THE MINERALS OF IDAHO 51 



total production in Idaho from the earliest discoveries in 1861 to 

 the end of 1917 as having a mint value of $130,910,969, which amounts 

 to approximately half a million pounds of metallic gold. While 

 many lode mines have been developed in the area and have made 

 large production, the greatest bulk of the gold yield has come from 

 placer mines. 



Auriferous veins are widespread in occurrence. The most im- 

 portant mines, however, fall into several groups, the largest group 

 being that in the west-central part of the State mainly in Boise 

 County, but extending into Ada and Elmore counties and including, 

 among others, the Black Hornet, Mclntyre (Boise), Gambrinus, 

 Deadwood, Quartzburg-Grimes Pass, Summit Flat (Pioneerville), 

 Westview (Willow Creek or Pearl), Atlanta, Black Warrior, Neal, 

 Pine Grove, and Rcfcky Bar districts. The country rock in all of 

 these districts is the granite of the western part of the central Idaho 

 batholith and near the veins this granite is usually very greatly 

 altered by thermal solutions with abundant development of sericite. 

 The lodes occur along well-defined fissures and vary from clean cut 

 filled veins to mineralized shear zones containing seams and stringers 

 of auriferous material. The gangue is, for the most part quartz 

 which, where unaffected by oxidation, contains greater or less 

 quantities of sulphides, while in certain mines the sulphides occur 

 as solid masses practically without quartz. At and near the sur- 

 face the sulphides are in most cases thoroughly oxidised, the ore 

 then consisting of quartz containing much soft and limonitic material 

 which incloses the abundant spongy gold in free milling and easily 

 recoverable form. As greater depth is attained in mining, however, 

 the unaltered primary ore is encountered, in which the gold is as- 

 sociated with or contained in the prevalent sulphides, chief among 

 which are pyrite, arsenopyrite, sphalerite, and galena. Bismuth 

 sulphide or bismuth-lead sulphide (locally called antimonite) are 

 locally characteristic of the richest ores. The primary sulphide 

 ("base" or "refractory") ores are not amenable to the simpler and 

 less expensive methods of treatment and the encountering of such 

 unoxidized ore has led to the temporary or permanent shutdown 

 of a large number of mines in this area. 



The second major group of gold lodes is located in Idaho and 

 Clearwater Counties and includes the Elk City, Dewey, Dixie, 

 Florence, Newsome, Oro Grande, Buffalo Hump, Warren, and 

 Pierce districts. These districts are occupied either by the northern 

 continuation of the granitic batholith or by pre- Cambrian meta- 

 morphic rocks cut by granitic dikes. In most of the districts of 

 this group the lodes are clean-cut quartz veins without great quanti- 

 ties of sulphides and the gold, even in the unoxidized primary ore, 

 is for the most part free milling. 



