MANGANESE 



395 



sound generalizations as to how much might be pres- 

 ent in given areas or volumes of rock. Perhaps this 

 section should be entitled "Guidelines for future 

 prospecting" rather than "Speculative resources." 



Presumably, future prospecting will be for medi- 

 um- to large-scale deposits; when compared with 

 potential reward, the expense of looking for small 

 deposits is prohibitive. In terms of national security, 

 only those deposits that can make a significant con- 

 tribution to requirements are worth considering ; the 

 long lead time in developing significant production 

 emphasizes the need for an adequate stockpile. 



For Jurassic and younger deposits, the few vol- 

 canogene deposits of economically interesting size 

 and grade are all near the leading edge of the moving 

 tectonic plates, and therefore this type of deposit 

 should be sought in that environment. The large 

 higher grade sedimentary deposits in this age range, 

 however, are all either on the trailing edge of the 

 continental plates or on intracontinental platforms. 

 We cannot yet make similar informed generalizations 

 about pre-Jurassic sedimentary deposits, for the 

 former configurations of the tectonic plates have not 

 been deciphered. Many of the older sedimentary 

 oxide deposits were clearly formed in a platform 

 environment. 



In the United States, the largest concentration of 

 manganese is in the carbonate nodules and the en- 

 closing manganiferous rocks of the Cretaceous 

 Pierre Shale in South Dakota. (The resource esti- 

 mate in table 77 is based on rock with less than V2 : 1 

 stripping ratio; the manganiferous beds are very 

 much more extensive. This resource is essentially 

 unworkable at present because of mining, ore dress- 

 ing, and metallurgical problems ; development would 

 perforce hasten silting of important reservoirs.) The 

 source of this enormous quantity of manganese is 

 still unknown. Precambrian iron-formation is in 

 many localities accompanied by or closely associated 

 in time and in space with concentrations of man- 

 ganese. Some of the iron-formations in the Lake 

 Superior region, notably the Cuyuna range, are high- 

 ly manganiferous. Although most bedrock in central 

 and western Minnesota is now hidden by Pleistocene 

 glacial deposits, it is known that underlying rocks 

 cause magnetic anomalies and also that a long ero- 

 sional interval with semitropical or tropical climate 

 took place there during Cretaceous time. The authors 

 think that the manganese in the Pierre Shale may 

 have been derived by intensive chemical weathering, 

 during the Cretaceous Period, of now-buried man- 

 ganiferous iron-formations or manganese deposits in 

 central or western Minnesota or contiguous areas to 

 the south. The Cretaceous weathering episode may 



also have created manganese oxide concentrations at 

 that weathering surface. It is of course possible that 

 Pleistocene glaciation may have scoured away such 

 supposed surficial concentrations, which in unglaci- 

 ated terrane occur on topographic highs. 



It is also possible that Precambrian sedimentary 

 deposits of manganese carbonate such as those that 

 gave rise to secondary oxide deposits in Africa, Asia, 

 and South America were present in eugeosynclinal 

 rocks of the Precambrian shields of North America; 

 such deposits are now known in several other Pre- 

 cambrian shields. The characteristically high 

 graphite content of the wallrocks of such deposits 

 might be utilized in geophysical prospecting if cover 

 were not too thick. 



Finding of the so-far unique Molango deposit in 

 Mexico raises question as to whether a similar de- 

 posit might be present in miogeosynclinal environ- 

 ments in the United States. The gray manganiferous 

 rock at Molango looks like most somewhat-organic 

 limestones and could easily be overlooked; it was 

 discovered during development of small bodies of 

 secondary manganese oxide formed from it by 

 weathering. Most analyses of limestone made in the 

 United States do not determine the manganese con- 

 tent of the rock, and our knowledge of the man- 

 ganese distribution in limestone in the country is 

 spotty and incomplete. Several carbonate formations 

 in the United States are known to be slightly man- 

 ganiferous, notably the Shady Dolomite in the East- 

 em Appalachians. This formation has never been 

 systematically sampled, mainly because it crops out 

 poorly and is in many places covered by debris from 

 other formations. By careful sampling, patterns of 

 manganese distribution in this or other manganifer- 

 ous carbonate formations might be found, and an- 

 other Molango might be located. A careful regional 

 study of the Molango area to discover the controls 

 of manganese deposition there would be helpful. 



An area of the North American continent that 

 might carry a significant deposit of high-grade man- 

 ganese oxide is the part of California and northwest- 

 ern Mexico where the Pacific Rise intersects the 

 continent at the head of the Gulf of California. In 

 Pliocene time the Lucifer manganese deposit (and 

 also the adjacent Boleo copper deposit) in Mexico 

 formed in a basin, probably closed, beside or on this 

 rift zone. The Colorado River has dumped enormous 

 quantities of more recent sediment on this part of 

 the continent, and wells to tap thermal fluids in these 

 sediments in the Salton Sea area and areas to the 

 south in Mexico have brought to the surface steam 

 and water at high temperature and pressure highly 

 charged with mineral salts, including manganese; 



