130 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



stals. 11 Pyrite is a persistent mineral in all the deposits in the north- 

 western part of the county. In the walls of the late Tertiary veins 

 abundant cubes of it accompany the metasomatic alteration. 110 In 

 a prospect at the head of Little Fall Creek near the head of Lost 

 River well-developed pyrite crystals are associated with galena. At 

 the Red Bird mine in the Bay Horse district on the No. 8 level a 

 mass of pyrite 17 meters (55 feet) wide has been opened. This pyrite 

 occurs as fine-grained masses and also as porous aggregates of plumose 

 groups of crystals which resemble marcasite but are of pyritohedral 

 form. 



ELMORE COUNTY 



Pyrite is common in all mines. Small pyritohedrons are dissemi- 

 nated in silicified and mineralized granite from the Boise-Rochester 

 mine, Atlanta district. 



IDAHO COUNTY 



Pyrite is a constituent of the gold ores of most Idaho County dis- 

 tricts and is usually auriferous. It occurs scattered through the 

 quartz of the ores in only moderate amount. Large imperfect crys- 

 tals, grains, and masses accompany minute amounts of galena and 

 chalcopyrite in ore from the Hercules and Blue Ribbon mines, Elk 

 Cit} r district. It is the principal sulphide in the Black Pine mine, 

 also in this district, where it is disseminated in quartz with galena, 

 tetrahedrite, sphalerite, and native gold. The ore of the Mineral 

 Zone mine consists of granular auriferous pyrite in rusty quartz. 



In the Buffalo Hump district sparsely distributed auriferous 

 pyrite in quartz forms the gold ore of the Concord (Mother Lode) 

 and other mines. 



In the Penman and other mines of the Oro Grande district pyrite, 

 which is practically the only sulphide disseminated in quartz, is again 

 the ore of gold. 



In the Thunder Mountain district in the Dewey mine pyrite- 

 nodules occurring in volcanic tuff had coarse leaf gold at their center. 12 



LEMHI COUNTY 



In Lemhi County pyrite appears in all the deposits, and in the gold 

 veins it is auriferous. In the Mackinaw district it is the most 

 abundant mineral, occurring as coarse cubes included in the quartz, 

 and makes up about one-third of the bulk of the gold ore. A typical 

 specimen of ore from the A. D. & M. mine, Gibbonsville, containing 

 $150 a ton in gold, shows pyrite as the only sulphide as small brilliant 

 cubes in a gangue of quartz and calcite. 



» J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 54, 1917. 



»• J. B. Umpleby. U. S. Geol. Survey, Bull. 539, p. 53, 1913. 



1= J. B. Umpleby and D. C. Livingston. Idaho State Bur. Mines and Geol., Bull. 3, p. 6, 1920. 



