SUPERFICIAL ALTERATION OF ORE DEPOSITS 185 



In very many cases that have come under the writer's notice chalco- 

 pyrite and pyrite carrying copper are seen altering to a soft, black, sooty 

 substance which soils the fingers and is pulverulent when dry. This 

 proves to be cupric sulphide when tested in the laboratory, but it is not 

 crystalline. Its formation is readily understood if we accept the expla- 

 nation just given of the alteration of chalcopyrite. Chalcopyrite being 

 Cu 2 S, Fe 2 S 3 , the iron sulphide molecule yields more readily to attacking 

 solutions of ferric sulphate (which by hydrolysis are acid) than the cupric 

 sulphide, and the iron is removed and Cu 2 S left as the amorphous powder 

 seen. This is said to occur because copper has a " very much greater 

 affinity for sulphur than iron.' 1 * After the greater part of the iron is 

 gone, or rather converted to sulphate, the ferric sulphate solution attacks 

 the Cu 2 S, forming the amorphous powder seen, and removes it in turn, 

 and it is carried a greater or less distance before it is redeposited. In 

 the Gold Hill (Rowan county, North Carolina) specimens the copper 

 sulphide has not been carried far, and in a large measure has by the 

 gradual lowering of the limit of oxidation been converted to oxide and 

 carbonate and native copper. Hand specimens collected by me show 

 all the stages of this alteration. 



As the sulphate of lead is virtually insoluble in water, it can only be 

 carried downward to form transposed, redeposited sulphides by an in- 

 termediate change to the carbonate through carbonate of lime, namely, 

 PbS0 4 -f CaC0 3 = PbC0 3 -f- CaS0 4 . This carbonate is soluble in waters 

 charged with carbon dioxide, but is precipitated by carbonate of lime 

 as lead carbonate. Galena is also dissolved by copper sulphate, as 

 shown by experiments in which, after eleven months' immersion, the 

 galena was etched, and lead sulphate and a subsulphate of copper was 

 formed.f If the vein contains much pyrite, supplying an abundance of 

 iron sulphate, the upper part of the vein will be leached of all its valu- 

 able metals, including gold and silver, as it is well known that ferric 

 sulphate dissolves with great ease, not only the copper sulphides, but 

 also the small content of precious metals of the ore, the reactions being 

 similar to those just given. { If the amount of ferric sulphate present 

 is relatively small, copper oxide and carbonates will be formed in the 

 lower part of the gossan and in cracks and fissures in the underlying 

 sulphide ores, be they original or secondary. 



That the metals are leached out of the gossan and go into solution is 

 well known at almost all copper mines. For other metals the evidence 



*The series is Hg Ag Cu Sb Sn Pb Zn Ni Co Fe As Mn (E. & M. Jour., Oct. 25, 1890, p. 484). 

 See F. Sandburger : Untersuchungen fiber Erzgange, 1882, and Vogt : Zeitschrift fi'ir Praktisehe 

 Geologie, vol. i, 1893, p. 262. 



fCompte Rendu, vol. xx, 1845, pp. 1509-1530. 



X Vogt : Zeitschrift fi'ir Praktisehe Geologie, July, 1899. 



