『Abstract
Some of the world's largest and most valuable manganese resources
formed as shallow-marine chemical sediments. We hypothesize that
manganese deposition was on the margins of black shale (and related)
facies of stratified seas. Manganese was deposited when deep anoxic
water with high manganese solubility mixed with shallower oxygen-bearing
water with low manganese solubility. The deposits show little
or no evidence of volcanic contribution of manganese from nearby
sources. The size of the manganese deposit varies with the vigor
and dimensions of mixing and with the disslved manganese content
of anoxic deep water, the dilute ore-forming solution. The purity
of the deposit depends on prior removal of dissolved iron from
the system, primarily by pyrite precipitation in basinal environments,
and on the absence of clastic dilution. Deposits may form one
or two members of a zonal spectrum that includes sulfides, phosphorite,
and barite. In many deposits there is evidence of great organic
productivity, probably a by-product of vigorous mixing across
stratification surfaces in the water column. Also common are primary
sedimentary pisolites or oolites, glauconite, and biogenic silica.
Manganese oxide facies deposition is preserved on oxic substrates
in shallow water. Carbonate facies deposition can apparently be
either on these oxic substrates or on 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 may show landward oxide facies
and basinward reduced carbonate facies. Depositional regression
may preserve the water column redox interface as ba contact between
overlying oxide facies and underlying carbonate 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 are described.』
Purpose
Marine Geochemistry of Manganese and Iron
Depositional Models
Temporal Variations an Anoxia and Sea Level
Analogues from Modern Environments
Black Sea
Baltic Sea
Stratified fjords
Other restricted marine basins
Lacustrine environments
Oxygen minimum zones of open marine environments
Some Ancient Deposits as Examples
Molango, Mexico
Moanda, Gabon
Groote Eylandt, Australia
Nikopol and related deposits, U.S.S.R.
Imini, Morocco
Batesville, Arkansas
Chamberlain, South Dakota
Conclusions
Acknowledgments
References