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UNITED STATES MINERAL RESOURCES 



manganese in igneous rocks occurs in such diadochic 

 substitutions, largely in mafic minerals. True man- 

 ganese minerals are most rare in such rocks. Mafic 

 minerals break down readily under weathering, and 

 the contained ionic manganese is liberated. Under 

 reducing or near-neutral conditions, bivalent man- 

 ganese is relatively soluble and is commonly trans- 

 ported by subsurface or surface waters to sites 

 where for some reason the waters have higher oxy- 

 gen content. There the mnaganese may be deposited 

 at higher valences in the relatively insoluble oxide 

 form. Given proper conditions, the manganese may 

 also be deposited in bivalent carbonate form. Bi- 

 valent manganese may also substitute for the cal- 

 cium ion in limestone or dolomite, forming man- 

 ganoan limestone or dolomite or deposits of even 

 more complex manganiferous carbonate minerals. 



Volcanic rocks subjected to large-scale alteration, 

 such as molten lava extruded into relatively shallow 

 sea water or solid volcanics altered by hydrothermal 

 solutions, may also lose considerable quantities of 

 manganese by destruction of mafic minerals. The 

 manganiferous solutions may then transport the 

 manganese to sites where the ambient conditions 

 abruptly change because of variations in wallrock, 

 redox potential, intermixing ground water, rapid 

 cooling, or other factors. There the manganese may 

 be precipitated in veins, replacement deposits, or 

 hot-spring deposits, or, in marine or lacustrine 

 basins, as sedimentary deposits. 



Manganese derived from cooling magma tends to 

 concentrate in pneumatolytic and hydrothermal 

 solutions. Pegmatites locally contain a widely vary- 

 ing suite of complex manganiferous minerals of no 

 economic interest as a source of manganese. In 

 veins and replacement deposits of magmatic origin 

 deposited at relatively low temperatures, manganese 

 as the carbonate or silicate may replace wallrocks 

 or form the gangue of precious- and base-metal 

 deposits, as at Leadville, Colo., Pioche, Nev., and 

 Philipsburg, Mont. In many locahties the manganese 

 occurs in zones outside the main center of base- or 

 precious-metal deposition. The manganese so de- 

 posited may be produced as a byproduct or coprod- 

 uct of the base-metal mining, as at Butte, Mont. 

 Low-temperature veins of manganese oxide are also 

 formed from such solutions, but these are of minor 

 economic significance. 



Manganese may be carried to lakes or the sea in 

 ionic solution or in colloidal or very finely particu- 

 late form. Certain bacteria precipitate manganese 

 as part of their life cycle, and bog and some lake 

 deposits apparently are due to such organic action. 

 Other lake or basin deposits are apparently due to 



sudden increases in the oxygen content of the water, 

 typically in near-shore environments, resulting in 

 long narrow deposits of manganese oxides closely 

 related to facies changes of the host sedimentary 

 rocks. Manganese that reaches the sea may be do- 

 posited in a platform or near-shore environment, 

 either as rather pure manganese oxide mixed to a 

 greater or lesser extent with detrital terrigenous 

 sediments, or as manganese carbonate or complex 

 manganese-calcium-magnesium carbonate minerals 

 if the' environment is more reducing. 



The exact environmental conditions causing the 

 deposition of such sedimentary manganese in any 

 particular spot have never been satisfactorily ex- 

 plained. We know the general mechanism and en- 

 vironment of deposition but not the localizing fac- 

 tors; that is, we often know how a deposit was 

 formed but not why it was formed in a particular 

 spot and not in some other spot. This is a critical 

 question for the economic geologist. 



Many deposits of sedimentary and other origins 

 are in rocks later subjected to metamorphism. If 

 the original deposits are relatively pure oxides or 

 carbonates, they remain generally unchanged; if 

 intermixed alumina and silica are available, the 

 original manganese reacts with them to form bodies 

 of recrystallized manganese oxide or carbonate with 

 some intermixed manganiferous silicates. Examples 

 are the oxide deposits of the Central Provinces of 

 India and the manganese carbonate bodies of Ghana 

 and Brazil. If the original deposit is high in inter- 

 mixed alumina and silica, all the original manganese 

 carbonate or oxide reacts with the impurities to 

 produce extensive zones of manganese silicates, 

 called gondite, useless for industrial purposes and 

 from which only small and low-grade secondary 

 deposits form. 



In the deep sea, and locally at the intermediate 

 depths, where sedimentation of detrital and organic 

 material is very slow, nodules containing from about 

 10 to 35 percent manganese and from 5 to 35 per- 

 cent iron cover extensive areas. Some believe that 

 most of the manganese in these nodules is derived 

 ultimately from terrigenous sources; others believe 

 that contributions from submarine volcanic activity 

 are large. The manganese in these nodules commonly 

 occurs as complex oxide minerals. These nodules 

 may be the ultimate sink for the manganese and 

 iron and some other metals introduced into the sea 

 water from rivers and submarine volcanics. Their 

 equivalent has not been recognized in land deposits, 

 but their included metals may be recycled as sea-bed 

 deposits which are carried to great depths in the 

 mantle in subduction zones and there are melted to 



