spencer.] TREADWELL ORE DEPOSITS, DOUGLAS ISLAND. 83 
METASOMATrC ALTERATION. 
As already stated, the Treadwell ore bodies are dikes of albite-dio- 
t-ite filled with reticulating veinlets of quartz and calcite and perme- 
ited with metallic sulphides carrying small amounts of gold. 
From the structure of the deposits it is evident that the dikes were 
subjected to pressure which caused fracturing, whereby they became 
porous, affording channels of easy circulation for underground waters, 
rhe minerals in the ores and their mutual relations suggest that car- 
bonated and mineral-bearing solutions found the broken dikes and 
continued to move through them for a very long period. In transit 
ihese waters attacked the minerals of the albite-diorite, decomposing 
:hem, and in some cases effecting more or less complete metasomatic 
replacement. As a rule, the hornblende and mica of the original rock 
lave entirely disappeared, their place being taken by aggregates of 
secondary minerals, sometimes including metallic sulphides. A few 
specimens of relatively unaltered material indicate that the original 
•ock characteristically contained two sorts of feldspar, albite-oligo- 
clase, and microperthite. The first occurs in phenocrysts of fairly 
lefinite form, often showing concentric structure, and always clouded 
iy decomposition products, excepting for clear rims, which are usually 
larrow. The microperthite, which has the characteristic mottled 
appearance of this minute intercrystallization of albite and orthoclase, 
s entirely interstitial as regards the albite-oligoclase. It is usually 
learly free from decomposition inclusions, and is ordinarily accom- 
)anied by some clear albite. When pyrite occurs in such slightly 
ltered material it lies in or next to decomposed hornblende crystals, 
dost of the rock has suffered extreme alteration, and pyrite occurs 
hroughout interstitial groundmass. Its introduction has apparently 
►een accompanied by breaking down of the microperthite, for this 
nineral, so abundant in the comparatively fresh rock, is usually 
ntirely absent when the sulphide occurs outside of the decomposed 
ornblende — that is to say, in the interstitial feldspar. In the most 
ltered rock the place of the microperthite is taken by an aggregate 
f small albite crystals, and this mineral is regarded as a secondary 
eplacement of the original feldspar. In some cases the replacement 
as gone so far that the crystals of albite-oligoclase have been attacked, 
his feature is relied on in part to prove the secondary nature of the 
lbite, but more conclusive evidences that the albite is of secondary 
rigin are its occurrence in veinlets cutting the old feldspar, the fact 
lat it is found intercrystallized with calcite, both in veinlets and 
aroughout the rock itself, and the fact that where albite forms the 
iterstitial material instead of microperthite, pyrite, and often r utile, 
re present, embedded either in the feldspar or in the evidently con- 
miporaneous calcite. 
