1914] 



Lawson: Ore Deposition by Meteoric Waters 



233 



The batholiths, on the other hand, are probahly developed at such 

 great depths as to be always below the sedimentary zone, although 

 with their growth they may rise through it. With the upward advance 

 of a growing batholith the water contained in the overlying sediments 

 would gradually in part be driven off and in part be incorporated in 

 the crystals (mica, etc..) formed by the thermal metamorphism, but 

 it would never have an opportunity of leaching the granite or its deep 

 differentiate of magmatic sulfids. On the hypothesis of meteoric 

 waters as the agency concerned in the deposition of contact ores we 

 have, therefore, a consistent and satisfactory explanation of the 

 absence of such ores on the margins of the great batholiths and their 

 presence in and about the periphery of the minor laccolithic bodies. 



SYNCLINAL STRUCTURE AND ORE DEPOSITION 

 Butte. — But such contact ores are not characteristic of all lac- 

 colithic intrusions. Certain structural conditions appear to obtain 

 in several of the more important instances of copper deposits which 

 have been referred to magmatic waters. I have elsewhere 20 called 

 attention to the synclinal structure of the region in which the so-called 

 •Boulder "batholith" lies and pointed out that it is probably a laccolith 

 resting upon a concave floor of sedimentary rocks. Knopf in his 

 recent review gives partial support to this view. He says : 30 



"To the northwest, however, the prolongation of the intrusive coincides with 

 a well-developed synclinal structure in the sedimentary rocks. The contact 

 here, as elsewhere, is of a transgressive nature, but it is not impossible that at 

 greater depths the sedimentary rocks pass beneath the granite and constitute a 

 synclinal basement upon which it rests. ... It may be conceded nevertheless 

 that as a working hypothesis, the conception advanced by Professor Lawson 

 is entitled to much weight, perhaps to as much weight as the batholithic 

 hypothesis. ' ' 



The recognition of the probable synclinal encasement of the Boulder 

 intrusive led me to review the structural conditions at other well- 

 known localities where copper ores have been ascribed to magmatic 

 waters. I have found that at Bingham, Ely, Bisbee, Clifton-Morenci, 

 and probably at Cananea, the intrusive with which the ores are 

 genetically associated in every case invades a syncline. 



Bingham. — Keith and Boutwell have given us an excellent account 31 

 of the general relations of the ore deposits of the Bingham Alining 



20 Univ. Calif. Publ. Bull. Dept. Geo!., vol. 8, no. 1, 1914. 

 so Op. ext., p. 397. 



31 Economic Geology of the Bingham Mining District, Utah, U. S. G. S. Pro- 

 fessional Paper no. 38, 1905. 



