SUPERFICIAL ALTERATION OF ORE DEPOSITS 183 



Emmons has ascribed the origin of the great bodies of rich copper 

 sulphides at Butte, Montana, to secondary deposition and transporta- 

 tion, and gives a very clear outline of the possible changes. For copper 

 ores, however, there seems to have been for many years a general belief 

 in secondary deposition based upon the generally accepted explanation 

 of the origin of the black copper (sulphide) ores of Ducktown, Tennes- 

 see, as derived from the material leached out of the part of the vein now 

 forming the gossan. 



Kemp also says that the upper 400 feet of the Butte veins is leached 

 of copper, while below are found the bornite and chalcocite bodies of the 

 zone of enrichment. 



Upon reviewing the literature of ore deposits the writer has been im- 

 pressed with the almost total lack of evidence to show that secondary 

 minerals might be formed by such leaching or decomposition of vein 

 matter. In many cases bodies of rich ore are mentioned and their 

 structural features and occurrence described, but no evidence of second- 

 ary derivation is given. 



Chemical Changes 



leaching of gossan zone 



The superficial alteration of ore deposits, by which the upper part of 

 a mineral vein is altered and decomposed, has been discussed by many 

 writers and has recently been fully treated by Penrose.* It is, however, 

 necessary to consider briefly the reactions involved, since the origin of 

 the secondary sulphides lying between the gossan and the unaltered, 

 original vein matter is usually due to descending — that is, surface — 

 waters. In general, it may be stated that in many instances the sec- 

 ondary ores are deposited above the level of deep seated water, the so- 

 called " permanent water level," but in others the surface waters descend 

 by water-courses and channels below the general level of the uprising 

 deep waters, and all such waters eventually mingle with them. 



Surface waters descending through the relatively porous and open tex- 

 tured gossan of a vein are normally oxidizing, and on passing downward 

 usually attack the unaltered sulphides and deepen and extend the zone 

 of the gossan. As the waters descend they are robbed of their ox}^gen 

 by the sulphides which they decompose, and percolating further down- 

 ward the waters which at first were strongly oxidizing in character are 

 now charged with various salts and frequently with free sulphuric acid.f 



* Jour, of Geology, vol. for April-May, 1894, p. 288. See also Emmens in Engineering and Min- 

 ing Journal, vol. 54, December 17, 1892, p. 582. 



f As, for instance, mine waters run through precipitating tanks at Butte, the zinc-bearing waters 

 of Missouri, etcetera. Am. Jour. Sci., vol. xliii, May, 1892, p. 418. 



