54 
FIFTEENTH REPORT. 
GOLD DEPOSITS OF PORCUPINE DISTRICT, ONTARIO. 
BY REGINALD E. HORE, MICHIGAN COLLEGE OF MINES. 
In the fall of 1909, important discoveries of gold were made in the 
Porcupine district, Ontario, about 100 miles northwest of Cobalt and 
360 miles north of Toronto. Development work has proven some of 
the deposits to be large and of very profitable grade. Two mines, Dome 
and Hollinger are now producing on a large scale and several are con- 
tributing smaller amounts. About $2,000,000 was produced in 1912 and 
a much larger output is expected for 1913. 
The ore is native gold in pyritic quartz. The quartz occurs 1 (1) as 
single fissure fillings or veins; (2) a series of fissure fillings running 
nearly parallel — vein system; (3) quartz-ferrodolomite lodes in which 
quartz veins penetrate bands of ferrodolomite or iron-calcium-magnesium 
carbonate; (4) quartz masses of irregular form, chimneys, kidneys, etc., 
and some more or less parallel, thick, lenticular masses. 
The quartz is partly coarse grained, but much of it is very fine grained 
and has evidently been crushed. Granulation is indicated by the ap- 
pearance of thin sections and strain phenomena are common. Some of 
the quartz shows a ribboned structure with banding in direction of the 
strike of the veins. 
The gold is mostly in fine particles, but much also is in coarse grains 
and in the ore is frequently visible to the naked eye. Most of the visible 
gold is in parts of the quartz near the wall rock or masses of enclosed 
rock, and assays indicate a similar distribution for the invisible gold. 
Pyrite is almost always present in both quartz and wall rock and is 
more abundant in the latter. The gold is intimately associated with 
the pyrite and sometimes intergrown with it. Much of the visible gold 
near pyrite, however, is not actually in contact with it. There is some 
calcite in the ore, but it is not present in large quantity. Tourmaline, 
generally in aggregates of small crystals, is often found in the quartz. 
Sclieelite has been found in a few veins. 
MICROSCOPIC CHARACTERISTICS OF THE GOLD ORE. 
The examination of numerous thin sections shows that there are cer- 
tain differences in the ore from the several deposits, but these seem to 
be of a minor nature and the following description of gold-quartz from 
the Dome Mine may be taken as fairly typical of the Porcupine Mines. 
The quartz is not uniform in grain and one may distinguish readily 
between the part that is coarse and the part that is fine. The coarser 
grains are commonly 0.5mm. to 1.0 mm. in diameter, while the finer are 
about 0.05mm. The coarse has numerous small cavities partially filled 
with liquid inclusions, and shows marked strain shadows; the fine has 
fewer inclusions and strain effects are not so marked. Fine grained 
descriptions of four typical deposits were given in Canadian Mining Journal, Nov. 1, 1910, pp. 
649-656. 
