246 
ECONOMIC GEOLOGY OF SILVERTON QUADRANGLE, [bull. 182. 
Barstoiv mine. — This mine, formerly known as the Bobtail, was the 
only mine in the Red Mountain district working and producing ore 
in 1899 and 1900 on any considerable scale. It is situated about a 
mile west of the Guston, on a prominent lode running up Commodore 
Gulch, on which the Iranstan, Bobtail, Hector, Saratoga, and Moscow 
claims are located. The Iranstan and Bobtail were worked early in 
the eighties, but have been idle for the last few years, until work was 
resumed by the Barstow Mining and Milling Company. The mine is 
opened by an oblique crosscut tunnel and all the work is on one level. 
The lode, which has a general northwest-and-southeast strike, and 
dips southwest at about 50°, is a large one, 30 to 40 feet wide, but the 
ore is confined to one or two rich streaks in its middle portion. The 
lode contains much soft, altered country rock and is traversed by clay 
gouges, making it exceedingly treacherous and difficult to work by 
overhand stoping. The mass of the lode contains pyrite, chalcopy- 
rite, sphalerite, and some bunches of low-grade galena. The gangue 
is chiefly quartz, often cavernous and drusy, with a little fluorite. 
The best ore is a light-gray metallic mineral called "gray copper" at 
the mine, which, when pure, assays as much as 2,000 ounces of silver 
and 25 ounces of gold per ton. The gold always accompanies and is 
proportional in amount to the silver. Chemical examination shows 
that the so-called "gray copper" is in this case an argentiferous lead 
sulphobismuthite containing almost no copper, probably the mineral 
cosalite. But the present material obtainable (sp. gr. 7.04 at 30° C.) 
proves on chemical investigation to be a mixture of the lead sulpho- 
bismuthite with pyrite and some telluride. Under these circum- 
stances it was inadvisable to spend time in making a complete 
quantitative analysis. From his preliminary tests, however, Dr. W. F. 
Hillebrand has kindly supplied the following rough approximation 
to the composition of the mixture : 
Approximate composition of mixture from Barstow mine. 
Constituent. 
Per cent. 
Constituent. 
Per cent. 
Pb 
45.5 
22.5 
6.0 
.06 
Cu 
.3 
Bi 
Fe 
3.5 
Ag 
S 
14.1 
Au ... 
1.5-2.0 
The lead of cosalite is known to be sometimes partly replaced by 
silver and copper. In this particular case the silver is evidently 
accompanied by some gold, which is probably combined with tellurium 
and not directly with the sulphur and bismuth. The sulphobismuth- 
ite is often closely associated with chalcopyrite, which carries little 
value. The rich ore streaks are more or less lenticular in form and 
are usually separated from the mass of the lode by wet clay gouges. 
The best ore occurs nearer the footwall. As the workings are con- 
