542 



UNITED STATES MINERAL RESOURCES 



The estimates of identified platinum-group metals 

 in the basal zone of the Stillwater Complex, Montana, 

 are based on published reserves (Dayton, 1971) of 

 ore tonnages and the existence of a 25-mile-long 

 zone, averaging 300 feet in width and workable on 

 the average to 2,000 feet downdip with a probable 

 average of 0.5 ppm platinum-group metals. Similar 

 estimates were made for the chromitite, using aver- 

 age platinum-group-metal content of the A, G, and 

 H zones. 



The 250-mile-long Merensky Reef of the Bushveld 

 Complex, South Africa, has been mined locally down- 

 dip 6,000 feet, with an average stoping thickness of 

 28.5 inches (Mertie, 1969). The grade ranges from 

 0.25 to 0.35 oz per ton. If a grade of 0.25 oz per ton 

 persists throughout the reef's extent and the reef 

 were mined to 10,000 feet, about 656 million ounces 

 of platinum-group metals would be available in addi- 

 tion to the 200 million ounces estimated by Ageton 

 and Ryan (1970, p. 657). Mertie (1969) implied that 

 only 70 miles of reef could be productive under pres- 

 ent economic conditions, and so our estimate repre- 

 sents a large proportion of subeconomic resources. 



The other area of identiiied resources is in the 

 Goodnews Bay mining district and the contiguous 

 offshore areas. Exploration there by mining com- 

 panies has shown the presence of anomalous 

 amounts of platinum metals in Goodnews Bay and in 

 the beaches and offshore sediments. A recent study 

 by Allen L. Clark (unpub. data) defined anomalous 

 platinum-metal zones within the nearby Red Moun- 

 tain ultramafic mass. 



The hypothetical resources (table 112) of Alaska 

 are large, primarily because all the ultramafic bodies 

 studied to date (Clark and Greenwood, 1972) contain 

 anomalous amounts of platinum (table 113). In addi- 



Table 113. — Concentrations, in parts per million, of platinum, 

 palladium, rhodium, and iridium, southeastern Alaska 



tion, the concentrically zoned complexes of south- 

 eastern Alaska are geologically very similar to the 

 platiniferous bodies of the Ural Mountains, Russia, 

 and therefore may have a comparable potential. One 

 ultramafic body at the Salt Chuck mine yielded a 

 large amount of platinum and palladium as a by- 

 product of copper mining, indicating the likelihood of 

 undiscovered economic concentrations. 



Other belts of ultramafic rocks exist in Alaska, 

 and where studied, have all been found to contain 

 anomalous amounts of platinum metals. Frequently, 

 placer mining for gold has recovered small amounts 

 of platinum metals from streams draining these 

 ultramafic belts. 



T. P. Thayer (U.S. Geol. Survey, 1957) estimated 

 chromite reserves in alpine ultramafic rocks and in 

 beach sands in Oregon, California, and Washington ; 

 these deposits, in turn, are estimated to contain 0.03 

 ppm platinum-group metals and could, then, yield 

 1,660 hypothetical ounces of platinum-group metals 

 as a byproduct. 



The U.S. Bureau of Mines (1967) estimated the 

 potential production of gold from placers in Cali- 

 fornia, Oregon, Washington, and Montana as 

 81,068,000 ounces. Comparison of gold and platinum 

 production figures and estimates by Logan (1919) 

 and Pardee (1934) suggests that for each ounce of 

 gold about 0.0007 of an ounce of platinum-group 

 metal is recovered. This suggests that there are 

 57,000 ounces of potential platinum metals in these 

 placers. 



SPECULATIVE RESOURCES 



Limited knowledge of the geochemistry and geo- 

 logic environments of platinum metals makes it diffi- 

 cult to speculate about potential resources. With the 

 assumption that platinum metals are derived from 

 the mantle, then one may speculate that any igneous 

 rocks that have a direct known or postulated origin 

 in the mantle may contain resources of platinum 

 metals (table 112). Ore deposits associated with 

 some basalts or with some types of alkalic rocks and 

 their associates— for example, carbonatites — might 

 be worth investigating for platinum potential. Of 

 course, known but unexplored stratiform mafic and 

 ultramafic complexes, such as the Dufek in Antarc- 

 tica (Ford, 1970), and known but inadequately ex- 

 plored belts of ultramafic rocks, such as the central 

 Goias Precambrian serpentine belt in Brazil (White 

 and others, 1971), would have to be included as 

 speculative resources. As noted by Wright and 

 Fleischer (1965), platinum metals are sometimes 

 concentrated by late-stage magmatic and hydrother- 

 mal processes and therefore pegmatites may be a 

 potential source of platinum. 



After erosion and weathering, platinum metals ap- 

 parently act in two ways: (1) as heavy minerals and 

 (2) by dissolving. Any locality in which heavy min- 

 erals are concentrated from potential platinum-bear- 

 ing terranes, and localities where salines are found 

 in drainage basins draining potential platinum ter- 



