Chapter 29 



SEDIMENTARY ROCKS AS HOSTS FOR ORE DEPOSITS 



John S. Brown 



Chief Geologist, St. Joseph Lead Co. 



Bonne Terre, Missouri 



LIMITATIONS OF TOPIC 



This discussion is limited, by agreement, to a consideration of sedi- 

 mentary rocks as receptacles for the subsequent deposition of ores 

 and of the factors that determine what kinds of rocks are effective 

 hosts in particular circumstances. Some of these features, naturally, 

 result from the sedimentation processes that formed the rocks and, 

 therefore, are germane to a general consideration of problems of sedi- 

 mentation. Obviously excluded from this survey are all syngenetic 

 bedded ores, which are covered in another part of this book. Like- 

 wise excluded, since they are covered in another chapter, are the so- 

 called sedimentary iron ores, even though sometimes there may be a 

 difference of opinion as to whether these are strictly syngenetic or in 

 part epigenetic. 



Thus delimited, there remain within our purview three quite dissimi- 

 lar types of deposits which occur abundantly in sedimentary rocks: 

 (1) residual concentrations, (2) vein deposits, (3) primary replace- 

 ments. 



RESIDUAL CONCENTRATIONS 



Ores form by residual concentration in both igneous and sedimentary 

 rocks but are commonest in the latter. They comprise principally 

 ores of the commoner metals which are widespread in the earth's crust, 

 notably iron, manganese and aluminum. However, they include im- 

 portant deposits of less abundant minerals such as phosphates and 

 barytes, and occasional important bodies of non-ferrous metals, par- 

 ticularly copper, zinc, and lead in oxidized forms. 



The residual ores of iron, aluminum, phosphorus, and, frequently, 

 manganese form mainly by the concentration of uneconomic amounts 



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