26 S. F. EMMONS THEORIES OF ORE DEPOSITION 



alluded to will be attempted. His discussion is practically confined to ore 

 bodies deposited from aqueous solutions, which, he considers, embrace the 

 larger proportion of workable deposits, and he holds that the waters from 

 which these deposits have been made are chiefly of meteoric origin. 

 Their circulation is in part descending, in part lateral moving, and in 

 part ascending, and during each of these movements they may take up 

 or deposit metallic minerals according as conditions favor either action. 

 This circulation takes place in openings in rocks, mostly produced by 

 fracture, and hence is confined to the outer portion of the crust, which 

 he has denned as a zone of fracture as distinguished from a deeper zone, 

 that of flowage, where, under accumulated pressure, deformation pro- 

 duces no macroscopic openings. Its general tendency is to concentrate 

 from the small openings into larger or trunk channels. The deposits 

 from these waters are distinguished as concentrations (1) from ascend- 

 ing waters alone, (2) from descending waters alone, and (3) first from 

 ascending and second from descending waters. In prevailing composit- 

 ion the first class are sulphides, tellurides, etcetera ; the second oxides 

 or oxide salts, while the third are chiefly the one or the other, according 

 as they were formed above or below the ground water level. 



Emmons and Weed, coming to the subject from a different but some- 

 what narrower standpoint — that of a practical field study, extending 

 over several years — explained the frequent occurrence of bonanzas, or 

 exceptionally rich portions of deposits just below the oxidized zone or 

 ground water level, as the result of leaching by surface waters of the 

 upper portions of these deposits and their redeposition as sulphides in 

 contact with preexisting metallic sulphides (especially pyrite) in the 

 zone below. Through similar processes of chemical reasoning and with 

 a similar disregard of Posepny's assumption that the ground water level 

 forms an effective barrier separating the action of the surface or vadose 

 waters from that of the deep circulation, all three arrived at the same 

 general conclusion with regard to the continuance of rich ore '.in depth 

 a question which has occupied the attention of geologists and miners 

 since the days of Werner. This conclusion was that in most ore deposits 

 a deeper region exists beyond the influence of surface waters in which 

 the ore is of comparatively low and uniform grade. Van Hise even 

 went so far as to say that in depth all deposits would become low grade 

 pyritic ores, and that all veins would eventually wedge out. 



De Launay, in his generalizations on Mexican deposits, had already 

 recognized three zones: (1) an upper oxidized zone, (2) a middle zone 

 of rich sulphides, and (3) a lower zone of low grade sulphides. He as- 

 sumed the enrichment of the middle zone had been by descending waters, 

 but placed it above the groundwater or hydrostatic level, which in many 

 veins had probably been displaced since their original formation. 



