THE MINERALS OF IDAHO 373 



Compared with published analyses, the above, like most analyses 

 of sericite and related secondary muscovites, is low in potash and cor- 

 respondingly high in water. The molecular ratio of water to total 

 alkalies is 3^ to 1 instead of 2 to 1, as in the formula given above 

 for muscovite. The very low content of low-temperature water is 

 noteworthy. 



Sericite, as indicated above is abundant in connection with practi- 

 cally all of the veins of Boise County. A number of specimens have 

 been seen, among which the following may be mentioned: 



Specimens of ore from the Enterprise vein, Blackbird tunnel, 

 Diana Mines Co., Pioneerville district, consist of cream-white silky 

 sericite which is the gangue of pyrite and sphalerite, and a specimen 

 from the stope below the tunnel level of the Golden Age mine, in the 

 same district shows much sericite replacing the granite adjacent to 

 quartz containing sulphides. 



Typical silky sericite occurs in small amount coating one side of a 

 specimen of heavy sulphide ore from altered granite porphyry from 

 the Coon Dog No. 4 mine. Silky brownish iron-stained sericite 

 coats fractures in quartz of high grade ore from the Lucky Boy mine, 

 Idaho City district. In a specimen from the Lincoln mine, Pearl 

 district, silky sericite occupies cavities in masses of dark brown 

 sphalerite. 



CUSTER COUNTY 



In Custer County sericite is meagerly developed in the Alder 

 Creek district. It has been detected in specimens of the granite- 

 porphyry only from the vicinity of the Iron Tunnel. In trachyte- 

 porphyry dikes it is abundantly developed after orthoclase. 80 



LEMHI COUNTY 



In Lemhi County, although sericite is probably widespread in 

 occurrence, it has been noted only in specimens from the Indian 

 Creek, Mineral Hill, Mackinaw, and Parker Mountain districts. In 

 the latter district it occurs both in the quartz oi the veins and in the 

 wall rock adjacent thereto. In the quartz it is probably, in part at 

 least, derived from adularia. It invariably occurs as foils and shreds 

 of microscopic size. 81 



OWYHEE COUNTY 



Sericitization of the walls of veins and adjacent rocks is a pro- 

 nounced feature in the Silver City and other silver and gold mining 

 districts of Owyhee County as pointed out by Lindgren, 82 Lindgren 

 found that the abundant white clay which accompanies the ores of 

 the Silver City and other districts varied in composition, in some 



so J. B. Umpleby. U. S. Oeol. Survey, Prof. Paper 97, p. 54, 1917. 



»i J. B. Umpleby. U. S. Geol. Survey, Bull. 528, p. 79, 1913. 



« Waldernar Lindgren. U. S. Qeol. Survey 20th, Ann. Rept., pt. 3, p. 610, 1900. 



