1914] 



Lawson: Ore Deposition by Meteoric Waters 



235 



1. Disseminated pyrite and chalcopyrite in the monzonite. 



2. Replacements of limestone by pyrite and chalcopyrite roughly 

 parallel to the bedding at various horizons of the limestone formations. 



3. Argentiferous galena in fissures traversing all the rocks of the 

 district, both sedimentary and intrusive. 



It is obvious that these deposits are genetically related to the 

 injection of the laccolith. Boutwell thinks all three classes of ores 

 were deposited by waters escaping from the magma. In regard to the 

 first class, however, Keith is of the opinion that the disseminated 

 sulfids are of pyrogenic origin ; and he is not clear as to the source of 

 sulfids in the two other classes. He says : 



"It is not clear whether the sulfid material in the mineralized bodies was 

 derived from the monzonite or came through it in solution from some more 

 remote source. If the solutions merely pass through, however, a general deposi- 

 tion throughout the mass of the monzonite would be less likely than local concen- 

 trations such as appear in the limestone and quartzite. Far the greater part 

 of the sulfid material is disseminated through the monzonite in grains, and was 

 crystallized directly from the magma, like the other minerals. The amount of 

 the sulfids in the monzonite as a whole is vastly greater than that in the sedi- 

 ments, and the monzonite is on the whole a source of the sulfids. ... It seems, 

 therefore, most likely that the sulfid minerals in the limestones and quartzites 

 were derived from the larger quantity of their kind which formed part of the 

 intrusive mass and that they were deposited by solutions that passed from the 

 monzonite to the enclosing strata. ' ' 



Since he thus regards the sulfids of the monzonite as pyrogenic and 

 the source of the other ores, the latter must have been derived by a 

 process of leaching. Although I agree with Keith as to the probable 

 truth of the view above expressed, it is not so much my purpose to 

 pit him against Boutwell in order to minimize the force of the latter 's 

 conclusions as to the agency of magmatic waters as it is to point out 

 that the structural conditions support Keith's general interpretation. 

 The Bingham laccolith doubtless rests upon a floor of sedimentary 

 rocks and these clip down beneath it, so that both the water contained 

 in the underlying sediments and that which would flow from the 

 limbs of the syncline to make good any withdrawal, would tend to rise 

 through the mass of the hot laccolith when the latter after consolida- 

 tion became shattered and faulted by shrinkage ; and these waters, as 

 Keith holds, would find an abundant supply of material for peripheral 

 deposits in the pyrogenic sulfids. 



Ely. — The copper deposits of Ely are chiefly in the form of sec- 

 ondary chalcocite disseminated through a decomposed and sheared 



