PRECIOUS METAL* 77 



113 miles. Tin- mines are situated in Amador, Calaveras, El 

 Dorado, Mariposa and Tuolumne Counties. These counties 

 furnish three-fourths of the milling ores of the State. The aver- 

 age recovery per ton is much less than in other counties where the 

 veins are smaller and richer. The average recovery from all the 

 counties in the Mother Lode district is less than $4 per ton while 

 in Nevada County the amount exceeds $10 in gold and silver per 

 ton of ore mined. 



One characteristic of the Mother Lode is the permanancy of 

 the ore with increasing depth. In Amador County the mines 

 are now 3500 ft. deep and the ore is as good as that found at the 

 surface. The ores occur in fissure veins in steeply dipping slates 

 and altered volcanics of Carboniferous and Jurassic age. The 

 ores are found at so great a distance from the granitic rocks of the 

 Sierra Nevadas that they are supposed to bear no genetic rela- 

 tion to them. The veins occur both in the slates and at their 

 contact with diabase dikes. The veins show a remarkable ex- 

 tent and uniformity. In the tilted layers of the slate there lay 

 planes of weakness which the mineral-bearing solutions followed. 

 The chief gangue mineral is quartz, and the ore is native gold and 

 auriferous pyrite. 



Nevada County: The Grass Valley district of Nevada County 

 still continues to be the leading quartz-mining section of the State. 

 None of the other counties, even those of the famous Mother 

 Lode, approach it in its production of gold. The deep mines of 

 the county are yielding per annum about 2,000,000 tons of free 

 milling ores and 500,000 tons of auriferous copper ores that are 

 treated at the smelters. The veins are quartz and occur along 

 the contact between a grano-diorite and diabase prophyry. They 

 also cut the igneous rocks. Two systems of fissuring are known. 

 The gold is either native or associated with metallic sulphides. 

 The width of the vein seldom exceeds 3 ft. The lode ore oc- 

 curs in well-defined bodies or pay shoots. W. Lindgren believes 

 that the ores were leached out of the rock at a considerable depth 

 and deposited by hot solutions while the wall rocks contained 

 the rare metals in a disseminated condition. 



The Alaska Field. The region may be divided into: (1) The 

 Sitka district; (2) the Juneau and Douglas Island district North- 

 east of Sitka; (3) the Fairbanks district in the central part of 

 Alaska and; (4) the Seward Peninsula in the western part of 

 Alaska, as shown in Fig. 57. 



