MICHIGAN ACADEMY OF SCIENCE. 
57 
bonates, quartz and chlorite. The light colored wall rocks" are largely 
made up of the first three minerals. In the darker ones, there is much 
chlorite. Fyrite occurs abundantly and is commonly well crystal ized. 
The wall rocks are commonly not highly auriferous except where pen- 
etrated by quartz stringers. Where thus silicified, the rocks sometimes 
contain payable quantities of gold. 
ORIGIN OF THE DEPOSITS. 
We have in the Porcupine district pyritic gold quartz, deposits en- 
closed in rocks characterized by an abundance of ferrodolomite, sericite 
and pyrite. The nature of the ore and the wall rocks suggest that 
the gold was introduced into the fissures along with the chief consti- 
tuents of the minerals mentioned. The solution,* * which contained the 
gold, probably contained also in some form, iron, sulphur, silica, potas- 
sium, and carbon dioxide. From the solution, practically all the potas- 
sium and carbon dioxide escaped into the wall rocks and aided in the 
formation of sericite and ferrodolomite. Part of the iron and sulphur 
also escaped into the wall rocks and there formed pyrite crystals and 
contributed iron to the formation of ferrodolomite. Part of the iron 
and sulphur and nearly all of the gold and silica was deposited in the 
fissures themselves. It appears that the walls were more readily pene- 
trated by some constituents than by the others, 4 and in this way much of 
the C0 2 , S, and Fe escaped. In proportion as these constituents escaped, 
the solubility of the gold in the remaining solution would be decreased 
and the deposition therefore aided by removal of solvent as well as by 
lowering of temperature. The pyrite, first formed in the veins, was 
comparatively poorly crystallized and was probably formed quickly. The 
pyrite in the wall rocks and some pyrite in the veins, that is probably 
of secondary origin, is in well formed crystals and evidently formed 
slower, or at least, under some more favorable conditions than did the 
original auriferous pyrite of the quartz veins. The gold and pyrite were 
not evenly distributed originally. Evidently in the first crystallization, 
they tended to segregate here and there, and the especially favorable 
place for deposition was near the walls or around masses of enclosed 
rock. 
After the filling of the fissures with quartz, gold and pyrite, the veins 
were shattered and the quartz granules strained or crushed. In the 
crushed zones, a secondary set. of minerals including sericite, chlorite, 
calcite, ferrodolomite and pyrite and some gold were deposited. These 
probably originated in the vein and wall rocks. The gold thus formed 
is in coarse grains which probably grew by slow accretion of small 
particles by a process continued over a long period. It is probable that 
this coarse gold grew at the expense of the fine gold contained in the 
quartz in its immediate neighborhood, thus leaving much very low 
grade quartz in the vicinity of the spectacular specimens. The coarse 
gold, to which a secondary origin is here attributed; while showy, is 
3 Microscopic description of several of the rocks is given by C. W. Knight and A. G. Burrows in the 
Bureau of Mines report, 1911. Mr. John Stansfield described rocks and ores from Vipond Mine in 
Canadian Mining Journal, Feb. 1.5, 1911. The wall rocks of Dome, Hollinger and Rea mines were 
described by the writer in Trans Cen. Min. Inst., 1911, pp. 173-178. 
*For discussion of transport of gold in solutions containing such constituents see Macaren Gold, 
PP 4Cf.°Lindgren, Characteristic features of California gold quartz veins. Bull. Geol. Soc. Am., 1895. 
