244 



UNITED STATES MINERAL RESOURCES 



mineralizing solutions for some deposits is clearly 

 associated with nearby igneous intrusions. For a 

 significant number of major districts, however, the 

 source of mineralizing solutions is equivocal. The 

 Mississippi Valley and the Tri-State districts, in 

 which gallium and germanium are relatively highly 

 concentrated, are included in the group of deposits 

 which have no closely associated intrusive bodies. 

 The unique Tsumeb deposit in South West Africa is 

 a pipelike body cutting a dolomitic sequence ; it con- 

 sists of a core of aplitic or pseudoaplitic rock sur- 

 rounded by a shell of replaced carbonate rock con- 

 taining lead, copper, and zinc sulfides with sparse 

 amounts of germanite and renierite intergrown with 

 galena and tennantite. 



From the sparse data available, the occurrence of 

 gallium, germanium, and indium in sulfide deposits 

 is governed by their ability to coprecipitate with zinc 

 in environments favorable to metal sulfide deposi- 

 tion. Apparently the concentration of gallium, ger- 

 manium, and indium in mineralizing solutions is 

 small, and in only a few deposits, notably at 

 Tsumeb, has the concentration exceeded the maxi- 

 mum level at which they are able to enter the crystal 

 lattice of sphalerite and other sulfide minerals. 



In addition to occurring in zinc deposits, germa- 

 nium is notably concentrated in some sulfide ores 

 of copper, especially in deposits containing enargite, 

 bornite, or tennantite, such as those at Butte, Mon- 

 tana; Chuquicamata, Chile; Cerro de Pasco, Moro- 

 cocha, Quiruvilca, and Casapalca, Peru ; and Bor, 

 Yugoslavia. The germanium content of enargite 

 typically may be higher than it is in many zinc 

 deposits. Complex sulfide copper ores containing tin, 

 antimony, and arsenic, as well as many tin ores such 

 as those in Bolivia, contain significant indium and 

 some germanium and gallium. Lead ores and com- 

 plex lead-zinc-copper ores also may be potential 

 sources. These ores of metals other than zinc in 

 general have not provided major sources for recov- 

 ery of germanium or gallium but have been the 

 source of some indium production. 



Gallium is also concentrated in bauxite, which 

 forms through intensive weathering of different 

 kinds of aluminous rocks. For the Arkansas bauxite 

 deposits, a slight increase in the gallium-aluminum 

 ratio during bauxitization has been shown in com- 

 paring a series of analyses of fresh nepheline sye- 

 nite, partly bauxitized rock, and high-grade piso- 

 litic bauxite. This increase has been attributed to 

 the lower solubility of gallium hydroxide compared 

 with aluminum under reducing conditions. Typical 



gallium contents of bauxites range from 20 to 100 

 ppm, the higher values being found in bauxites de- 

 veloped from granitic and alkalic rocks. There are no 

 systematic studies of the lateral variation of gallium 

 content in bauxite deposits, but the few data avail- 

 able suggest that lateral variation could be very 

 slight. 



In the less intensely weathered high-alumina clays 

 that have been considered as potential resources of 

 aluminum, the gallium content is generally in the 

 range of 30-60 ppm. 



COAL DEPOSITS 



Germanium and gallium both are found in very 

 small concentrations in living plants, but no specific 

 function has been attributed to their presence. Plant 

 residues typically show some enrichment in both 

 elements. The germanium and gallium content of 

 many coal beds is notably high and is greatest in 

 the uppermost or lowermost parts of individual coal 

 seams. The germanium content of coal typically 

 ranges from a few to a few hundred parts per mil- 

 lion, and it may increase to a few tenths of a per- 

 cent in the ashes formed by combustion of the coal. 

 Gallium content in coals is not as variable, but it 

 also increases from a few tens of parts per million 

 in most coals to a tenth of a percent or more in 

 selected combustion products. 



The variability of germanium content in coal 

 seams is largely attributed to the reaction of com- 

 plex humic acid hydroxyl groups with germanium- 

 bearing water in such a way that any germanium 

 released by plant decay or entering from outside 

 the coal-forming environment is promptly incorpo- 

 rated or adsorbed in the organic complex. The gal- 

 lium content presumably is similarly enhanced dur- 

 ing coalification. The higher content of both metals 

 in the upper and lower parts of coal seams is attrib- 

 uted to the greater volume of water passing through 

 or adjacent to these parts of the coal beds during 

 their formation because of the low permeability of 

 the enclosing clayey strata. 



RESOURCES 



The known reserves of gallium, germanium, and 

 indium are closely associated with currently ex- 

 ploited zinc and aluminum ores and with coal de- 

 posits that are supplying fuel to a few power plants 

 in Great Britain. Additional potential resources are 

 known, but much of the ore from which these metals 

 might be recovered currently is processed without 

 separation of these minor constituents. The extent 

 of recovery in the future depends both on better 

 recovery technology and on increased demand for 



