COPPER. 137 



acidic, light-colored granite, which consists of quartz and ortho- 

 clase feldspar with a very little biotite. Dikes of quartz porphyry 

 penetrate the basic granite, and dikes of rhyolite are also found 

 associated with the ore bodies. Tongues of rhyolite are met, ap- 

 parently offshoots of the butte. 



2.04.06. East of the butte and in the coarse granite are found 

 the older mines along two strongly contrasted east and west zones. 

 A second, later-developed, group is west of the butte in the acidic 

 granite. Of the former, the southern affords argentiferous copper 

 ores, consisting of chalcocite, bornite, chalcopyrite, enargite, and 

 pyrite in a siliceous gangue. The northern zone contains silver 

 ores, chiefly sulphides of silver, lead, zinc, and iron, in a siliceous 

 gangue with much rhodonite. Strangely enough, hardly any cop- 

 per occurs in the silver zone, and no manganese is met in the copper 

 zone, except in the Gagnon vein, which also contains zinc. The 

 silver zone is mentioned again under " Silver." The zone west of 

 the butte is silver-bearing with manganese. The occurrence of 

 these two parallel systems of fissures in the same country rock and 

 not far from each other, yet filled with such contrasted ores, is a 

 very remarkable phenomenon and points to different and at least 

 for one of them deep-seated sources of the ores. 



2.04.0V. The ore bodies do not, in general, present very sharply 

 defined walls, but the ores fade into the wall rock. From this 

 S. F. Emmons has suggested that they have formed along a series 

 of small fissures marking some line of disturbance, and not from 

 a general faulting, and have enlarged the original channels by re- 

 placement of the walls. It would seem probable that the frequent 

 dikes are connected with the butte, or with the same parent body 

 that sent it off. The same eruptive activity probably shattered the 

 rock along the zones. In this event it must have been from an 

 easterly offshoot of the western rhyolite area, and have operated 

 with the same fissuring action which attends earthquakes from the 

 intrusion of subterranean dikes. The Butte district alone rivals 

 the Lake Superior copper mines in output, and has in recent years 

 surpassed them. Reference will again \Q made to this region 

 under " Silver." l 



1 " Butte Copper Mines," Engineering and Mining Journal, June 19, 

 1886, p. 445; also April 24, 1886, p. 299. S. F. Emmons, "Notes on the 

 Geology of Butte, Mont.," M. E., XVI. 49. Rec. Richard Pearce, "The 

 Association of Minerals in the Gagnon Vein, Butte City, Mont.," M. E.> 



