CHAPTER II. 

 THE ORE DEPOSITS IN GENERAL. 



GENERAL FEATURES. 



Throughout the Boise Ridge and the Idaho Basin the primary gold 

 deposits present a certain similarity. They are all contained in gra- 

 nitic rocks or associated dikes. They are all either fissure veins or 

 impregnations connected with fissures. Nearly all of these fissures 

 have a direction ranging from east- west to northeast-southwest, the 

 chief exceptions to this rule being "found in the Black Hornet district. 

 The dip is ordinarily to the south at angles of from 45 to 89, but in 

 the Willow Creek and Rock Creek districts similar dips to the north 

 are found. The prevailing direction of the fissures is the same as 

 that of an often well-developed system of joint planes commonly seen 

 in the Boise Ridge. Finally, the ores are, on the whole, of a similar 

 character, consisting chiefly of auriferous pyrite, arsenopyrite, zinc- 

 blende, and galena in a gangue of quartz or, more rarely, calcite. 

 The fresh ores from deeper levels contain a variable percentage of free 

 gold. Rarely, however, is more than 60 per cent of the total value 

 caught on the amalgamating plates. Gold predominates largely in 

 the value of the ore, though seldom by weight, for in the ordinary 

 ores the weight of the silver considerably exceeds that of the gold. 

 The alteration of the country rock in the vicinity of the veins is 

 throughout of the same character. 



ALTERATION OF THE COUNTRY ROCK. 



A marked change appears in the rock in the vicinity of the veins. 

 The dark constituents, biotite and hornblende, are bleached or disap- 

 pear, and the feldspar is altered to a soft, white, opaque material, 

 only the quartz remaining unaltered. Besides abundant iron pyrite, 

 arsenopyrite also appears in small, scattered, perfect crystals. The 

 width of the altered zone may be from a foot up to 50 or 60 feet. 

 This alteration of the country rock has been noted by Messrs. George 

 H. Eldridge, 1 J. B. Hastings, 2 and F. D. Howe, but has ordinarily 

 been described as kaolinization. The soft, white substance, which 

 often has a greasy feel, is also referred to by the miners as "talc." 

 This change in appearance and composition is without the slightest 



1 Sixteenth Ann. Rep. U. S. Geol. Survey, Part II, 1895, pp. 225, 252. 



2 Idaho Mining News, Vol. L, No. 1, p. 15. 

 638 



