ransome.] ORIGIN OF THE LODE AND STOCK ORES. 139 
Chalcocite may be produced from pyrite, chalcopyrite, or bornite, 
as 
follows 
Cupr.ms sulphate + pyrite + oxygen = chalcocite + cuprous sulphate + sulphurous anhydride. 
Cu 2 S0 4 + FeS,+ O, = Cu 2 S -h FeS0 4 + S0 2 
Cupric sulphate+chalcopyrite + oxygen=chalcocite+cuprous sulphate 4-sulphurous anhydride. 
CuSo 4 +CuFeS 2 + 2 = Cu 2 S -f FeS0 4 -;- SO, 
Cupric sulphate + bornite +oxygen = chalcocite + ferrous sulphate +sulphurous anhydride. 
CuS0 4 +Cu 3 FeS 3 -f 0, = 2Cu 2 S + FeS0 4 + S0 2 
Of course it is understood that these sulphides overlap one another. Before the 
iron sulphide has wholly been replaced by chalcopyrite bornite may appear. At 
the place where bornite has become reasonably abundant chalcocite may be found. 
However, certain general statements may be made. If the dominating material is 
iron sulphide, the copper mineral which is present is likely to be chalcopyrite 
rather than the richer sulphurets. Chalcopyrite, on the one hand, is likely to 
be associated with the pyrites, and on the other hand with bornite, or even chal- 
cocite. Bornite and chalcocite are likely to be associated with each other and with 
chalcopyrite, but with the first two compounds iron sulphide is likely to be sub- 
ordinate or absent. 1 
When the sequence of formation, deduced largely on theoretical 
grounds by Van Hise, is compared with the facts of occurrence in the 
Yankee Girl and Guston mines, the close correspondence affords 
striking confirmation, no1 only of the probable truth of the theory of 
secondary sulphide enrichment, but also of its applicability to these 
deposits. As far as the cupriferous sulphides and pyrite are con- 
cerned, the relations are exactly those which might be expected as a 
result of the downward percolation of solutions of copper sulphates 
through a body of low-grade pyritic ores. 
The iron sulphates, of course, can not act directly, as do the copper 
sulphates, in enriching I lie ore. Their role is limited to an attack upon 
the previously formed sulphides, which converts them into sulphates 
and redeposits them in part as sulphides below the zone of oxidation. 2 
They thus aid, although in a different manner, the copper sulphates in 
effecting a secondary concentration of the ores. It is probable that, 
owing to the slight solubility of its sulphate, the lead is deposited as 
sulphide at an early stage in the downward movement of the solu- 
tions, and that, through a constant and prolonged continuance of this 
process, as the outcrop of the ore body is lowered by erosion, there 
have resulted the masses of nearly pure galena ore which occurred 
above the chalcocite or stromeyerite. 
The application of the theory of secondary sulphide enrichment to 
any ore body naturally presupposes the existence of small amounts 
of the valuable metals in the low-grade ore of primary deposition. 
But this fact is not always easily determined, as mining operations are 
not often pushed very far beyond the depth at which the ore ceases to 
1 Van Hise, op. cit., p. 112. 
2 The secondary enrichment of ore deposits, by S. F. Emmons: Trans. Am. Inst. Min. Eng. 
Vol. XXX, 1900, p. 210. 
