34 Mineralizers in Ore Segregations [232 



by. the Tofo deposit north of Coquimbo, Chile, being worked 

 by the Bethlehem Steel Company. This consists of a large 

 mass of comparatively pure magnetite forming the top of a 

 hill on the east side of the coast range and occurring within a 

 large area of gabbro rock. The igneous mass has undergone 

 considerable differentiation and various rock types are repre- 

 sented in the vicinity of the ore body from highly feldspathic 

 to almost pure ferromagnesian silicate rocks, some of which 

 occur as dikes. The broader relations of the ore body are 

 such as to suggest at once a magmatic segregation; but, at 

 the same time, there are many features that suggest pneuma- 

 tolysis. Adjacent to the ore, there are numerous stringers of 

 magnetite in the country rock, many of them of no greater 

 thickness than a knife blade, which traverse it in such a 

 way as to preclude the entrance of molten oxides and that can 

 be explained only on the basis of the high liquidity at lower 

 temperature that would be imparted by the presence of 

 abundant mineralizers. 



The argument for the participation of mineralizers in the 

 formation of magmatic deposits is a plausible one ,also from 

 a general standpoint of ore genesis. Processes in nature 

 representing different stages of a sequence from a given 

 starting point are not usually separated by a hiatus. It is 

 generally accepted today that igneous magmas are the pri- 

 mary sources of the metals and modern genetic classifications 

 group ore deposits according to their position or relation to 

 the original sources. It has been customary, however, to- 

 draw a sharp line between one group of deposits which it was 

 held segregated from the molten magma and solidified with 

 it, and such groups as represented deposition of material 

 extracted from the magma by mineralizers and constituting 

 the pneumatolytic and hydrothermal deposits. DeLaunay's 

 classification recognizes no such hiatus in the sequences of 

 mineralization, but postulates a gradually increasing partici- 

 pation of mineralizers and hence a gradual gradation from 

 one stage to the next. It goes even a step further and indi- 



