Economic Geology Vol. 83, 1988, pp. 93-117 Depositional Model for Shallow-Marine Manganese Deposits around Black Shale Basins ERIC R. FORCE AND WILLIAM F. CANNON U.S. Geological Survey, National Center, Mail Stop 954,Reston, Virginia 22092 Abstract Some of theworld's largest and most valuable manganese resources formed as shallow- marine chemical sediments. We hypothesize that manganese deposition was onthemargins of black shale (and related) facies of stratified seas. Manganese was deposited when deep anoxic water withhigh manganese solubility mixed with shallower oxygen-bearing water withlowmanganese solubility. The deposits show littleor noevidence of volcanic contri- bution of manganese from nearby sources. Thesize of themanganese deposit varies with thevigor and dimensions of mixing and withthe dissolved manganese content of anoxic deep water, the dilute ore-forming solution. The purity of the deposit depends onprior removal of dissolved iron from the system, primarily by pyrite precipitation in basinal environments, and on the absence of clasticdilution. Depositsmay form one or two members of a zonal spectrum that includes sulfides, phosphorite, andbarite. In many deposits there isevidence of great organic productivity, probably a by-product ofvigorous mixing across stratification surfaces in thewater column. Also common are primary sedi- mentary pisolites or oolites, glauconite, and biogenic silica. Manganese oxide facies deposition is preserved on oxicsubstrates in shallow water. Carbonate facies deposition can apparently beeither on these oxic substrates oron reduced substrates in slightly deeper water. The reduced carbonate facies apparently forms by replacement of calcareous substrates by anoxic waters saturated with MnCO3, just below the water column redox interface. A single zoned deposit mayshow landward oxide facies andbasinward reduced carbonate facies. Depositional regression maypreserve the water column redox interface as a contact between overlying oxide facies and underlying carbon- ate facies manganese deposits. Stratigraphic evidence indicates that the deposits formed most commonly during high sea level stands in narrow time intervals when ocean anoxia was widespread. Eight manganese- precipitating modern environments and seven manganese deposits aredescribed. Purpose MOST of the world's conventional manganese re- sources are sedimentary rocksformedin shallow- marine environments. Thispaperpresents a deposi- tional model for a large number of sedimentary manganese deposits that apparently have in com- mon formation during times of high sealevel and stratified seas (e.g., blackshale basins), andmanga- nese precipitation at intersections of horizontal oxi- dation-reduction interfaces with cratonic shallow- marine substrates. We refer to these as stratified basin margindeposits. We have previously published a brief description of some of these deposits (Cannon and Force,1983) along with a generalized theoryof their origin. This paper treats in greater detail thebehavior of manga- nese in modern stratified water bodies, temporal variations in oceanstratification, and a few descrip- tions of basin margin deposits. Some independently proposed models of manganese deposition (Sapozh- nikov, 1970, 1971; Degens and Stoffers, 1976, 1977; Stamm and Thein, 1982; Frakes and Bolton, 1984; Bolton andFrakes,1985) are sufficiently sim- ilar to be considered stratified basin marginmodels also.Someproposed models for sedimentary iron deposition (Borchert, 1960; Drever, 1974; Cannon, 1982; Maynard, 1983; Holland, 1984; Van Houten and Arthur, 1984) are of the sametype, and the proposed chronologies of deposit formation (Van Houten, 1985) are similarto ours. Marine Geochemistry of Manganese and Iron The formation of a valuable manganese deposit requires thattwogeneral conditions be met: manga- nese must be concentrated to at least 200 times its average crustal abundance of about0.15 percent, andiron cannot be concentrated to anyappreciable degree from its average crustal abundance of about 5 percent. The solubility of iron andmanganese compounds in aqueous solutions was determined by Hem (1963, 1972). In Figure 1, whichsuperimposes two of Hem's diagrams, it isevident thatbothMn and Fe formvirtually insoluble oxides or carbonates at Eh andpH conditions typical of modern ocean water. 0361-0128/88/773/93-25 $ 2.50 93