454 abstracts: geology 



GEOLOGY. — The enrichment of sulfide ores. W. H. Emmons. Bulle- 

 tin U. S. Geological Survey No. 529, pp. 252. 1913. 

 The theory of sulfide enrichment is stated as follows: 



No metallic sulfide that is long exposed to air and water remains 

 unaltered. Iron sulfides, which are present in practically all sulfide 

 ores, are changed by weathering to iron oxides, and the changes are 

 attended by the liberation of sulfuric acid. ]\Iany of the metals form 

 soluble sulfates with sulfuric acid, and when conditions favor their 

 migration downward they are carried in solution to depths where air 

 is excluded. Unoxidized rocks are in general alkaline. Acid solutions 

 that encounter such rocks in the regions where air is excluded will lose 

 acidity, and as the solutions approach a neutral or alkaline condition 

 some of the metals they contain are deposited. If the solutions of 

 the metallic sulfates encounter metallic sulfides in depth precipitation 

 may take place, or there may be an interchange between the metals 

 in solution as sulfates and the metallic sulfides. Thus as a result of 

 precipitation or chemical interchange the metals are redeposited and 

 certain portions of the ore bodies become enriched. 



The effects of physical conditions on the process of enrichment are 

 discussed and the chemistry of the process is approached from botii 

 the experimental and the theoretical side, the chemical relations of min- 

 erals of copper, silver, gold, lead, zinc, and iron being discussed in turn. 



The literature of the principal deposits of copper, gold, and silver 

 sulfide ores in North America is reviewed with special reference to the 

 extent, character, and distribution of the secondary ores in each. Of 

 copper deposits with deeply enriched zones none are known to carry 

 pyrrhotite or abundant sphalerite. The data indicate that, in general, 

 ores containing abundant pyrrhotite and sphalerite become more 

 thoroly oxidized than those containing pyrite and chalcopyrite without 

 pyrrhotite or sphalerite; but oxidation and enrichment are not likely 

 to extend as deep in pyrrhotite or sphalerite ores. 



Many auriferous deposits in the surficial zone appear not to have 

 been enriched by secondary agencies, while in others there is no evi- 

 dence of solution and re-precipitation of gold. It is concluded that 

 the solution of gold depends in the main on the presence, simultane- 

 ously, of manganese and chlorides. Its migration depends on the per- 

 meability of the lodes and the material of the primary ores. Of the 

 common minerals calcite and pyrrhotite will probably precipitate gold 

 from auriferous solutions most rapidly. All the districts of silver-gold 

 deposits in which gold is assumed to have migrated include mangan- 

 iferous ores. In deposits carrying both metals, especially where chlor- 



