644 IDAHO MINING DISTRICTS. 



SiO* 24.33 



A1 2 3 - -.. -- - 14.26 



Fe 2 3 ---- 67 



FeO .44 



K 2 4.19 



H 2 --- 2.16 



Sericite.. 46.03 



Hygroscopic water ... .36 



Total 100.07 



The sericite would have the following composition : 



SiO* - 52.82 



A1 2 3 30.96 



Fe 2 O 3 1.46 



FeO 96 



K 2 9.10 



H 3 O._ 4.70 



100. 00 



The altered wall rock from the Black Hornet mine (60, 63 Boise 

 sheet collection) is a grayish- white, very quartzose rock impregnated 

 with small grains and crystals of arsenopyrite. Though adjoining 

 ore contained from $10 to $20, it was found upon assay to contain 

 neither gold nor silver in appreciable quantities. Under the micro- 

 scope No. 60 appears as a crushed granite or coarse granite-porphyry. 

 The quartz grains are converted into coarse aggregates; the unstri- 

 ated feldspar is filled with sericite fibers and grains of arsenopyrite, 

 some of which have a little adhering chlorite. No. 63 contains unal- 

 tered idiomorphic andalusite, besides crushed large quartz grains and 

 sericitized microperthite, and also a little soda-lime-feldspar. Scat- 

 tered grains of pyrite are associated with foils of sericite. 



The ore of the Black Hornet consists of massive quartz with inclosed 

 sulphides. The preponderating quartz is granular and shows strong 

 evidence of pressure. The ores consist of small, perfect crystals of 

 arsenopyrite and anhedral grains of dark-brown, scarcely translucent 

 zinc blende; inclosed in the latter are many small, distinct foils of 

 sericite. Smaller shreds of this mineral are also scattered through 

 the quartz. 



The granite and porphyries adjoining the veins of the Idaho Basin 

 are altered in a similar manner. Sericitization always takes place, 

 and is often accompanied by the formation of carbonates. A specimen 

 from a seam in the hanging wall of the Boulder mine (19 Idaho Basin 

 sheet collection) consists of a quartz vein 2-g- inches wide, of which 1 

 inch is pure quartz and the rest quartz with much finely divided iron 

 pyrite. This vein is inclosed in a greenish- white, bleached granite, 

 impregnated with a little pyrite and arsenopyrite. 



An assay of the quartz vein gave 2.25 ounces of gold and 1 ounce 

 of silver to the ton, or a total value of $47.21. An assay of the 



