SECONDARY PRECIPITATION OF GOLD tT) 
most abundant source of ferrous sulphate in mine waters is found 
in the iron sulphides, which, upon partial oxidation, give rise to 
ferrous sulphate and sulphuric acid. These are almost always 
present in the zone where the sulphides are undergoing oxidation, 
and, because of the downward movement of the waters, and the 
presence of reducing agents lower down, ferrous salts are likely 
to be abundant below this zone. In such an ore body gold cannot 
migrate far, since ferrous salts, even in small amounts, almost 
prohibit the transportation of gold in solution.t. Ferrous salts 
in solution are more abundant in mine waters than is hydrogen 
sulphide, and are doubtless a more important factor in the secondary 
precipitation of gold. 
McCaughey?’ showed the effect of ferrous salts on the solubility 
of gold in concentrated solutions of ferric salts, and doubtless an 
equilibrium condition exists between the concentrations of ferrous, 
ferric, and gold salts in solution, in contact with metallic gold. 
The equation may be written in the ionic form: 
3Fettt+Au=3Fett+Auttt 
whence the equilibrium equation: 
[Fet+]}3x[Autt+] 
[Fet ++} 
McCaughey’s results were not sufficient to give the value of 
this constant, but more complete experiments would doubtless 
show such a relationship—indeed the form of curve obtained shows 
qualitatively that some such relationship exists. 
If the condition of equilibrium were satisfied, fluorite may 
disturb it by going into solution and giving rise to the fluoride 
ion, which forms a little dissociated complex with the ferric ion,’ 
practically removing it from the system and allowing the ferrous 
ion to cause precipitation. This case, though theoretically possible, 
is probably not of great importance in nature on account of the 
very low solubility of gold in ferric salts at concentrations obtaining 
in mine waters. 
*McCaughey, Jour. Am. Chem. Soc., XXXI, 1261 (1909). See also Emmons, 
loc. cit. 
aa oc. cit. 
3 Stieglitz, Qualitative Analysis, Part I, 271 (New York, 1912). 
