E. R. Force, J. B. Maynard, 1991. "Manganese: Syngenetic Deposits on the Margins of Anoxic Basins", Sedimentary and Diagenetic Mineral Deposits: A Basin Analysis Approach to Exploration, Eric R. Force, J. James Eidel, J. Barry Maynard
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Manganese has been mined from vein-type deposits, from weathering profiles, and from sedimentary rocks; only the last type is discussed in this chapter. Some sedimentary manganese deposits formed in the deep sea and are clearly volcanic related, some are of deep-sea origin with no local volcanism, and some formed in shallow-marine (<500 m) environments on continental platforms, generally with no local synchronous volcanism. The shallow-marine sedimentary manganese deposits dominate world manganese resources and current production (DeYoung et al., 1984). Therefore for this volume, with its focus on major deposits, we will emphasize the shallow-marine type of manganese deposit. For reasons that will become apparent, these have also been called stratified-basin-margin or, colloquially, bathtub-ring manganese deposits.
Manganese exhibits one of the most pronounced solubility contrasts between reduced and oxidized waters in the behavior of the elements forming sedimentary precipitates. Figure 11.1 shows the stability fields of the most common manganese oxides and carbonates as a function of Eh and pH. The boundaries can shift with CO2 content of the water and the concentration of manganese, but the diagram shows that in oxygenated neutral water such as today's seawater, manganese should be present as solid oxides, consistent with extremely low (about 1 ppb) dissolved Mn contents of normal seawater. At low Eh, however, reduced water at neutral pH contains manganese in solution, or as a solid carbonate if alkalinity is increased. This is consistent with the fact that reduced seawater, rather rare in today's seas, can contain three orders of magnitude more
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Major oil companies have been utilizing techniques of quantitative basin analysis in exploration for a decade or more. Ore-forming processes in stratiform, sediment-hosted ore deposits commonly involve sedimentary processes, diagenesis, basinal brines, and paleohydrology. Like the maturation and migration of hydrocarbons, their formation is an integral part of basin history. Consequently, applying comprehensive basin analysis to mineral exploration is a logical and helpful approach to understanding sediment- hosted ore deposits and predicting their occurrence, location, and origin.
When the Society of Economic Geologists' Short Course Committee contacted the writer in 1985 to develop a short course on sedimentary processes of ore formation, ft seemed to me that such a course would provide an excellent opportunity to introduce the concept of comprehensive basin analysis as an exploration tool for sediment-hosted mineral deposits. As Sawkins pointed out (1990, p. 333), “Meaningful exploration in extensional tectonic paleo-environments will increasingly require the integration of surface, subsurface, and geophysical data, and enlightened programs of basin analysis similar to those practiced by the petroleum industry will be increasingly needed.”
Sediment-hosted ore deposits include sedimentary gold and other heavy mineral accumulations; evaporites; syngenetic to late diagenetic base metal and barite deposits in clastic and carbonate rocks, including epiclastic volcanic rocks; banded iron formations; Clinton-minette-type iron and manganese ores; unconformity-related and sandstonetype uranium deposits; and Mississippi Valley-type leadzinc deposits. Some sediment -hosted ore deposits were formed at various stages of basin history and are multistage. This short course focuses on (1) the types of basins in which major sediment - hosted ore deposits occur, and (2) the controls of basin types on ore-hosting sedimentary environments and ore-forming processes.
The precise role of sedimentary processes in the formation of ore deposits has been debated by geologists around the world; this debate has affected the manner and success of exploration program s . Skinner (1979, 1987) traced the origins of the polarization of thought on the genesis of ore deposits to Agricola, who expounded on lateral secretion and precipitation of metals from circulating ground waters, and to Descartes, who perceived the earth as an outgassing star and believed that metals were not derived from host rocks. The neptunist theories of Werner (1750-1817) may have evolved from Agricola and the plutonist theories of Hutton (1726-1797) from Descartes. L. C. Graton, whom the Graton-Sales volume Ore Deposits in the United States 1933-1967