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UNITED STATES MINERAL RESOURCES 



which yielded over 100,000 tons of copper in ore 

 averaging nearly 10 percent, is believed to have been 

 derived from the nearby metavolcanic rocks of the 

 Nelson Formation of Late Mississippian age (Coats 

 and Stephens 1968). Volcanic rocks of Mississippian 

 to Permian age are widespread in northwestern 

 Nevada, and these and nearby sedimentary rocks 

 offer numerous prospecting targets for high-grade 

 copper deposits (Hewitt, 1968). 



Metal-rich brines in the Salton Sea area of south- 

 ern California and the Boleo copper deposits in 

 Pliocene rocks in Baja California, Mexico, suggest 

 a prospecting area. Upper Tertiary to Holocene 

 rocks and sediments in and along both sides of the 

 Gulf of California rift zone and its northern projec- 

 tion in the Salton trough may have been or may be 

 in the process of being mineralized by metal-rich 

 solutions rising through this rift zone. 



In the Appalachian Mountains, the possibility of 

 finding large porphyry copper deposits of Paleozoic 

 age is intriguing. Ore-bearing porphyries are be- 

 lieved to be emplaced only at relatively high levels 

 in the crust, during the final or postorogenic stages 

 of a tectonic cycle. Thus pre-Mesozoic fold belts 

 contain few of these deposits, because erosion in 

 these areas has progressed to the point where most 

 of the upper part of the crust has been removed. 

 Special conditions, however, such as the deep burial 

 and preservation of rocks of an early tectonic cycle 

 by geosynclinal deposits of a later cycle, may result 

 in the preservation of some older porphyry-type 

 deposits such as at Catheart Mountain in Maine and 

 those of the U.S.S.R. in central Asia. 



Guild (1971) and Silhtoe (1972) discussed the 

 relationship between porphyry copper deposits and 

 the boundaries of major crustal plates and con- 

 cluded that many deposits are related to the process 

 of magma generation along the subduction zone 

 where one crustal plate slides beneath another. 

 Porphyry copper deposits tend to occur along the 

 concave side of arc-trench systems resulting from 

 the process of subduction. Recognition of ancient 

 crustal-plate boundaries of this type would enable 

 us to outline episubduction regions where porphyry 

 copper deposits might occur. The Piedmont Province 

 of the Appalachian Mountains may be such a region. 

 H. L. James ("The Metallogenic Concept," Presi- 

 dential address. Soc. Economic Geologists, Nov. 13, 

 1970, unpub. rept.) pointed out that this province 

 has an arrangement of ore-bearing zones suggest- 

 ing and eastward-dipping subduction zone. If por- 

 phyry copper deposits still are preserved they would 

 lie in the Piedmont near the present edge of the 

 Coastal Plain. Copper, lead, and zinc have been 



noted in several old mines forming a trend extend- 

 ing 400 miles northeastward from central Georgia 

 to the North Carolina- Virginia boundary. Within 

 this trend, several low-grade disseminated copper- 

 molybdenum occurrences have been explored by 

 drilling programs in recent years. Porphyritic in- 

 trusive rock has been reported from only the Ma- 

 gruder mine area in Lincoln County, Ga., but geo- 

 logic reports on the old gold mines and prospects 

 are mostly sketchy and limited to descriptions of 

 the veins and the closely associated rocks. Some of 

 the mines and prospects are reported to have areas 

 of abundant disseminated pyrite. 



In southwestern Alaska and the Aleutian Islands, 

 geologic conditions similar to those of the Southwest 

 Pacific permit the possibility of discovery of por- 

 phyry copper deposits. Quartz diorite intrusions 

 and wide areas of disseminated pyrite described on 

 Amchitka Island (Powers and others, 1960, p. 539, 

 541) suggest that in at least one locality a favor- 

 able environment exists for porphyry copper pros- 

 pecting. Projection of the porphyry copper provinces 

 of British Columbia and Yukon Territory into cen- 

 tral Alaska suggests that large resources might be 

 found there. 



In the Antilles, an extensive copper province is 

 suggested by the occurrence of porphyry copper 

 deposits in Puerto Rico. The Puerto Rican deposits 

 were formed during a period of vulcanism and in- 

 trusion in the early Tertiary (Barabas, 1971) and 

 are overlapped by middle and upper Tertiary 

 coastal plain carbonate rocks. A small molybdenum 

 prospect in an Eocene pluton in the Virgin Islands 

 (Ratte, 1971) suggests an eastward extension of 

 this mineral province. In the limestone Caribbees, 

 lower Tertiary plutonic rocks are exposed on St. 

 Martin and Antigua. Hydrothermally altered rocks 

 in St. Martin, Guadeloupe, Martinique, and other 

 islands suggest interesting drilling targets, some of 

 which were being explored by private companies 

 during 1971. It is possible that another million tons 

 of copper might be discovered in these islands, but 

 land-use conflicts between mining and tourism may 

 make their recovery in the near future doubtful. 



West of Puerto Rico the lower Tertiary mineral 

 province is interrupted. Volcanic rocks and poten- 

 tially productive intrusive bodies are absent in the 

 lower Tertiary sequence of Santo Domingo. How- 

 ever in the Montagues Noires of Haiti, the Blue 

 Mountains of Jamaica, and the southern Oriente 

 Province of Cuba, lower Tertiary volcanic rocks are 

 present. Quartz-bearing intrusive rocks and altera- 

 tion zones that may be present in these areas repre- 

 sent high-priority targets for porphyry copper 



