364 abstracts: geology 



b}'^ well-marked mineral association, but other ores occur which are not 

 typical but are a mixture of both kinds. 



The wall rocks of the veins contain copper. Chemical analyses of 

 fresh unaltered material show that the Butte quartz monzonite holds 

 copper, though in less amount than the altered rock. 



The question as to the source of the copper in the lodes is complex, 

 as the amount of copper present in the altered rocks is in excess of that 

 in the fresh rock and there is no body of leached rock from which this 

 excess of copper could have come. The evidence seems to indicate that 

 the alteration of the rock and its impregnation with copper are due to 

 a common cause, and that copper has been added to the rock, together 

 with silica and iron, the two other constituents which are so abundant 

 in the altered granite. 



Tho the evidence is not conclusive, it is believed to indicate that 

 the copper and other metals of the quartz-pyrite veins are derived 

 from magmatic emanations coming from a deep-seated mass of igneous 

 magma. The rhyolite porphyry intrusion appears the most probable. 

 A second and later mineralization occurred at the time of the rhyolitic 

 eruption, forming the silver veins and the ores of the Blue vein and 

 other fault vein series. 



The sequence of the vein systems is considered to be as follows: (1) 

 East-west or Anaconda system (oldest); (2) Silverbow system; (3) Blue 

 Vein system; (4) silver veins; (5) Steward system; (6) Mountain View 

 fractures; (7) Rarus fault zone. The enargite veins are of different ages, 

 but are all younger than the Anaconda system. 



The veins of the Anaconda and Silverbow systems are quartz-pyrite 

 veins; those of the Blue and Steward systems are mineralized fault veins. 

 Many of the most productive lodes of the district are compound or 

 composite veins, due to a combination or superposition of the two types 

 in one vein — that is, a reopening of an old vein with deposition of new 

 material. As many of the older veins have been reopened in this way 

 they have the characters of both the quartz-pyrite and fault veins. The 

 four systems embrace practically all the productive veins of the dis- 

 trict. It is not certain, however, that all the veins having a similar 

 direction are of the same age. It is known that the district has been 

 an area of movement and fracture ever since the time of the granite 

 intrusion, and has continued so to the present day. The early fractures, 

 tho cemented by mineral matter, have been lines of weakness and 

 have been reopened either by direct fracture or during the straining of 

 the rock mass by the faulting of blocks by other crossing fractures. 



W. H. W. 



