MANGANESE 



391 



might well become practical. Several major Ameri- 

 can and foreign companies and governments are novs? 

 engaged in scientific and technological research 

 toward this goal. 



Extensive areas of the ocean floor have concen- 

 trations of nodules that range from a few milli- 

 meters to several tens of centimeters in diameter 

 and are roughly spherical to ellipsoidal. The nodules 

 are composed of roughly concentric layers of ma- 

 terial of varying composition, largely oxides of iron 

 and manganese, deposited around nuclei of broken 

 nodules, fragments of volcanic ejecta or other clas- 

 tic material, and, rarely, fossils. It is estimated that 

 these nodules grow at the rate of 1-5 mm per million 

 years. 



Nodules are most abundant and highest in tenor 

 in parts of the oceans that receive minimal contri- 

 butions of sediments; these are the deeper parts, 

 ranging from about 2,500 meters to about 6,000 

 meters in depth. Lower-grade deposits are found in 

 great abundance on the Blake Plateau, off Florida, 

 at much shallower depth. Although nodules buried 

 in the sediments of the bottom are found, the first 

 30 cm down from the interface is believed to con- 

 tain about 65 percent of the useful material; the 

 deposits should be envisioned as very widespread 

 but very thin accumulations of consolidated nodules 

 resting in and on a usually unconsolidated and very 

 fine clay matrix. 



The metallic content of the nodules varies widely 

 from ocean to ocean, regionally and to some extent 

 systematically within the individual oceans, and 

 from place to place within the regions. Variations 

 in nodule population may be very abrupt. Various 

 authors have presented generalized information on 

 composition and population, but the detailed data 

 on which mining plans and resource calculations 

 can be based, resulting from prospecting in progress, 

 are not publicly available. It is understood, how- 

 ever, that areas, each of 2,600 square kilometers or 

 more, have been located in which the nodules aver- 

 age more than 25 percent Mn, 1.0 percent Cu, 1.0 

 percent Ni, and 0.25 percent Co, with smaller values 

 in other metals. The iron is not economically recov- 

 erable. At 1972 prices, the value of the metals in 

 such nodules would be more than $150 per dry ton. 

 The richer areas may contain about 13,500 tons per 

 square kilometer or even much more; the average 

 in deposits of minable size would probably be sig- 

 nificantly less. Although astronomical figures as to 

 the total tonnage of nodules on the sea floor have 

 been published, resources in areas having a minable 

 population of nodules are probably far less, but still 

 undoubtedly very large. Extraction of the metals 



from the nodules demands innovative metallurgical 

 practice ; dredging of the nodules from great depths 

 demands innovative mining practice. 



The genesis of these manganiferous nodules is 

 hotly debated. Some oceanographers believe that the 

 metallic content is derived from submarine volcan- 

 ism or alteration of submarine volcanic rocks; 

 others, that it comes from reactions in the upper 

 part of the sediments by which the metals are re- 

 leased during diagenesis of the sediments; and still 

 others, that the metals are derived by subaerial 

 weathering of the continental masses, being trans- 

 ported to the ocean basins by rivers, and to the sites 

 of deposition by ocean currents. The presence of ex- 

 tensive deposits on the Blake Plateau on a limestone 

 substrate, far from volcanic activity, suggests the 

 third theory as the preferred model there. However, 

 a sole source for all the deposits does not seem re- 

 quired. 



VOLCANOGENE DEPOSITS 



Bcause many small-scale and some medium-scale 

 deposits of manganese are closely related in space 

 and time to volcanism, the conclusion that such 

 deposits are somehow genetically related to volcan- 

 ism seems unavoidable. There is no general agree- 

 ment as to the mechanism by which the manganese 

 has been separated from the volcanic material and 

 concentrated; several processes may be involved. 

 This uncertainty, as well as the wide variation in 

 types of deposits (ranging from clearly sedimentary 

 to vein-type deposits) that are closely related to 

 volcanism, makes use of the term "volcanogene" 

 appropriate. This type of deposit is gradational both 

 with the sedimentary type already discussed and 

 with the hypogene type discussed below, and no 

 clear-cut separation can be made. 



Volcanogene deposits are closely associated with 

 volcanic rocks, of both pyroclastic and flow origin, 

 and either subaerial or submarine. In many in- 

 stances the pyroclastic rocks have been deposited in 

 lakes or the sea and are closely associated with 

 clastic and chemical sediments or with limestone. 



Major volcanic piles, such as those in eastern Cuba 

 or the Olympic Peninsula in Washington, are fa- 

 vored sites for this type of deposit. Peculiarly, al- 

 though such piles may have a maximum thickness 

 of thousands of meters, the manganese deposits in 

 many of them are confined to widespread zones a 

 few meters to a hundred meters thick, indicating 

 that the concentration of manganese by volcanic 

 processes or from the volcanic rocks was not a 

 continuing and normal process but demanded spe- 

 cial conditions, the nature of which remains uncer- 



