26 



ECONOMIC GEOLOGY 



incumbent strata. (See Fig. 18.) In the process of segregation 

 of the ecomonic minerals three classes of ore deposits are formed: 

 (1) The metals; (2) the oxides of the metals; and (3) the 

 sulphides of the metals. Following the separation of the metallic 

 content, the oxides and the sulphides of the metals, there appears 

 the solidification of the ferro-magnesian minerals as the pyroxenes, 

 the amphiboles, etc. After the solidification of the ferro- 

 magnesian minerals comes a more acidic product as the feldspars 

 and the last to separate is quartz. (See Fig. 19.) The causes of 

 segregation are: (1) Fractional crystallization of the various 



$ Mica Syenite 

 Porphyry 



FIG. 19. Vein of mica syenite-porphyry, showing marginal segregation ot 

 ferro-magnesian minerals and iron ores. (After J . H. L. Vogt.) 



constituents of the magma. (2) The separation of the magma 

 into two immiscible solutions, that is, solutions that will not 

 mix, like water and oil. The completeness of the segregation 

 may be determined by the rate of cooling of the magma as a 

 whole. (3) The degree of viscosity. The viscosity will depend 

 largely upon the chemical composition of the masses, varying 

 with the relative amount of the ferro-magnesian and acidic 

 minerals present. A segregation of the magma is not impossible 

 before its appearance as an intrusive and this gives rise to ore 

 bodies not unlike those segregating from homogeneous magmas, 

 but the segregation is less complete. 



