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Manganese: Syngenetic Deposits on the Margins of Anoxic Basins

By
E. R. Force
E. R. Force
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J. B. Maynard
J. B. Maynard
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Published:
January 01, 1991

Abstract

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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Contents

Reviews in Economic Geology

Sedimentary and Diagenetic Mineral Deposits: A Basin Analysis Approach to Exploration

Eric R. Force
Eric R. Force
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J. James Eidel
J. James Eidel
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J. Barry Maynard
J. Barry Maynard
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Society of Economic Geologists
Volume
5
ISBN electronic:
9781629490120
Publication date:
January 01, 1991

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