spencer: chalcocite deposition 71 



showed that SO2 is actually one of the products when pyrite 

 undergoes oxidation. In a recent discussion of this subject Tol- 

 man/ who was associated with H. V. Winchell, has used the results 

 of these two investigations as the basis for an understanding of 

 the chemistry of secondary chalcocite. Previously, however, it 

 had been argued by Lindgren,^ that the scheme suggested by 

 Winchell is open to criticism because no indication is given 

 as to how SO2 generated in the upper oxidizing zone could reach 

 the situs of chalcocite deposition, since it would be destroyed in 

 transit by contact with products of pyrite decomposition. 



On its face this reasoning against the Winchell hypothesis 

 appears to be adequate, but the fuller analysis here presented 

 tends to show that SO2 may still be an actual factor in the reac- 

 tion. Since the writer does not have that unwavering faith 

 which would find in every balanced chemical equation a truth 

 of fundamental value, it is recognized that the equations of the 

 series here given afford no demonstration. They do however, 

 furnish a convenient shorthand to assist in analytical discussion, 

 and some of them are allowable expressions of known actions. 



Chalcocite, deposited in molecular replacement of primary sul- 

 fides, is a sid^ product of the culminating reaction in a series of 

 oxidizing effects which occur in situations where water and air 

 penetrate from the surface to cupriferous sulfide-bearing rock or 

 vein stuff. The full series of reactions may be divided conven- 

 iently and logically into three groups, assignable to a higher, an 

 intermediate, and a lower position in the body of sulfide-contain- 

 ing material. If considered with respect to the minerals which 

 suffer decomposition these reactions present a succession of oxi- 

 dations, while with respect to the active solution the changes are 

 of course as consistently in the direction of reduction. Group 

 I comprises the reactions of complete oxidation on the upper 

 edge of an ore body where free oxygen is present. Group II 

 comprises reactions ensuing at slightly greater depth and involv- 

 ing oxidation of sulfides (both primary and secondary) by con- 

 tact with ferric sulfate. Group III, occurring at still greater 



* Min. Sci. Press 106: 42. 1913. 



* Copper deposits of Clifton-Morenci. Prof. Paper U. S. Geol. Survey No. 43, 

 p. 184. 1905. 



