1916] Umpleby: Ore on the Limestone Side of Garnet Zones 35 



ore-bodies occur in the limestone close to or adjoining the granite, but 

 ore also occurs in the granite, "often at considerable distance from 

 the limestone." "The constituent minerals, and general character of 

 the ore-bodies in the two formations, are very similar. ' ' The principal 

 copper minerals are bornite and chalcopyrite, which are associated 

 either with magnetite and hematite or with garnet, augite and tremo- 

 lite. "All the ore-bodies of this (the latter) class, so far discovered, 

 occur in the limestone, close to the granite, and are often separated 

 from the granite by a zone of more or less completely replaced lime- 



Fig. 10. Section through Alberta Mine, Mackay, Idaho, showing occurrence 

 of ore in the central part of a garnet-diopside mass. 



stone." 18 This relationship is clearly brought out by the geologic 

 maps of the Grafter Mine and the Best Chance ore-body. Endo- 

 morphism has resulted in a considerable development of garnet, 

 pyroxene and epidote in the granitic mass. 



Interpretation 



The extent to which the occurrence of ore on the limestone side 

 of garnet zones, as illustrated in the seven well known districts cited 

 above, represents a general law cannot be definitely stated at present. 

 Indeed, the object of this paper is merely to set it forth as a working 

 hypothesis in the hope of arousing discussion and stimulating obser- 

 vation. It is highly probable, however, that there are many more 

 examples of the relationship. In papers on four of the districts cited 

 one writer has clearly stated or illustrated the relationship, but in the 

 writing of another on the same deposit I have been unable to find 

 a quotable statement. 



The occurrence of ore on the limestone side of garnet zones, 

 whether it be a constant relationship or not, must be explained for 



is ibid., p. 22. 



