THE MINERALS OF IDAHO 63 



The Oro Grande district is 12 miles south of Elk City. The veins 

 are mostly large and low grade, although rich native gold ore was 

 found on the Higan claims in 1905. 



The Robbins of Buffalo Hump deposits consist of a network of 

 quartz seams along joints in granite. The gold occurs in white 

 quartz with scattered masses of pyrite, galena, chalcopyrite, and 

 tetrahedrite. The Jumbo mine in 1902 was producing 2,000 tons of 

 ore a month. The district contains some placers. 



The Salmon River placers or Simpson district contain gold bearing 

 gravels which are worked by tunneling. One lode mine — the 

 McKinley Gold Mining Co. — is operating. The placer gold of the 

 district averages 0.836 fine, being worth $17.28 an ounce. The 

 production was $4,627 in 1908 and $3,152 in 1909. 



The Thunder Mountain district exhibits several deposits of 

 unusual character, the chief properties being the Sunnyside and the 

 Dewey. In the Sunnyside the gold occurs in a mineralized rhyolite 

 breccia containing a considerable amount of tuff, the metal being 

 mixed with clay along fractures and joints. The mineralized flow 

 is from 15 to 30 feet thick and is underlain by rhyolite and overlain 

 by volcanic mud, both of which are barren. The breccia averages 

 $4 to $11, although some lots run $35. The gold is alloyed with 

 silver in approximately the proportions of electrum as placer gold 

 from the outcrop is worth only $11 an ounce and the ore assays 2 

 to 3 ounces of silver a ton. Geological conditions are similar at the 

 Dewey mine, where the gold occurs in seams in a rhyolite tuff. 

 The rock between seams is barren. The gold is sometimes found 

 associated with pyrite and nodules of pyrite may contain leaf gold 

 in the center. Some very rich pockets of ore were extracted near 

 the surface. The tuff contains charred logs of wood up to 2 feet in 

 diameter and 60 feet long, which are frequently impregnated with 

 native gold. The gold is much more abundant near the surface and 

 is in all probability due to secondary enrichment of low grade rock. 14 

 A sample of the gold concentrated by panning from the volcanic 

 breccia (Cat. No. 74439, U.S.N.M., gift of Victor C. Heikes, 1902) 

 consists of fine, mossy grains which, under a lens, are seen to be 

 made up of dendritic or arborescent groups of crystals similar to 

 the "wire" copper and "wire" silver of the Coeur d'Alene district. 



The Warren district recorded a production of $15,000,000 in gold 

 previous to 1896, although after 1870 the placers were worked 

 chiefly by Chinese. Vein mining began in 1866 after the richest 

 placers were depleted and the lode mines had produced $125,000 up 

 to 1871. The placers were mostly comparatively shallow and are 

 now largely exhausted except certain deeper gravels ialong Meadow 



» J. B. Umpleby and D. C. Livingston. Reconnaissance of South Central Idaho, Idaho Bureau 

 Mines and Oeol. Bull. 3, pp. 5-6, 1920. 



