ORIOIh <>r ORE DBP081 47 



Weathering changes elTected in the upper part of lode 

 in part oxidation and in part reduction. The upper part com- 

 prises a zone of oxidi/rd minerals. Beneath this then- is a /one 

 of enriched ores, often the richest portion of the entire lode, l>e- 

 neat h which there is a zone of permanent value. 



In the oxidation of chalcopyrite, ferrous sulphate is formed, 

 which is readily oxidized to ferric sulphate. The ferric sulphate 

 reacts upon the chalcopyrite, reducing the mineral to chalcocite, 

 and by further influence of the ferric sulphate the chalcocite is 

 converted into the sulphate of copper, according to the equation : 

 Cu 2 S+5Fe 2 (SO4)3+4H 2 O=2CuSO4+10FeSO4+4H 2 SO4. 



The copper sulphate in solution is transferred downwards to be 

 reduced by the pyrite or other sulphides according to the equa- 

 tions 



7CuSO 4 +4FeS 2 +4H 2 O = 7CuS+4FeSO 4 +4H 2 SO4 

 1 1 CuSO 4 + 5CuFeS 2 + 8H 2 O = 8Cu 2 S + 5FeSO 4 + 8H 2 SO 4 . 



The copper sulphate in the presence of either pyrite or troilite 

 may be converted into chalcopyrite, as, for example, on Vancouver 

 Island. The carbonates of copper are less soluble than the sul- 

 phate. If calcium carbonate in solution is transported dowmva r< 1 

 in the copper lode, malachite or azurite and calcium sulphate 

 would be formed with the liberation of carbon dioxide. The 

 chalcanthite in solution may be reduced to tenorite, 2CuSO 4 + 

 2CaCO 3 = 2CuO+2CaSO 4 +2CO 2 , which in contact with ferrous 

 sulphate solutions would be reduced to cuprite, Cu 2 O, and ulti- 

 mately to native copper. 



Cerussite and anglesite, the carbonate and sulphate of lead, 

 are similarly formed direct from galenite, while native silver is 

 often produced by the reducing action of decomposing pyrite 

 upon the sulphate of silver. 



Detrital Deposits. Detrital deposits are those resulting from 

 the disintegration of rock masses through atmospheric agencies. 

 The more resistant the parent rock the shallower the deposits 

 become. If the gradient of the stream in the valley is small, the 

 deposit will concentrate near the source of the ore. The higher 

 the gradient and the greater the velocity of the stream, the fart her 

 down the valley the metalliferous minerals will be carried (see 

 Fig. 35). The composition of the detrital deposit will depend 

 upon the nature of the overlying country rock and the assorting 

 power of the associated waters, as shown in Fig. 36. 



