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University of California Publications in Geology [Vol. 8 



At Butte the quartz-monzonite had solidified and it was traversed 

 by two distinct intrusives, the first aplite and the second quartz- 

 porphyry, before the veins were formed: 



"The vein and fault-fracture systems intersect the granite, aplite, and 

 quartz-porphyry dykes alike, and all these are similarly affected by the general 

 alteration processes accompanying later vein formation, "is 



' ' In the Wasatch the ore bodies found at the points of actual contact of 

 country rock with eruptive masses have not proved to be so important econom- 

 ically as those which lie at some distance from the contact zones and have no 

 evident and direct connection with them. "i<> 



At Bingham the descriptions which Keith and Boutwell 20 have 

 given us afford no warrant for the statement that the occurrence of 

 ore at points of limestone projecting into the intrusive mass is a 

 general phenomenon. Neither do I find such occurrences described in 

 the most valuable memoir on the Clifton-Morenci district 21 ; but I do 

 find that important ore bodies occur in fissures in the porphyry, and 

 the evidence that these are of later age than the ore in the metamorphic 

 contact zone is not conclusive. 



' ' Fissures and extensive shattering developed both in porphyry and altered 

 sediments after the congealing of the magma and these fissures and seams were 

 cemented by quartz, pyrite, chalcopyrite, and zinc blende, forming normal veins 

 largely of the type of replacement veins. . . .As far as the metallic minerals 

 are concerned, there is a striking similarity between the veins connected with 

 the porphyry and the contact metamorphic deposits. ... A relationship is also 

 clearly seen in the remarkable action of the vein solutions on the adjoining 

 wall rock wherever this is limestone, tremolite and diopside being formed by 

 metasomatic action. On the whole, iron and silica are the main substances 

 added during contact metamorphism as well as during vein formation. "22 



If, then, the waters that formed the fissure veins at a period 

 subsequent to the consolidation of the porphyry laccolith replaced the 

 limestone walls by tremolite and diopside, why may not the contact 

 deposits have been also formed by similarly late waters? And what 

 ground is there for rejecting so absolutely the possibility that these 

 were meteoric waters heated by the cooling intrusion? 



It would seem, therefore, that the assertion regarding the gener- 

 ality of the formation of ore in included slabs and projecting points 

 of limestone requires some elaboration and specification before it can 



is Sales, Ore deposits at Butte, Mont., Bull. A. I. M. E., no. 80, August, 1913. 

 is S. F. Emmons, IT. S. G. S. Professional Paper no. 38, p. 25. 



20 IT. S. G. S. Professional Paper no. 38. 



21 Lindgren, U. S. G. S. Professional Paper no. 43. 



22 Op. cit., pp. 219-220. 



