164 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



siderite. Tetrahedrite is also the principal mineral in the silver ores 

 of the Weber and other mines south of Lakeview on the south end 

 of the lake. 



CUSTER COUNTY 



Tetrahedrite is exceedingly rare in the Alder Creek district, having 

 been observed only in small particles in some of the partly oxidized 

 ore from the Copper Bullion tunnel. 49 Tetrahedrite forms the prin- 

 cipal valuable mineral in the ores of the Bayhorse district, which has 

 perhaps produced more tetrahedrite ore than any area of similar size 

 in the world. Ore, in which tetrahedrite is the only valuable metallic 

 mineral, to the value of $6,250,000 has been mined from the four 

 principal mines of the district. 50 The tetrahedrite is practically 

 always massive, dark gray, and with splendent luster on fresh fracture, 

 which becomes metallic upon moderate exposure. The three prin- 

 cipal mines producing tetrahedrite ore are the Ramshorn (product, 

 $3,000,000), Sky Lark, and Silver Bell. The ore bodies are filled 

 fissures in slates. The typical ore consists of about three-fourths 

 siderite and tetrahedrite makes up about one-third of the remainder 

 or about 8 per cent of the vein filling. A little arsenopyrite, galena, 

 and chalcopyrite accompany the tetrahedrite. Sometimes a central 

 vein an inch or two wide, of tetrahedrite, is bordered on either side 

 by a layer of chalcopyrite a half inch or more in width; (Cat. No. 

 56521, U.S.N. M.) . Other specimens show large masses of pure tetra- 

 hedrite from the Ramshorn mine, in siderite. In small vugs in the 

 ore crystals of tetrahedrite occur associated with crystals of arsenopy- 

 rite and siderite. The crystals are usually coated with a mossy crust 

 of minute cr} r stals of chalcopyrite. A specimen from the Hoosier 

 mine, Bay Horse, contains masses of this mineral, 1 to 2 inches in 

 diameter, in granular pyrite. Specimens from the Skylark mine 

 contain masses of tetrahedrite up to 4 inches through in oxidized 

 gangue and smaller veins and masses partly altered to cuprite, 

 malachite, azurite. A specimen from the No. 3 tunnel, Silver Bell 

 mine, contains some residual masses in oxidized material. The 

 tetrahedrite is sometimes altered to crusts of yellow-green or yellow 

 waxy bindheimite, associated with azurite and velvety tufts of 

 malachite (Cat. No. 51863, U.S.N.M.). Specimens from the Bull of 

 The Woods, Bullion, and other mines are similarly largely massive 

 tetrahedrite partly altered to malachite and azurite (Cat. No. 56518> 

 U.S.N.M.). 



A specimen of very pure tetrahedrite from the Ramshorn mine 

 was analyzed for Umpleby by R. C. Wells. The results of this 

 analysis, together with the ratios, are given in the following table: 



<» J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 55, 1917. 

 so J. B. Umploby. U. S. Qeol. Survey, Bull. 539, pp. 34-35, 1913. 



