ORIGIN OF ORE DEPOSITS 25 



in the desert regions of the Mexican plateau situated many miles 

 Inmi the mountains and far away from igneous rocks. 



Metallic Content of Meteorites. Nearly all of the common 

 metals have been found in meteorites and gold has been reported. 

 As these particles reach the earth they mingle with the soil, 

 oxidize in the presence of moisture, pass into solution and are 

 transported to a deeper zone by the circulating ground waters. 

 It is probable that a part at least of the widely diffused metallic 

 content of sedimentary rocks is of meteoric origin, and that 

 this extraterrestrial material is the primary source of many vadose 

 ore deposits. 



Segregation. Many ore bodies are so intimately associated 

 with masses of igneous rocks as to lead the observer to the 



FIG. 18. Example of a contact deposit between two different kinds of rock. 



conclusion that such deposits resulted without doubt from the 

 solidification of homogeneous or heterogeneous magmas through 

 various causes of differentiation. Such deposits are so closely 

 related petrographically to the rock masses as to lead to the 

 conclusion that they form one heterogeneous complex. Magmas 

 may be either homogeneous or heterogeneous. If the magma 

 be homogeneous under the same conditions of temperature and 

 pressure, there is uniform mobility in the various portions of 

 the material. If the magma be heterogeneous then the more 

 fluid portions may move readily toward the surface and the 

 heavier and more basic material migrate toward the lower 

 portion of such a magma. These magmas in which a perfect 

 segregation may occur are deep-seated deposits brought into 

 view through the erosion of a considerable amount of super- 



