weed.] ORE DEPOSITS AT BUTTE, MONT. 177 
ing 5 per cent copper. The amount of arsenic (and antimony) pres- 
ent is very large, it being estimated that over 32,000 pounds a year 
pass off in smelter fumes. Tellurium is present in very small 
quantity in the ores, amounting to 2^ ounces, or 0.008 per cent, in 
the crude copper upon the converters. It is recovered in electrolytic 
refining. 
Ore deposit/ion. — Three distinct periods of ore deposition are recog- 
nizable in the deposits of Butte. As many of the ore bodies are of 
composite character and derive their contents in part from each one 
of these periods, a careful study is necessary to discriminate the 
1 evidence and results of each period. In general it is necessary to 
differentiate primary deposits, or those formed of material brought 
to and deposited in the veins from outside sources, and the so-called 
"secondary" deposits of transposed and redeposited material. The 
former constitute the normal vein filling, the latter both the bodies 
of rich ore that have made the district famous and masses of low- 
i grade, concentrating ores. As a general statement, it may be said 
that the deposits of copper glance are secondary. 
The original source of the metallic contents of the primary deposits 
is still an unsolved question. It has been inferred by Mr. Emmons 
that, in the lack of direct evidence, "It is probable that circulat- 
I ing waters have somewhere in the depths extracted the metals from 
parts of the granite mass." To the writer the mineralogic evidence 
1 and the intimate connection between periods of ore deposition and 
igneous activity indicate a possible derivation from magmatic ema- 
nations — so-called mineralizing agents in waters partly of magmatic 
origin, mingled perhaps with predominating meteoric waters 
In general it may be stated that the original mineral-bearing solu- 
tions were probably hot and ascended through fractures in the granite. 
I The copper deposits are almost entirely replacement deposits formed 
I by waters ascending through mere cracks and attacking and replac- 
ing, particle by particle, the adjacent rock. The silver veins, on the 
contrary, are in large part due to the filling of open fissures, though 
replacement deposits also occur. In the replacement deposits there 
is a general lack of definition between country rock and ore, a wide 
zone of altered decomposed granite alongside of the vein, and com- 
monly an impregnation of the rock between the individual veins of a 
lode with ore minerals. This is especially noticeable in the eastern 
part of the copper area, in Leonard, Rarus, and adjacent mines. In 
the former an ore body is stoped out for 135 feet in width, consisting 
of altered granite, sheeted and intersected by a multitude of small 
veins crushed by later movements and impregnated by primary 
minerals in part replaced by secondary glance. 
In the central part of the copper area fresh unaltered granite is 
uncommon. There has been local development of intense thermal 
activity. The rocks are closely fissured as a result of several periods 
Bull. 213—03 12 
