540 



UNITED STATES MINERAL RESOURCES 



layers in the upper gabbroic rocks of the complex 

 (Howland and others, 1936 ; Page and Jackson, 1967 ; 

 Page and others, 1969) . 



CONCENTRIC COMPLEXES 



The concentrically zoned ultramafic complexes and 

 associated mafic bodies of southeastern Alaska con- 

 tain only one known platinum lode, developed by the 

 Salt Chuck mine. Both platinum and palladium were 

 recovered as a byproduct of copper mining. The ore 

 minerals occur as small masses and disseminations 

 in pyroxenite, gabbro, and gabbro pegmatite (Mertie, 

 1969, p. 76) . The platinum metals are closely as- 

 sociated with bornite and chalcopyrite. The form in 

 which the platinum metals occur is not known, but 

 sperrylite has been identified in one specimen. 



Potential platinum lodes may exist within the con- 

 centrically zoned ultramafics, as shown by anomalous 

 concentrations of platinum as high as 29 ppm in the 

 chromite-bearing dunite core of the ultramafic com- 

 plex at Union Bay. 



ALPINE COMPLEXES 



Alpine complexes may or may not represent poten- 

 tial platinum resources, judged on relatively few 

 modern analyses. Eighteen dunites and harzburgites 

 from Burro Mountain, Calif., average 12.8 ppb Pt, 

 3.7 ppb Pd, and <5 ppb Rh (Loney and others, 1971) . 

 Analyses of dunites and harzburgites from Red 

 Mountain and New Idria in California, and of similar 

 rocks from Twin Sisters and Cypress Island in Wash- 

 ington, average <10 ppb Pt, <4 ppb Pd, and 

 <5 ppb Rh. Chromites from these occurrences aver- 

 age slightly higher — 36 ppb Pt, 10 ppb Pd, and 9.5 

 ppb Rh (Page, 1969) . 



OTHER MAPIC-ROCK ENVIRONMENTS 



Minor platinum-metal deposits associated with 

 copper, nickel, and gold in mafic and ultramafic rocks 

 are probably insignificant in terms of platinum-metal 

 resources but illustrate other environments where 

 platinum metals are found. For example, deposits in 

 the Bunkerville district, Nevada (Beal, 1965), have 

 a reported range in grade of 0.14 to 0.25 oz/ton. 

 Another example is in the Revais Creek district, 

 Montana, where 0.23-0.54 ppm Pt and 0.10-0.79 ppm 

 Pd (eight samples from the Copper King deposit) ai'e 

 found in a gold-silver-copper ore that consists of 

 chalcocite, bornite, and chalcopyrite. The ore is in a 

 hornblende gabbro dike and in the Ravalli Group 

 (Precambrian) that it intrudes. The Rambler and 

 Centennial mines, Wyoming, produced a small 

 amount of platinum-group metals with a grade 

 between 1 and 1.3 oz/ton from deposits of irregular 



shape in mafic Igneous and metamorphic rocks (Mer- 

 tie, 1969 ; Theobald and Thompson, 1968) . 



Platinum-group metals occur in many placers 

 within the United States. Minor amounts of the 

 platinum group have been recovered from the gold 

 dredges of California and from the gold placers of 

 Oregon, Washington, Montana, Idaho (Logan, 1919), 

 and Alaska, but significant amounts have been pro- 

 duced only from the placers of the Goodnews Bay 

 district, Alaska. 



Platinum-metal-bearing placers of the Goodnews 

 Bay mining district occur in two paystreaks in the 

 valley of the Salmon River (Mertie, 1969, p. 80). 

 Total production from 1927 to 1969 is believed to 

 have been considerably more than half a million troy 

 ounces, with a maximum output exceeding 30,000 

 ounces in 1938 (Mertie, 1969, p. 79). The weighted 

 mean percentages of metals mined from 1936 to 1970 

 are 82.31 percent platinum, 11.28 iridium, 2.15 osmi- 

 um, 0.17 ruthenium, 1.29 rhodium, 0.38 palladium, 

 and 2.24 gold. 



The platinum metals of these placers are contained 

 mainly in two alloys. The major alloy is ferroplati- 

 num ; microprobe analyses by G. A. Desborough gave 

 the results shown in table 111. The minor alloy con- 



Table 111. — Composition, in iveight percent, of ferroplatinum 

 in 50 placer grains from Goodnews Bay, Alaska 



Mean (standard deviation) 



Range 



Pt 86.1 (1.8) 82.7-88.6 



Fe 9.3 (.6) 8.2-11.2 



Ir 2.26 (1.70) .1- 2.0 



Rh .9 (.5) .1- 2.0 



Pd .65 (.3) .1- 1.4 



Cu .7 (.3) .1- 1.1 



Os .3 (.3) .1- .8 



Ru .1 .1- .1 



sists dominantly of iridium and osmium, with small 

 amounts of platinum, and still smaller amounts of 

 ruthenium and rhodium. In addition to these two 

 alloys, minute amounts of five platinum minerals — 

 cooperite, laurite, sperrylite, mertieite, and a yet- 

 unnamed iridium-palladium sulfide (table 111) — 

 have been identified. 



MINOR OCCURRENCES 



Platinum metals are associated with alkalic in- 

 trusive rocks (syenites) that have copper minerali- 

 zation, but very little is known about this environ- 

 ment. The Copper Hill mine. La Plata mining dis- 

 trict, Colorado (Eckel, 1949), is in an ore zone of 

 chalcopyrite that contains platinum and palladium. 



