HE SECONDARY PRECIPITATION OF GOLD 
IN ORE BODIES 
ALBERT D. BROKAW 
University of Chicago 
Secondary enrichment of ore deposits has been recognized as 
a subject of fundamental importance to the economic geologist, 
and in recent years has received considerable study, both in the 
field and in the laboratory. The chemical reactions involved 
are, in many cases, less complex and more readily susceptible 
to laboratory study than many of the other great problems of 
earth chemistry. The temperatures, pressures, and concentra- 
tions involved are well within the range easily obtainable in the 
laboratory; the reacting substances are less numerous, and_ their 
chemistry is better understood than is the case with the substances 
and reactions involved in petrogenesis. 
In December, 1909, Emmons’ published a paper calling atten- 
tion to the relationship between placer deposits and manganiferous 
and manganese-free gold ores, suggesting manganese as a deter- 
mining factor in the solution of gold. About the same time the 
writer began a series of experiments dealing with the solution of 
gold in secondary enrichment, the results of which were published 
in May 1910.2 In October, r910, Emmons published a somewhat 
comprehensive review of the chemical and geological data available, 
with special reference to the gold deposits of the United States.’ 
Very little material bearing on the subject has appeared since that 
time, and at the suggestion of Dr. Emmons the writer undertook 
a series of experiments with the hope of clearing up some important 
points in connection with the chemistry of the precipitation of 
gold, and in particular, of establishing the peculiar set of conditions 
under which gold and manganese dioxide may be precipitated 
1W. H. Emmons, Min. and Sci. Press, XCIX, 751-54, and 782-87 (1909). 
2A. D. Brokaw, Jour. Geol., XVII, 321-26 (1910). 
3W. H. Emmons, Trans. Am. Inst. Min. Eng., XLII, 1-73 (1911). 
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