526 brown. SEDIMENTARY HOSTS FOR ORE DEPOSITS [Ch. 29 



also, warm climatic conditions are most favorable. However, rainfall, 

 topography, and plant cover must combine in most situations to pre- 

 vent appreciable erosion so that the residual products will not be re- 

 moved as they are formed. 



Since the area exposed to leaching and oxidation is greatest the 

 more nearly the rocks are flat-lying, and also since erosion is least 

 the flatter the topography is, it follows that most important bodies of 

 residual ores occur in areas of relatively flat-lying rocks and extend 

 only to shallow depths, usually not beyond 100 or 200 feet, often 

 much less. They may, however, be covered by a mantle of uneconomic 

 overburden, due to excessive leaching, subsequent alluvial cover, or 

 other factors. Contacts between unlike formations, as between lime- 

 stone and shale, or even sandstone, often provide favorable surfaces 

 for the concentration of residual deposits. 



The carbonate and oxide ores of lead, zinc, and copper are an ex- 

 ception to the preceding rules, for they may form at considerable 

 depths, especially in arid regions. 



There are many interesting chemical and physical problems con- 

 nected with the formation of these deposits, and they have received 

 much attention, which has proved quite rewarding. However, they 

 seem to have no very pertinent connection with the importance of sedi- 

 ments as hosts for ore and are hardly appropriate to this discussion. 

 Good summaries of residual processes will be found in several text- 

 books, notably Lindgren (1933) and Bateman (1942). 



VEIN DEPOSITS IN SEDIMENTARY ROCKS 



Sedimentary rocks are important as hosts to veins of infinite variety, 

 embracing virtually the entire category from pegmatites and quartz 

 veins, with or without economic accessories, through the carbonate 

 series, mineralized or barren, to barite, fluorite, etc. All of these may 

 occur in non-sedimentary rocks as well, and in many localities similar 

 veins occur in both igneous and sedimentary rocks. Indeed, it has 

 long been recognized that sedimentary rocks in the vicinity of in- 

 trusive igneous bodies are in a highly perferred position for receiving 

 the mineralizing and vein-forming materials which so often follow 

 closely the final phases of igneous activity. This is due in part, no 

 doubt, to their contrasting physical and chemical nature with respect 

 to the igneous rock, but perhaps even more to the sharp cooling effect 

 they exert on emanations escaping from the heated areas of igneous 

 activity. 



