•encer] TREADWELL ORE DEPOSITS, DOUGLAS ISLAND. 81 
GANGUE MINERALS. 
Feldspar, calcite, and quartz are the three important nonmetallic 
ninerals of the Treadwell ores. Part of the original feldspar of the 
ntrusive diorite remains in the ore, and with a considerable amount of 
secondary feldspar forms the principal gangue mineral. Other min- 
erals of the unaltered rock were hornblende and mica, but these are 
^resent in relatively small amounts, as is epidote, which has been 
! ormed as a product of alteration from them. Calcite and quartz 
)ccur in veinlets penetrating the diorite, and make up perhaps one- 
ifth of the material which is mined. Calcite is also found dissemi- 
lated irregularly in the more altered parts of the diorite, unaccom- 
)anied by quartz. When sulphides and calcite are both present, they 
re almost invariably in contact with each other, but the secondary 
eldspar also carries a great deal of pyrite. 
The occurrence of ferruginous calcite is common in the superficial 
workings, where it may have been formed by the action of iron-bear- 
ng solutions upon the primary calcite of the deposit. It occurs also 
n small amounts in deeper parts of the mines, where it is possibly an 
riginal mineral. A small amount of pink carbonate, probably a 
lixture of calcite and rhodochrosite, has been observed in the open 
its of the Ready Bullion. 
METALLIC MINERALS. 
As shown by the mill records, the metallic minerals, or u sulphides," 
|onstitute about 2 per cent of the Treadwell ores. They consist mainly 
f iron pyrites, but a considerable amount of magnetite is also present. 
Pyrite occurs both in the rock and in the veinlets, but the position 
f the sulphides has no apparent influence on the gold content. In 
le rock it invariably has the form of minute cubes, ranging from a size 
arcely visible to the unaided eye up to about one millimeter, rarely 
rger. It is distributed sparsely through the diorite accompanying 
e secondary minerals, especially the albite and calcite, though where 
ese are not present it is associated with epidote and uralitic horn- 
ende. In the reticulating veinlets the pyrite occurs either as sepa- 
te cubes, often several millimeters across, or in bunchy aggregates, 
rming " turkey-egg rock," which usually contains more than aver- 
se values. 
Magnetite occurs only in the form of minute grains outside the 
inlets. Part of it appears to have been an original constituent of 
le diorite, but much of it was deposited secondarily along with the 
yrite, perfect cubes of which it sometimes surrounds. 
Rutile occurs in minute needles, and though seldom visible to the 
iked eye the microscope shows that it is widely distributed in various 
arts of the mines. As a rule, it occurs embedded in calcite, but it is 
mietimes in the secondary albite. Pyrrhotite often accompanies or 
Bull. 259—05 6 
