MANGANESE 



389 



form new igneous rocks with associated ore bodies, 

 completing and renewing the geochemical cycle. 

 Thus may be explained the association of volcano- 

 gene manganese with volcanic rocks ringing the 

 Pacific. 



To summarize, the manganese in the earth's ore 

 deposits originated from manganiferous mafic min- 

 erals in igneous rocks or from magmas, being re- 

 leased by weathering and hydrothermal alteration 

 of those minerals or by fluids driven off from the 

 crystallizing magma. The manganese was carried to 

 the site of deposition in very dilute solution and 

 was there deposited in significant concentration be- 

 cause abrupt change of the environment rendered 

 the dissolved manganese insoluble. Changes in the 

 oxidation-reduction potential were probably the most 

 important control in the precipitation of manganese. 

 There is reason to believe that biologic agents were 

 involved in deposition of major carbonate-facies 

 manganiferous sedimentary deposits, but there is 

 no evidence of this for major oxide-facies deposits. 



Much disagreement exists as to the relative im- 

 portance of the various sources of manganese dis- 

 cussed above; the enormous size of the major sedi- 

 mentary deposits strains the capacity of direct con- 

 tribution from volcanic or magmatic sources. The 

 lack of volcanic rock closely associated with such 

 major deposits makes the weathering of igneous 

 rock a more likely source of manganese for the 

 larger deposits. The manganese in the higher grade 

 deposits has in many instances been through at 

 least two stages of natural concentration: (1) its 

 original deposition and (2) subsequent weathering 

 of low grade deposits, particularly the carbonates, 

 that removed dilutents. 



TYPES OF MANGANESE DEPOSITS 



Primary manganese deposits may be classified 

 into three types — sedimentary (including sea-floor 

 nodules), volcanogene, and hypogene — that to some 

 extent are intergradational. All are modified to 

 greater or lesser extent by supergene processes; in 

 oxide form, manganese is less soluble than many 

 other elements and is concentrated in the zone of 

 weathering. 



MANGANESE MINERALS 



Table 75 shows the more common manganese min- 

 erals formed in these various environments. 



SEDIMENTARY DEPOSITS 



A distinction must be made between sedimentary 

 deposits of manganese laid down as carbonates and 



Table 75. — Common manganese minerals 



[Supergene mineralogy essentially the same regardless of type or origin 

 of protore except possibly nsutite] 



Supergene minerals 



Pyrolusite ^ 



Cryptomelane ^ 



Psilomelane ' 



Coronadite ^ 



Nsutite^ (carbonate source) 



Birnessite ^ 

 Todorokite ^ 

 Chalcopbanite ^ 

 Manganite - 

 Lithophorite - 



Metamorphic minerals 



[Metamorphic mineralogy is largely dependent on metamorphic grade 

 and rock composition. Many oxides and rhodochrosite remain intact 

 during metamorphism in environments low in Al and Si. Many primary 

 oxides stable in low-grade metamorphism] ) 



Manganite -7 

 Bixbyite ^ (all grades) 

 Braunite ^ (all grades) 

 Hollandite^ (all grades) 

 Hausmannite ^ (higher grades) 

 Jacobsite =^ (highe 



Rhodochrosite * 



Rhodonite ^ 



Spessarite '^ 



Tephroite ^ (higher grades) 



Other Mn silicates ^ 



Hypogene Minerals 



Pyrolusite ^ 

 Cryptomelane * 

 Psilomelane ^ 

 Birnessite ^? 

 Coronadite ^ 

 Hollandite = 

 Braunite ^ 



Pyrolusite '7 

 Psilomelane ^7 

 Braunite ^ 

 Hausmannite ^ 

 Rhodochrosite * 



Pyrolusite ^ 

 Cryptomelane * 

 Psilomelane ^* 

 Birnessite '* 

 Todorokite ^* 

 Manganite ^ 



Jacobsite ^ 

 Hausmannite ^ 

 Rhodochrosite * 

 Mn-Fe-Mg-Ca carbonates 

 Black calcite * 

 Rhodonite ^ 



Volcanogene minerals 



Mn-Fe-Ca carbonates 

 Neotocite ^ 

 Bementite ^ 

 Mn silicates 



Sedimentary minerals 



Braunite ^ 

 Rhodochrosite * 

 Mn-Fe-Mg-Ca carbonates 

 Black calcite * 



Mn in lattice of magnetite ant 

 Fe silicates and carbonates 



' Higher oxide. 

 ' Hydroxide. 



* Medium and lower oxide. 



* Carbonate. 



' Silicate. 

 ' Other. 



• Nodule minerals. 



those deposited as oxides. In all instances for which 

 data are available, major deposits of high- and 

 medium-grade primary manganese carbonate are 

 known to be closely associated with carbonaceous 

 or graphitic rocks, often rather clayey, indicating 

 a strongly or moderately reducing sedimentary en- 

 vironment, in many examples in closed basins. Pri- 

 mary oxide deposits, on the other hand, are more 

 commonly associated with coarser clastic sediments 

 with little or no free or organic carbon, indicating 

 nearshore and strongly oxidizing conditions with 

 relatively free circulation of water. The Ukrainian 

 deposits in the U.S.S.R. are unique in that the inter- 

 tonguing transition from oxide facies to carbonate 

 facies can be observed to occur basinward over 

 distances ranging from some hundreds of meters 

 to a few kilometers. In Precambrian and lower 

 Paleozoic rocks, sedimentary manganese deposits 

 may be closely associated in space and time with 

 iron-formation; oxide-facies manganese deposits 

 may be interbedded with oxide-facies iron-forma- 

 tion, whereas the carbonate-facies deposits tend to 

 be separated stratigraphically (as at Moanda, 



