NOTES ON THE MINERALOGY OF THREE GOUGE 
CLAYS FROM PRECIOUS METAL VEINS. 
By Earu V. SHannon, 
Assistant Curator, Departinent of Geology, United States National Museum. 
INTRODUCTION. 
Ore bearing veins of the so-called ‘ true-fissure” type, whether 
the ore has developed by filling of open cavities or by replacement 
of wall rock adjacent to fissures, in a majority of instances are accom- 
panied by more or less of clayey material which is commonly called 
selvage, ‘talc,’ or clay gouge. From the relationships and appear- 
ance of this clayey material it would naturally be considered to be 
merely rock from the walls of the fissure which had been rendered 
clayey and plastic by extremely fine grinding by movements along 
the fissure. Were radical chemical changes not anticipated one 
would expect the clay gouges of veins in granitic of other igneous 
rocks to consist of finely comminuted fragments of quartz, feldspar, 
and ferromagnesian silicates with perhaps a little kaolin derived 
from the feldspar and perhaps some iron oxide or hydroxide derived 
from iron-bearing silicates. In slates, shales, and especially quartz- 
ites, rocks resistant to chemical action, the gouges would not be 
expected to differ appreciably either chemically or mineralogically, 
from the inclosing rock. For many years the clayey materials 
accompanying veins received little attention. It is only recently 
that students of ore deposits have learned that the gouges are most 
frequently radically different from the minerals which make up the 
rocks of the walls, and that, by their nature and composition these 
clays yield valuable information as to the character of the vein- 
forming solutions. Thus sericite, or finely divided fibrous muscovite, 
is known to be widespread as a constituent of gouges while kaolinite, 
alunite, serpentine, talc, and leverrierite are known from veins of 
certain types. In examining gouge materials from Idaho gold and 
silver veins in the United States National Museum the writer has 
recently analyzed mineralogically homogeneous clays from three 
mines. The results of these examinations are given in some detail 
below. 
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