Ch. 29] RESIDUAL CONCENTRATIONS 525 



of these materials originally dispersed in sediments where, often, they 

 may have been formed syngenetically. The ores of copper, zinc, lead, 

 and barytes, on the contrary, represent concentrations from epigenetic 

 fissure veins or replacements, often of originally uneconomic character 

 but not always so. 



The formation of ore deposits by residual processes depends usually 

 on some pronounced difference in the solubility in ground water of the 

 desirable constituent and its accompanying gangue. Either the desir- 

 able element ordinarily is much less soluble, or by oxidation it passes 

 into a less soluble form, while the gangue is dissolved and removed. 

 As limestone (including dolomitic types) is by far the most soluble 

 common rock, it follows that residual ores develop most often from 

 limestone terranes. Thus formations containing iron or manganese 

 carbonate are dissolved, the iron and manganese being converted, in 

 the process, to the less soluble oxides and left behind or transported 

 but slightly. Many deposits in the southern and eastern United States, 

 as well as elsewhere, are of this origin. When limestones contain 

 phosphate, which is less soluble than the carbonates, preferential solu- 

 tion produces residual phosphate ores, as in Tennessee and Florida. 

 Two of the important barytes districts of the United States, in Missouri 

 and in Georgia, produce mainly residual ores derived from lean fissure 

 and cavity fillings in limestone formations. Important zinc deposits 

 in the eastern United States from Pennsylvania to Tennessee have 

 consisted of carbonate ore formed from the oxidation of sulphides and 

 their redeposition as carbonate. This differs slightly from the usual 

 process in that it involves replacement of limestone at the base of the 

 weathered zone. Copper ores in the southwestern United States have 

 formed in somewhat similar process as carbonates. Sulphide iron can, 

 by oxidation, also form oxide, or carbonate, deposits, but these are 

 generally small, as large iron sulphide bodies in limestone are not 

 abundant. The Missouri sinkhole marcasite deposits are an exception, 

 and they are often altered to iron oxide. 



Aluminum ores such as bauxite and high alumina clays commonly 

 result from the alteration of various types of clay and shale or of cal- 

 careous rocks containing large amounts of aluminous impurities. To 

 form ore, the preferential removal of silica often is as essential as the 

 elimination of carbonate, and the inability to remove iron oxide often 

 results in a low-grade or uneconomic deposit. In some places, al- 

 luviation processes have affected the residual matter and perhaps form 

 a step in the process of ore formation. 



Rainfall, naturally, is an essential in all these residual processes, and 

 to form aluminous ores it apparently needs to be copious. Probably, 



