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Page: of 12
M.J. Brock
to great depth, were heated, and returned to the surface, where
their mineral content was deposited in the open spaces of the
fissures.
"The gangue minerals, the metals, and the sulphides of
the present veins were undoubtedly dissolved by these waters
from the deeper portion of the granitic mass; for although
these veins traverse different rocks, the mineral content of the
vein is practically the same, proving conclusively that the
minerals were not derived from the wall rocks of the vein.
As the solutions approaced the surface zone, their mineral
content was deposited by reason of the unbalancing of the chemical compounds by the absorption of certain elements through alteration of the wall rocks, by the mingling of chemically different waters in the different intersecting channels, and by
varying conditions of temperature and pressure. The effect of
the wall rocks and the intermingling of the solutions were probably of the greatest importance in causing the deposition of
the quartz, gold and metallic sulphides. It must here be remembered that the portions of the veins now being worked were,
at the time of their formation, probably a few thousand feet
I
below the ancient surface of the country."
ALTERATION OF WALL ROCKS
"As has been previously stated, some of the rocks of
the district have been altered over large areas by dynamic
metamorphism. The schists in the vicinity of the Brunswick
Mine which were derived from a normal diabase porphyrite
are an example of dynamic metamorphism; hydro-metamorphism is shown in the formation of the large areas of serpentine of the Idaho-Maryland area. Further, in the granodiorite
and other igneous rocks extensive sheeting and jointing has
taken place.
"The fissures themselves were probably formed by sudden
breaks in the rock, and not by long-continued movement along
the same line of weakness as in the veins of the Mother Lode.
The result has been the formation of a system of main fractures, with branching and minor fractures linking together
the main breaks, We therefore find, along the breaks, breccias (broken and crushed fragments of the wall rocks) which
have since been chemically altered by the vein solution and
finally cemented together by the vein-filling quartz.
"The chemical alteration of the wall rocks next to the
quartz varies in intensity to a marked degree, the greatest
amount of change having taken place in the case of the IdahoMaryland, a smaller amount in the case of the Empire and
North Star veins, and least in the case of the Omaha and Allison Ranch veins where comparatively fresh granodiorite is
found close to the quartz vein. The gold bearing ores of the
Grass Valley district are practically confined to the quartz
veins, and the altered wall rocks carry very little gold."
OUTCROPS
"in general the outcrops of the veins in this district are
inconspicuous, and as a rule they can not be traced for any great
distance on the surface. For example, the Eureka-Idaho-Maryland vein has been worked for a distance of 6000 feet along
the vein without a break, yielding about $19,000,000, and yet
this wonderful ore-body outcrops for but a few hundred feet
on the Eureka claim. This outcrop of quartz was of very low
grade, and pay-ore was not encountered until a depth of over 150
feet had been reached. The vein can not be traced on the surface east of the Idaho shaft, although, underground, some of
the richest ore has been taken from the pay-shoot for a distance
of 3000 feet east of the shaft.
"The famous Empire, Pennsylvania and North Star veins
can only be found at intervals on the surface. The reason for
this is that the veins are easily decomposed, forming a soft
reddish mass of limonite and quarts extending to an average
depth of 150 feet below the surface. The sequence of the deterioration seems to be, first, oxidation of the sulphides, with
the consequent liberation of the associated gold, followed by a
general loosening of the texture of the vein filling, As a rule
the disintegration of the vein is slightly more rapid than that
of the surrounding country rock; and, as the veins are small,
the decomposition of the country rock covers the vein, making it
exceedingly difficult to trace the lodes on the surface. This
necessitated considerable exploratory surface work to locate
them definitely."
(To be continued next week)
The Nevada County Nugget Wedne
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