186 



UNITED STATES MINERAL RESOURCES 



porphyries and other types of rocks that contain 

 disseminated sulfides. The use of geophysical meth- 

 ods in the search for massive sulfide ores is discussed 

 by several authors in Morley (1970). 



Recent developments in geochemistry and geo- 

 physics have made it possible to recover more in- 

 formation from diamond drilling than simply the 

 geology and copper assays of the core. Cheaper and 

 rapid analytical methods, principally in spectros- 

 copy, have made it more feasible to analyze large 

 numbers of samples for many elements. Slight varia- 

 tion in trace-element composition may be caused by 

 metals dispersed outvv^ard from an ore body not 

 intersected by the hole. Similarly, dovs^n-hole geo- 

 physical instruments of many types have been 

 perfected, enabling the geologist to extend his 

 know^ledge many feet beyond the walls of the drill 

 hole. 



PROBLEMS FOR RESEARCH 



Figure 22 shows the relationship between the size 

 and grade of ore deposits and copper production. 

 Annual U.S. production is about equivalent to a 

 deposit the size of Sierrita in Arizona, and annual 

 world production is about equal to a La Caridad 

 deposit or an average Zaire-Zambian sedimentary 

 deposit. It is important to know if we are mining 

 our copper deposits faster than we are finding them. 

 During the last 10 years, U.S. mine production was 

 about 13 million tons of copper (table 36). New 

 porphyry copper deposits discovered and (or) 

 brought into production during this 10-year period, 

 listed by Peters (1970), will yield slightly more than 

 14 million tons of copper. Assuming that several 

 million tons of copper is known in other, less im- 

 portant types of deposits and in some large but 

 unannounced discoveries, it appears that, in the 

 United States, exploration and development are 

 ahead of production by a small margin. Worldwide 

 exploration and development lead mine production 

 by at least 28 percent, again using Peters' list of 

 discoveries. 



The last decade was notable for intense explora- 

 tion activity in response to favorable political con- 

 ditions and high copper prices. Moreover, economic 

 geologists experienced a renaissance in geologic 

 theory during this period, in which new approaches 

 to problems of geochemistry, igneous and sediment- 

 ary petrology, and tectonics were found and applied 

 to ore deposits. However exploration geology is 

 very sensitive to economic cycles, and there is rea- 

 son to believe that this period of activity is over, at 

 least temporarily (Colen, 1972). This stop-and-start 

 exploration activity is extremely wasteful of human 



energy, and if we are to satisfy the long-term de- 

 mand for metal in a future world in which ore 

 deposits become increasingly more difficult to find, 

 new ways of organizing exploration activities must 

 be sought, especially long-range planning. 



Copper is a commodity for which a permanent 

 worldwide shortage is not easily foreseeable, but 

 even though the hypothetical and speculative re- 

 sources may be very large, an enormous amount of 

 geologic field work, analysis, and even intuition will 

 be required to transform these resources into proven 

 reserves. At the same time we must continue to 

 develop our scientific knowledge in order to replace 

 outdated exploration ideas with new ones. 



On the geological side of resource study, the need 

 for geologic and geophysical maps as a base for min- 

 eral exploration cannot be overemphasized. More 

 specifically, additional efforts must be applied to 

 mapping and other geologic studies of those parts 

 of the United States that have the highest mineral 

 potential. Southeastern Arizona is probably the 

 richest copper province in the world, yet geologic 

 mapping at a scale of 1 : 62,500 is complete for only 

 27 percent of the area, and nearly half the province 

 is unmapped. 



Large-scale metallogenic maps might be an effec- 

 tive exploration tool in southeastern Arizona to 

 detect the presence of deposits concealed beneath 

 younger volcanic and sedimentary rocks. 



Lowell and Guilbert (1970) and others pointed 

 out that almost all porphyry copper deposits have 

 a peripheral zone of small veins of base and precious 

 metals, manganese carbonates, and barite that may 

 extend outward for several miles from the deposit. 

 Careful mapping of these veins, including even the 

 smallest mineral occurrences, and careful classifica- 

 tion of them according to various mineralogic, struc- 

 tural, textural, and genetic schemes may direct the 

 exploration geologist to blind porphyry ore bodies 

 or to those covered by later deposits. This mapping 

 should be done at scales of 1:125,000 or greater 

 depending on the complexity of the geology and 

 spacing of the data points. Trace-element analyses 

 of the veins and of mineral separates from them 

 such as biotite or chlorite may aid in the interpre- 

 tation. Studies of salinity and temperature of for- 

 mation of fluid inclusions in the vein materials may 

 show the geologist the direction toward the source 

 of the mineralizing solutions. 



SELECTED REFERENCES 



Ageton, R. W., and Greenspoon, G. N., 1970, Copper, in Min- 

 eral facts and problems : U.S. Bur. Mines Bull. 650, p. 

 535-553. 



