396 



UNITED STATES MINERAL RESOURCES 



the metal content of the steam and water is much 

 higher in California than in Mexico. The source of 

 these metals, which is still unknown, might be the 

 sediments themselves, or underlying rocks, or mag- 

 matic bodies at depth. A careful study of the dis- 

 tribution of manganese in the fluids and the direction 

 of their movement might lead to prospecting guides. 

 Such a hypothetical deposit would, of course be very 

 expensive to find and more so to develop, as it would 

 involve underground mining in difficult conditions. 



On other continents, the most logical and promis- 

 ing areas are those in which Precambrian eugeo- 

 synclinal rocks have been exposed to one or more 

 cycles of erosion and where remnants of ancient ero- 

 sion surfaces are still presex'ved. All the significant 

 secondary manganese oxide deposits that formed by 

 weathering of primary sedimentary manganese car- 

 bonate deposits are in erosional remnants of such 

 ancient surfaces. In west and central Africa, rocks of 

 Birrimian age are the most favorable and are widely 

 distributed in the western and central parts of the 

 continent. In equatorial Africa, the Francevillian 

 rocks are also favorable. It would be surprising if all 

 the significant deposits have already been found. 



In South America the rain forest of the Amazon 

 region to the north and south of the central zone of 

 Paleozoic and younger rocks is possibly the most 

 favorable area; the eugeosynclinal Precambrian 

 rocks in the middle and headwater areas of the 

 north-flowing tributaries are also very favorable. 

 Many occurrences of manganese ores have been lo- 

 cated in these areas ; probably, commercial deposits 

 will be found, there too on remnants of ancient 

 erosional surfaces. The recent discovery of very ex- 

 tensive areas of oxide-facies iron-formation south of 

 the Amazon increases the possibility of discovery of 

 manganese or manganiferous iron ore associated 

 with these rocks. 



In western Brazil and eastern Bolivia at about 20° 

 south latitude, the large known reserves of bedded 

 manganese oxide ore associated with lower Paleozoic 

 oxide-facies iron-formation might be radically in- 

 creased by more detailed subsurface exploration, not 

 necessarily in the places now known to contain man- 

 ganese. Here the nearshore facies of the Jacadigo 

 Series is perhaps the prime target for exploration. 

 The known manganese beds are lenticular, although 

 individual lenses may be several square kilometers in 

 extent, and concealed lenses or lenses cropping out 

 obscurely may be of very significant size. 



In India the major manganese deposits are meta- 

 morphosed primary oxide sediments in lenses of 

 considerable extent. They have not been explored by 

 drilling, and deep drilling probaby will reveal a con- 



siderable tonnage of high-grade ore. The economic 

 viability of the deep ore will depend on increased 

 world prices, as such ore will be expensive to ex- 

 tract. India is a manganese-rich province, with de- 

 posits of different kinds, largely of sedimentary 

 origin. It would be surprising if further discoveries 

 were not made. 



PROSPECTING TECHNIQUES 



Prospecting for manganese deposits has been car- 

 ried on by the classical method of looking for surface 

 indications of manganese oxide. This approach is 

 very usable in areas of outcrop and residual soil be- 

 cause manganese oxide is relatively immobile under 

 most surface and near-surface conditions and be- 

 cause manganese carbonate oxidizes readily to man- 

 ganese oxide under near-surface conditions. 



The high but variable background content of man- 

 ganese in all rocks, soils, waters, and plants makes 

 geochemical methods difficult, if not unusable. Use of 

 other elements as pathfinders to manganese deposits 

 is hazardous, mainly because most large sedimentary 

 deposits are free of trace elements suitable for this 

 use. Plants may be useful in future manganese 

 exploration. Presence of certain plants, at least local- 

 ly in the Eastern United States and the Katanga, is 

 indicative of a manganese-rich environment (Can- 

 non, 1971, p. 245) . 



The common association of graphite and organic 

 carbon with sedimentary manganese carbonate de- 

 posits would cause anomalous electrical reactions; 

 therefore induced polarization (IP) and other elec- 

 trical geophysical methods should be able to detect 

 the carbonaceous wallrocks where these have not 

 been too highly altered. In tropical zones, however, 

 the carbonaceous material is normally oxidized to 

 some depth, so the electrical methods would be ap- 

 plicable only in areas of slight surface alteration. 



Once signs of a manganese deposit are located 

 visually, then drilling, test pitting, and exploration 

 of the deposit by adits or shafts are essential to 

 establish the size and grade, for the surficial expres- 

 sion of a deposit may be deceptively large and high 

 grade. 



PROBLEMS FOR RESEARCH 



GENERAL PROBLEMS 



1. The most important problem involved in man- 

 ganese supply for the United States today is the con- 

 tinuing maintenance of an adequate stockpile of 

 ferromanganese alloys or high-grade manganese ore 

 so that this country will never again be subject to 

 stringent and industrially crippling shortages caused 



