184 W. II. WEED — MINERAL VEINS ENRICHED BY SULPHIDES 



They are still descending waters of surface origin, but have lost all the 

 characters commonly ascribed to surface waters. This change is, of 

 course, due to the reactions involved in the changing of the vein min- 

 erals to gossan. The common metallic sulphides of veins are pyrite, 

 pyrrhotite,chalcopyrite,tetrahedrite,enargite,bornite,galena(with which 

 quartz is most commonly associated as a gangue mineral) and a little 

 less commonly calcite and other carbonates, and barite, etcetera. The 

 changes by which this mixture is converted into a mass of porous, more 

 or less pure limonite are briefly as follows: The pyrite alters to a mix- 

 ture of iron sulphates and sulphuric acid, which, reacting on more 

 pyrite, eventually forms a mixture of hydrated oxides (limonite ores). 

 The reactions commonly assumed are as follows : 



1. FeS 2 + 3 -f H 2 == FeS + H,SO. t . 



2. FeS + H 2 S0 4 = FeS0 4 + H 2 S. 



3. FeS 2 + O a + 2H 2 S = FeS + 2H 2 4- 3S. 



4. S 4- 8 + H 2 = H 2 80 4 . 



5. 2Fe 2 S0 4 4- + H 2 S0 4 = Fe 2 (S0 4 ) 3 4- H 2 0. 



6. FeS 4- Fe 2 (S0 4 ) 3 = 3FeS0 4 4" S. 



Pyrrhotite, if present, is attacked as in the second equation given. 

 Chalcop3 r rite and bornite are commonly assumed to consist of Cu 2 Sand 

 Fe 2 S a . The iron sulphide molecule is attacked and dissolved by the 

 ferric sulphate present in the water, leaving Cu 2 S as an amorphous, sooty 

 material. It is well known that ferric sulphate will attack and decom- 

 pose metallic sulphides. It has been shown* that the order of attack 

 will be first chalcocite, then galena, then blende, the reactions being of 

 the same character as those by which the iron sulphide is attacked. The 

 Cu.,S left by the alteration of the chalcopyrite is in turn attacked by 

 the ferric sulphate, forming cupric sulphate, ferrous sulphate, and sul- 

 phur. Lead, zinc, and other metallic sulphides are in their turn attacked 

 by ferric sulphate, forming a sulphate of the metal, together with ferrous 

 sulphate and sulphur, the latter, of course, oxidizing at once to S0 2 and 

 sulphuric acid. 



" It is therefore evident that all the metallic sulphides will be dissolved away, 

 and were it not for farther oxidation of the iron, gangue alone would be left. The 

 process has been stated to be 12FeS0 4 4- 60 4- H 2 = 4Fe 2 (S0 4 ) 3 4- 2Fe 2 3 , H 2 ; 

 that is, as no further supply of free sulphuric acid (or sulphur) is available, the 

 ferrous sulphate can not form ferric sulphate and is therefore precipitated as in the 

 equation and partly as a basic sulphate, 6FeSo 4 3 3Fe 2 3 (S0 3 ) 2 an insoluble yellow 

 substance precipitated from ferric hydrate and found also in nature." f 



* Kmitn'iis : Chemistry of gossan. Engineering and Mining Journal, vol. 54, December 17, 1892. 

 f Loc. bit., p. 583. 



