22 
SOME CHEMICAL CHANGES IN ROCKS, CAUSED BY SHEARING 
By H. C. Cooke 
Illustrations 
Page 
Figures 8 and 9. Straight-line diagrams illustrating results of shearing of carbon- 
aceous tuff, - 26, 27 
10. Straight-line diagram illustrating results of shearing of a greenstone 29 
Although the changes that take place in rocks during the processes 
of weathering, contact metamorphism, and hydrothermal alteration have 
been widely studied, and their chemistry is now fairly well known, little 
has yet been learned of the chemical changes that go on when a rock is 
converted into a schist under the influence of pressure only. One reason 
undoubtedly is, that up to recent years no one thought that mere pressure 
could cause chemical change, although rearrangement of the constitu- 
ents into new mineral combinations was recognized. But even when the 
possibility of chemical change has been recognized, the physical difficulty 
still confronts the investigator of securing material suitable for chemical 
analysis. The first requisite is a rock of fairly uniform composition, part 
of which has been rendered schistose. Having found this, it becomes 
necessary to secure specimens unaffected by weathering processes. As the 
cleavage planes of schist readily permit the ingress of surface waters, it is 
commonly impossible for a geologist, equipped only with a hammer, to 
get below the weathered zone, even in glaciated areas; and he must, there- 
fore, turn to the relatively small areas where mining is going on and he can 
go underground. But in these areas of ore deposition, sheared zones have 
commonly served as channels for the ascent of juvenile waters, have suffered 
more or less alteration from them, and have received additions of quartz, 
calcite, and other vein-forming minerals. Hence the opportunities for 
collecting material free from contamination, from one source or another, 
are extremely rare. 
C. K. Leith and W. J. Mead in their “Metamorphic Geology” have 
summarized most of the few known instances in which the sheared and 
unsheared varieties of the same rock have been analysed. These include 
three instances of the alteration of a quartzite to a sericite schist, three of 
a gabbro to a schist, two of a greenstone, and one of a granitoid gneiss. 
The writers conclude that shearing tends to eliminate from a rock those 
constituents in excess of the amounts required for production of the common 
schist-making minerals, such as the micas, chlorites, and hornblendes. 
During the summer of 1925 the writer examined the Canadian Asso- 
ciated Goldfields mine, situated on claim T. C. 699 in the southern part of 
McVittie township, Ontario, The ore-body in this mine has been formed 
by the replacement of Keewatin lavas and tuffs by fine-grained quartz and 
carbonates, accompanied by considerable amounts of ohgoclase, hematite, 
and auriferous pyrite and arsenopyrite. 1 A post-mineral fault, locally 
known as the graphite fault, cuts the rocks and ore-bodies. The fault is 
large, converting the rocks over widths of several feet into fissile schist; 
and it branches repeatedly, the branches re-uniting farther along the strike, 
iCooke, H. C.: Geol. Surv., Canada, Sum. Rept. 1923, pt. C I, p. 61. 
