138 



KEMPS ORE DEPOSITS. 



2.04.08. Example l7a. Gilpin County, Colorado. Veins 

 of pyrite and chalcopyrite, replacing gneiss (the rock may be 

 granite), and dikes of quartz-porphyry, and felsite along the planes 

 of joints, which cross the gneiss (or granite) perpendicularly to 

 the laminations. The veins are highly auriferous and are worked 

 primarily for gold, the copper being produced as a by-product. 

 The concentrates from the stamps are afterward treated for cop- 

 per. The veins occupy an area of only about a mile and a half 

 in diameter, centering about Central City. They show little indica- 

 tion of filling a fissure, as usually understood, but follow the cleav- 



PIG. 30. Cross section of the Bob-tail mine, Central City, Colo. After 

 F. M. Endlich, Hayderis Survey, 1873, p. 286. 



age joints of the gneiss and replace the country rock on each side 

 of them. The joints also cross the porphyry dikes, and the veins 

 are often in the latter rock. They are closely related in structure 

 and origin to the galena veins of the neighboring Clear Creek 

 County, which are referred to under "Silver," but the contrast 

 in mineral contents between the two is very marked. They were 

 the basis of the first extensive deep mining in Colorado, and were 

 located through the placer deposits in the neighboring gulches. 1 



XVI. 62. "On the Occurrence of Goslarite in the Gagnon Mine, Butte 

 City,"Proc. Colo. Sci., Vol. II., Part I., p. 12. E. D. Peters, Mineral Re- 

 sources of the U. S., 1883-84, p. 374. A. Williams and E. D. Peters, "On 

 Butte, Mont.," Engineering and Mining Journal, March 23, 1885, p. 208. 



1 S F. Emmons, Tenth Census, Vol. XIII., p. 68. The veins are de- 

 scribed as cited above. J. D. Hague, Fortieth Parallel Survey, III., p. 493. 

 The veins are called fissure veins by Mr. Hague. A. Lakes, Ann. Rep. 



