252 ALBERT D. BROKAW 
simultaneously, since manganese dioxide has been considered 
favorable, if not essential, to the solution of gold in secondary 
enrichment. 
The methods of precipitation here discussed are not necessarily 
limited to secondary enrichment. Gold from a primary deposit 
may be recrystallized; that is, may be dissolved and reprecipitated 
without appreciable transportation, and in this case no enrichment 
would be accomplished, though the chemistry of the precipitation 
might be essentially the same as that in which gold deposits are 
enriched. For this reason the writer does not hesitate to draw 
examples from deposits where the gold has been recrystallized by 
secondary processes even though the deposit is not thought to 
have undergone any appreciable enrichment. 
The substances in an ore body, capable of precipitating gold 
from a solution in which it is held as a chloride, may be grouped 
as follows: 
I. Native elements, copper, silver, mercury, tellurium. 
II. Sulphides, tellurides, etc. 
a) Simple—most of the common sulphides. 
b) Complex—sulph-arsenides and antimonides. 
III. Ferrous compounds. 
a) In solution derived from alteration of iron sulphides. 
b) Primary minerals, as siderite, iron bearing calcite, etc. 
IV. Manganous compounds. 
a) In solution, derived from alteration of manganese minerals. 
b) Primary, rhodochrosite. 
V. Other inorganic substances, sulphur dioxide. 
VI. Organic compounds. 
I. NATIVE ELEMENTS 
The metals preceding gold in the table of electrolytic solution 
tension’ are capable of displacing it from solutions of its salts. 
The reaction may be expressed ionically as follows: 
Aut+++3Me=Au+3Mer 
in which Me stands for one equivalent weight of the metal in ques- 
tion. Copper and silver are not uncommon in gold deposits and 
are capable of bringing about such precipitation. Doubtless 
t See Smith, General Inorganic Chemistry, p. 670; Walker, Introduction to Physical 
Chemistry, or any textbook of physical chemistry. 
