266 ALBERT D. BROKAW 
rather than a chemical, but amorphous carbon may be present 
in carbonaceous shales and supposably might cause such precipi- 
tation. Whether other amorphous substances are capable of 
causing such precipitation has not been rigorously investigated. 
SUMMARY 
If gold is in solution in waters circulating through an ore 
body many substances may cause precipitation of the gold. The 
ferrous compounds, derived from the partial oxidation of iron 
sulphides, are thought to be the most important, because they are 
formed comparatively readily from pyrite and other iron-bearing 
sulphides by the action of oxidizing waters," they are very often, 
if not usually present in mine waters, and they are very effective 
precipitants for gold. The effect of the iron-bearing sulphides 
themselves cannot readily be separated from the action of ferrous 
salts, since in precipitating gold they may lead to the formation 
of ferrous sulphate, which, of course may cause farther precipita- 
tion of gold, if any is present. In most gold deposits sufficient 
pyrite, or other iron-bearing sulphide is present in the primary 
ore, to precipitate any gold that may be brought to the zone of 
sulphides. Less commonly siderite or iron-bearing calcite may 
be important. Siderite is attacked by acids more readily than 
pyrite, and consequently gives up ferrous salts more easily, and 
these, in solution, may cause the precipitation. 
In comparatively rare cases, when the gold is in a solution 
containing manganous salts, contact with the country rock may 
neutralize the acids present and allow the manganous salt to reduce 
to gold. Rhodochrosite resembles siderite in its action except 
for the fact that no precipitation of gold can occur if the solutions 
are strongly acid, while in the case of siderite acidity does not 
inhibit the reduction of gold chloride to metallic gold. 
The other reducing agents considered are thought to be of 
comparatively little importance, as they may be applied, to only 
a very few unusual deposits. 
No attempt is made to explain the precipitation of gold from 
t Buehler and Gottschalk, Econ. Geol., V, 28-35 (1910), and VII, 15-34 (1912). 
