SOURCE OF THE METALS. 1033 



for the conglomerate deposits are even more important, but it is based 

 upon the fact that copper, both in the native form and to a less extent as a 

 sulphide, is sparsely disseminated throughout the basic Keweenawan lavas 

 of the entire Lake Superior Basin. This broad relation has much greater 

 significance to me with reference to the source of the metal than does the 

 association of the ore with an igneous deposit at a particular mine. 



Spurr notes substantially the same relations between the ore bodies of 

 the Monte Cristo district of Washington and the associated tonalite. He 

 says that metallic sulphides are everywhere noticeable throughout the tona- 

 lite; that scarcely a cross fracture occurs through which there has been 

 water circulation and not some concentration of sulphides; and that in the 

 solid tonalite itself there are local segregations of sulphides. Furthermore, 

 he says there are all g'radations from these bunches of sulphide in the 

 fractured rocks through a slight dissemination of sulphides to the ore 

 deposits along the major fractures, such as those of the Mystery and Pride 

 mines. Accepting this statement of fact, there is strong warrant for the 

 suggestion of Spurr that the ores are derived "directly by concentration 

 from the tonalite in which they lie."" 



While in many instances it is therefore believed that a contiguous 

 igneous rock is the main source of the metal for ore bodies, by some this 

 connection is over-emphasized, and the cases in which this is not true are 

 often ignored. Doubtless this is a consequence of the fact that gold and 

 silver are usually in mind when ore deposits are considered. Gold and silver 

 ores may rather frequently be derived largely from recognizable igneous 

 rocks, but to a very large extent, even for gold and silver, the immediate 

 source of the metal in the deposits heretofore exploited was not the original 

 igneous rocks. For instance, placers, which have yielded more gold to the 

 world than any other form of deposit, have derived their metals from veins, 

 from previous placers, and from the widely dispersed gold in many kinds 

 of rocks. Much of the gold of many placers has been worked over by the 

 processes of nature several times. 



If we turn to iron, a metal of vastly greater importance than gold and 

 silver combined, in few or no deposits now exploited can the metals be said 

 to be derived directly from igneous rocks. Again, the deposits of lead and 



a Spurr, J. E., The ore deposits of Monte Cristo, Washington: Twenty-second Ann. Rept. U. S. 

 Geol. Survey, pt. 2, 1901, pp. 828-829. 



