THE MINERALS OF IDAHO 53 



veins of appreciable gold content yet too low grade to have been 

 worked, and also of inconsequential lenses and stringers of auriferous 

 material which may have added greatly to the total but would not 

 have been of any commercial importance if discovered singly. While 

 the lighter constituents of the rocks have been gradually disintegrated 

 and reduced to fine particles and carried to great distances, the gold 

 and other heavy materials have remained behind. The granite, 

 especially where softened or sericitized by hydrothermal alteration, 

 is prone to be more rapidly removed than the vein quartz which makes 

 up a large proportion of the pebbles in the placer gravels. The 

 more productive placers are, for the most part, along the beds of the 

 present streams, although profitable mines have been worked in 

 high terrace gravels and in fossil stream channels which date from 

 some previous period in the drainage history of the region. The 

 placer gold is concentrated, usually, within a few centimeters or at 

 most a meter of bedrock and is associated with a variety of other 

 heavy minerals. The character of these heavy minerals varies from 

 place to place. 9 Magnetite, ilmenite, and garnet are widespread, 

 while monazite occurs in most of the placer regions in the Central 

 Idaho granitic area, especially in the Boise Basin. Colorless zircon 

 is also abundant, particularly in the Florence and Warren districts. 

 Corundum occurs in considerable quantity near Pierce and Resort 

 in Clearwater County with tourmaline, rutile, and ilmenite, and also 

 at the Rock Flat placer near Meadows in Washington County. The 

 latter locality is that mentioned above as yielding diamonds. Rare 

 earth minerals are known from many places. In addition to the 

 monazite just noted above, brannerite occurs in Stanley Basin, 

 Custer County, and poly erase, samarskite, columbite, etc., are 

 known from several placers in Boise County. Native amalgam 

 occurs in Stanley Basin, while cassiterite and cinnabar have been 

 found here and in the Pierce district. A heavy bismuth carbonate 

 occurs with gold in the Poncia placer, Centerville district, recalling 

 the occurrence of bismuth-bearing sulphides in the Gold Hill, Bel- 

 zazzar, and other mines. 



The richer placers along streams were naturally first discovered 

 and were worked by the simpler hand methods — by panning or with 

 arrastre, rocker, and sluice, and many of them yielded fabulous 

 returns by such methods. The high grade ground suitable for such 

 exploitation was limited, however, and it was necessary to construct 

 larger projects requiring miles of ditches and extensive pipelines 

 to work the higher gravels. Following the bonanza period many of 

 the placer camps were practically abandoned by white men in their 



» Earl V. Shannon. Mineralogy of some black sands from Idaho. Proc. U. S. National Museum, vol. 

 60, art. 3, 1921. 



