THOMAS B. NOLAN 57 



posits of metalliferous minerals, which greatly expand our capacity 

 for future production. The new lead-zinc deposits of New Brunswick 

 in Canada and in Tennessee, the new copper deposits in Arizona, the 

 iron and lead deposits of southeast Missouri, and the immense rare- 

 earth occurrence of the Mountain Pass district in California are exam- 

 ples of discoveries that, to a large extent, have been the result of re- 

 search-guided exploration using new techniques, such as airborne 

 geophysical instruments and geochemical prospecting methods. There 

 are sound theoretical grounds for believing that many additional de- 

 posits remain to be found by sharpened and improved concepts of 

 origin and by new and more elaborate exploration tools. Engel's re- 

 cent study " of variations in the ratio of the isotopes of oxygen in 

 some minerals associated with ore deposits, and the possibility that 

 such variations may reflect temperature gradients existing at the time 

 of ore deposition offers an exciting example of these techniques. 

 Barton ^^ similarly has opened up the possibility of predicting the 

 environment in which ore minerals may be deposited through his work 

 on the equilibrium relations on these minerals. 



The additions to our resource base now being made by better 

 knowledge of presently mineable orebodies and improved exploration 

 methods and techniques will be supplemented in the future to an in- 

 creasing extent by research on subgrade and ultra subgrade material. 

 The study of the distribution of elements in the earth's crust in con- 

 centrations that are too small to be presently workable is already 

 pointing the way to accumulations in which two or more elements or 

 substances are present in trace amounts but which, in combination, 

 may represent potential sources of very large magnitude. In addition, 

 increased requirements or new and improved recovery methods may 

 make much material merchantable that is presently below acceptable 

 grade. 



It appears that many elements may be distributed through- the crust 

 in such a way that there is an inverse relationship between the quan- 

 tity or tonnage of material containing the particular element and the 



1^ A. E. J. Engel and C. Patterson, Isotopic Composition of Lead in Leadville 

 Limestone, Hydrothermal Dolomite, and Associated Ore (Geological Society 

 of America Bulletin 68, 1957), p. 1723. 



18 P. B. Barton, Jr., "Some Limitations on the Possible Composition of the 

 Ore-forming Fluid," Economic Geology, Vol. 52 (1957), pp. 333-53. 



