminated mud associated with the dolostone. The euhedral sphalerite was predominantly red-brown, and no crystals larger than 1 mm in maximum dimension were observed.
Additional studies of the Decaturville cryptovolcanic structure in western Missouri, USA, in a new exposure, has revealed additional information on distribution of sulfide minerals and sedimentary features associated with this structure. The new roadcut exposes a nearly complete section of the Lower Ordovician Jefferson City and Cotter dolomites, permitting detailed examination of features about two km from the center of the structure along the inner margin of the ring fault which surrounds the structure. The formations consist almost entirely of dolomite with some sandstone, shale, and chert. Sandstones, in beds from 3 to 300 cm thick in this area, and breccias are interpreted to mean a closer proximity to the source area than other exposures in the Ozarks. In addition, the beds have a cyclic arrangement, whose pattern suggests oscillation from very shallow marine conditions to subaerial exposure. This feature is also not apparent in exposure in the Ozarks although poorer exposures may have obscured necessary details in other places. Sulfides occur both as stratiform blebs in a few layers and more particularly within the matrix and some breccia clasts in sedimentary dikes which occur in the upper part of the section. They make about 1% of the volume of the dike, sufficient to give the dikes a gray to dark gray color. The sulfides of the dikes consist of pyrite, marcasite, some galena and sphalerite. The sulfides occur in the following forms: (1) pyrite and marcasite spheres, containing framboids, (2) fragments of pyrite-marcasite with porous pyrite framboids, (3) isolated pyrite framboids, (4) minute fragments of fine-grained marcasite, pyrite or mixed pyrite-marcasite fragments, (5) fragments of colloform pyrite, (6) limonite-coated colloform pyrite fragments, and (7) fragments of sphalerite(?). Each of these occurrences is illustrated and a paragenetic sequence for these is given. It is postulated that the sulfides were transported into the dikes as detritals along with detrital sand and clay from source beds which can be defined for most dikes.
The role of diagenetic processes in the formation of mineral deposits is reviewed and their main traits are summarized. Diagenetic features are important criteria for the determination of deposits of sedimentary or exhalative sedimentary origin discussed in this chapter. In the first section the problem is approached from a historical angle and an answer is sought for the absence of considerations of diagenesis as a factor in ore genesis in "economic geology" textbooks until 1963. The second section shows that the concept of diagenesis (and the knowledge of diagenetic processes) has been very well developed before finding its way into the literature on mineral deposits. Again, a historical reason is sought. The keen interest of sedimentologists and stratigraphers in diagenesis since about 1868 is stressed. (The present volume is almost a "centennial of diagenesis"). The third section contains an outline of geometric evidence for diagenesis in mineral deposits. Various typical examples are briefly described and a number of them are documented by figures. This section is considered to be the "pièce de résistance" because the geometric evidence is the most direct and basic evidence. It is also the most powerful tool of the exploration geologist. Geochemical evidence, which is also very important, actually consists of an indirect geometric study, i.e., the distribution patterns of elements in space (through abundance curves, phase diagrams, etc.). Basically, genetic studies are always investigations of isomorphism or symmetry of geological bodies. The fourth section refers to the essential processes of a geochemical nature, pointing out that they are no different from those existing in common rocks. A brief literature review is given. The fifth section lists a number of important types of mineral deposits, and for each type, a number of pertinent publications on problems or observations on diagenetic processes. The sixth and last section is a case study, made by the second author, of diagenetic processes involved in the formation of oölitic iron ore deposits.
Decaturville sulfide breccia-a fossil mud volcanoe (Abstract)
  • G C Zimmermann
The polygonal structure at Decaturville, Missouri: new tectonic observations
  • R A Amstutz
Conference chm Annals of the New York Acad. Sci Diagenesis in sedimentary mineral deposits
In: Geological problems in lunar research. Conference chm. : J. GREEN, Ed. : H. E. WHIPPLE. Annals of the New York Acad. Sci. 123, p. 1050-1056 (1965b). - BUBENICEK, L.. Diagenesis in sedimentary mineral deposits. In: Diagenesis in sediments, vol. 8. Ed.: G. LARSEN and G. V. CHILINGAR. Elsevier, New York, p. 417-475 (1967).
Late pyrite often occurs in "globular" form; also in irregular shapes similar to worm forms Bibliography AMSTUTZ, G. C.: Tectonic and petrographic observations on polygonal structures in Missouri. In: Geological problems in l'mar research Conference chm
  • Fig
Fig. 10. Late pyrite often occurs in "globular" form; also in irregular shapes similar to worm forms Bibliography AMSTUTZ, G. C.: Tectonic and petrographic observations on polygonal structures in Missouri. In: Geological problems in l'mar research. Conference chm. : J. GREEN, Ed.: H. E. WHIPPT,E. Annals of the New York Acad. Sci. 123, p. 876-894 (1965a).
Decaturville sulfide breccia - a fossil mud volcanoe (Abstract)
  • G C Amstutz
  • R A Zimmermann
  • GC Amstutz