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Page: of 12
FORM AND STRUCTURE OF VEINS
"The main veins of the district are more or less regular
in strike; but local undulations occur, and a vein may branch,
and these branches may reunite or one may gradually die out.
On the dip of the vein the same branching may occur; or where
there are two parallel veins a short distance apart, a spur may
go off from one vein into either the foot wall or hanging wall,
and join the other vein. Thus a system of branching and linked
veins is formed; and unless the greatest care is exercised in
following the various branches, the main vein is likely to be
lost. On their downward course at different points, the Empire
Pennsylvania and North Star veins all branch, and in consequence a considerable amount of exploratory work was necessary before it was determined which was the main vein. In the
North Star Mine the drifts were driven followingthe vein; and when
a ‘split’ was encountered, as a rule the largest and most promising branch would be followed. It was often found, however,
that this branch would die out within a short distance, and that
the less likely-looking stringer would lead ultimately to an
ore-body. For this reason it is most important that extensive
exploratory work be continually carried on; and it is especially
advisable that cross-cutting be undertaken at frequent intervals, in order to be sure that the development work is being done
on the main vein. In the North Star Mine, the vein split on
its downward course and the lower vein was followed, and some
rich ore was stoped therefrom. Subsequently a cave-in of the
hanging wall revealed the fact that the hanging-wall stringer,
which had been practically overlooked, had become a large
and very rich vein. The stoping of this vein proved that a double
ore-shoot had been formed, with a horse of country rock between; and it was further found that the two branches reunited
at a point about 400 feet below their point of division.
"The walls of the vein are as a rule undulating curved
surfaces, and the quartz is separated from them by a few
inches of ‘gouge,’ or finely comminuted material derived by
minor movement along the fissure subsequent to the formal on
of the quartz vein,
"The veins vary greatly in width; the distance between the
walls may range from a mere seam to a maximum of ten feet.
Between the walls the vein filling is seldom composed of solid
quartz. It is generally a mixture of brecciated, altered wall
rock and calcite and quartz stringers, with a vein of quartz
from one foot to three feet in width usually found on the hanging
or foot wall, but sometimes on both, The average width of
the North Star quartz vein is about two feet two and a half feet
in the area stoped; in the Empire, the vein averages only about
eighteen inches in width. The maximum width of workable ore
attained in these mines is from eight to ten feet. These swells
in the ore-body generally occur at the intersection of two veins
or where two brances of the vein follow a nearly parallel course,
separated by a few feet of altered or shattered wall rock.
"Underground, the veins have been followed by drifts
in the different mines for distances varying from 1000 feet to
a maximum distance of 6000 feet inthe case of the Eureka-IdahoMaryland Mines, In each mine, however, the vein would, at
History of Nev
Published i
various levels, close down to a mere seam for several hundred
feet, to finally open out again into a pay-shoot. In the North Star
Mine the 1900 level followed such a seam, which sometimes
was so indistinct that it was followed only with the greatest difficulty by keeping the general course of the vein, At a distance
of about 1000 feet this seam intersected other seams and developed a large and rich ore-body several hundred feet in length
along the strike of the vein, and which was mind from the 2300
and the 1100 level. Similar conditions are found in all the veins
of the district."
RELATION 10 COUNTRY ROCK
"As has been previously stated, both the Empire and North
Star veins pass from the diabase-porphyry into the underlying
granodiorite, the Empire at a depth of 1700 feet and the North
Star at a depth of 3700 feet to 4000 feet, measuring along the
dip of the vein. In both of these mines this change in the character of the wall-rocks seems to have little effect upon the tenor
of the ore-bodies or upon their size, with the possible exception that in the latter characteristic they seem, in the grano-