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UNITED STATES MINERAL RESOURCES 



Gabon) or spatially (as in Minas Gerais, Brazil) 

 from the (usually) carbonate-facies iron-formation. 



Most large sedimentary deposits of manganese 

 were deposited in sea water, as indicated by fossils 

 and associated rocks. Many smaller deposits, gen- 

 erally oxide facies, were formed near the shores of 

 fresh- water lakes, as indicated by fossils (Autlan, 

 Mexico, and some deposits in Chile). Some sedi- 

 mentary manganese oxide deposits are closely asso- 

 ciated with gypsum and may well have formed near 

 the shores of closed evaporite basins (Three Kids, 

 Nev. ; Lucifer, Mexico ; and possibly some Chilean 

 deposits). 



The most favorable host rocks for carbonate-facies 

 sedimentary manganese deposits seem to be eugeo- 

 synclinal sediments with associated tuffaceous and 

 limy rocks or platform limestones or dolomites, de- 

 posited in local sea-floor depressions. The deposits 

 in the eugeosynclinal sediments are commonly com- 

 posed of rhodochrosite and to varying degrees are 

 diluted by clastic rocks, whereas those deposited 

 with platform or miogeosynclinal carbonates com- 

 monly contain manganoan calcite or dolomite or 

 complex calcium-magnesium-manganese carbonates 

 and are rarely diluted by clastic sediments. The 

 large deposits of very low grade manganoan car- 

 bonate rock, containing from half a percent to a few 

 percent manganese, are commonly miogeosynclinal 

 or platform deposits and do not characteristically 

 contain free carbon. Carbonate-facies iron-formation 

 may locally contain several percent manganese, 

 probably present as manganoan siderite. 



Sedimentary deposits are characteristically stra- 

 tiform and have widely varying degrees of lenticu- 

 larity. In many places, particularly the oxide depos- 

 its, these lenses are much longer than wide. Some 

 extend for many kilometers, as in the Ukraine and 

 in the Kalahari Basin, South Africa, but more com- 

 monly the lenses are a few hundred meters to a 

 few kilometers long and a few tens of meters to a 

 kilometer or more wide. In many deposits the 

 manganese is in a single bed; in others, such as 

 Chiatura, U.S.S.R., and Urucum, Brazil, the man- 

 ganese is in several separated beds. Thickness of 

 the beds ranges from a hairline to about 30 meters ; 

 except in the Kalahari field, few ore-grade primary 

 oxide beds are more than 3 meters thick. High-grade 

 carbonate beds tend to be thicker but to be smaller 

 in areal extent. Low-grade manganoan dolomite or 

 limestone may be very thick and very extensive. 



The grade of the sedimentary manganese deposits 

 also varies widely. Few unenriched oxide beds con- 

 tain more than 50 percent Mn ; more contain between 

 40 and 50 percent, and probably most contain be- 



tween 25 and 40 percent Mn before beneficiation. 

 Few beds containing less than 25 percent Mn are 

 now mined. The carbonate beds are lower grade 

 because manganese carbonate contains only 47.8 

 percent Mn at theoretical purity, which is rarely 

 found ; 35 percent Mn is considered to be high grade 

 for manganese carbonate in place, and most beds 

 contain between 15 and 30 percent Mn. 



Workable sedimentary manganese oxide deposits 

 range upward in size from a few tens of thousands 

 of tons. There are many such deposits with less than 

 a million tons, a substantial number with reserves 

 between 1 million and 10 million tons, several with 

 reserves between 10 million and 200 million tons, 

 and three known areas with even larger reserves, 

 ranging into the billions of tons. The Ukranian and 

 the Kalahari deposits each are known to exceed 3 

 billion tons, and indications are that they may be 

 several times this size, there is no economic incentive 

 to prove such larger reserves. 



No really large deposits of carbonate-facies man- 

 ganese ores of ore grade are now known, but ex- 

 ploration of such bodies has rarely penetrated far 

 below the zone of weathering, and this resource is 

 poorly known. High-grade oxide ore resulting from 

 the weathering of such beds rarely occurs in bodies 

 of more than 50 million tons, the most productive 

 districts being the Nsuta deposit in Ghana, the 

 Serra do Navio deposits in Amapa and the Meri- 

 dional deposits in Minas Gerais, Brazil, and the 

 Kisenge deposits in Zaire. The Moanda deposit in 

 Gabon, by far the largest of this type yet discovered, 

 has reserves of about 200 million tons of high-grade 

 secondary oxide ore. In many but not all such de- 

 posits, it may be presumed that the resource of un- 

 altered manganese carbonate at least equals the ton- 

 nage of oxide formed from it. 



MANGANESE NODULES 



An enormous potential future source not only of 

 manganese but also copper, cobalt, and nickel is on 

 the ocean floors in the form of discrete nodules, 

 locally welded together to form a pavement. Despite 

 much research into the nature, composition, and 

 distribution of these nodules, particularly since 

 sophisticated cameras and light sources permitted 

 photographing the sea floor, our knowledge of this 

 resource is in its infancy. In the last decade several 

 tens of millions of dollars have been spent by indus- 

 try and some governments in locating favorable 

 areas and in an attempt to develop modes of sub- 

 marine mining and extraction of the valuable metals 

 from the nodules. Within a decade or less, if political 

 obstacles can be overcome, economic extraction 



