26 Mineralizers in Ore Segregations [224 



usually basic in composition; (3) the gangue minerals of 

 these deposits are the same as the constituent minerals of 

 the enclosing rock; (4) the* ore body frequently passes by 

 gradual transition into the igneous rock by a decrease in the 

 amount of the ore minerals and increase in the amount of 

 the silicates. These deposits appeared to be an integral part 

 of the igneous rock in which they are found and to represent 

 an extreme facies of the product of rock differentiation, and 

 consequently were established as an independent group of ore 

 deposits to which the name magmatic segregation was applied. 

 The group was subdivided by Vogt into three divisions ac- 

 cording as the metal occurs in the native, oxidic or sulphidic 

 form. Segregations of native metals as primary ore deposits 

 are of little economic importance, but the placers derived 

 from such of native platinum in the Urals are our principal 

 source of that metal. Segregations of oxidic ores are our 

 only source of chrome ores, include countless deposits of 

 titaniferous iron ores, and important deposits of non-titan- 

 iferous iron ores. Segregations of sulphidic ores include the 

 nickeliferous and supriferous pyrrhotites and probably a few 

 copper sulphide deposits. 



The metallic content of the segregations of the native 

 metals is usually rather sparsely disseminated through the 

 rock, and on account of the few examples and their minor 

 importance, the propriety of regarding the metal as a segre- 

 gation and product of crystallization from a molten magma 

 has been little questioned. The segregations of oxidic ores 

 usually occur well within the igneous mass, and there are so 

 many admirable illustrations of gradation from ore-mineral 

 bearing rock to ore body that no particular significance has 

 been attached to the observation in a number of instances 

 that the ore minerals are later than the silicates, and the 

 conception of a segregation and solidification from a molten 

 magma has been rarely challenged. The position of the sul- 

 phidic deposits has, however, been somewhat dubious from 

 the start. They tend to occur on the periphery of the ig- 

 neous mass, the sulphides often penetrate into the wallrock, 



