One of the most interesting mines in the county is located in the rugged
mountains about 8 miles from the town of Azusa, in the San Gabriel
Cañon. It is commonly known as the Kelsey Mine, and has become famous as
a producer of silver ore of fabulous richness.
The country is made up almost entirely of metamorphic rocks, having
schistose, gneissoid, and massive structure. Both hornblende and mica
occur in these rocks abundantly, the former being frequently altered to
chlorite, or by further change to epidote. Dikes of porphyritic rock
have been intruded into the crystalline schists. In the immediate
vicinity of the Kelsey vein are intrusions of a dark green, much
decomposed, and shattered rock, probably diorite. Faults, great and
small, are numerous throughout the region. Within a few hundred feet of
the mine is a great fault, which may be plainly seen cutting the
mountain. The displacement must reach many hundreds of feet. It has
resulted in bringing in contact on a horizontal plane rocks of entirely
different character. On the south side of the fault the rocks are made
up of quite regularly bedded micaceous sandstones, more or less
schistose, and having a prevailing buff or light gray color. These rocks
dip east at an angle of 20° to 30°. On the north side of the fault the
rocks are harder, of a dark gray color, and containing considerable
hornblende. These rocks are more gneissoid and massive than schistose.
The dip is much less regular than on the south side of the displacement.
Large, lenticular masses of quartzose and feldspathic rock are of
frequent occurrence in the hornblende gneiss, evidently the result of
the segregation of the contained minerals. On the whole there is much
more evidence of the disturbance on the north side of the fault than on
the south side. It is in this area of greatly disturbed strata that the
Kelsey vein has formed.
The vein is of the fissure type and occupies the line of a fault plane,
that at first, perhaps, was a mere crack, but which has become enlarged
by the movement upon themselves of the rock masses forming the walls,
resulting in a grinding and crushing of the rocks by the attrition and
pressure incident to this movement. Into this crevice mineral waters
found their way, carrying in solution the minerals now constituting the
vein.
[Illustration: _FAULTING AND TORSION
OF THE
KELSEY VEIN_]
_The Ore._—The silver occurs as native and as glance (argentite),
possibly partly as chloride and in combinations with sulphur, cobalt,
and arsenic. The associated minerals are cobalt bloom (erythrite), a
hydrous cobalt arsenate, nickel arsenate (annabergite), carbonate and
silicate of copper (chrysocolla), iron oxide, and black oxide of
manganese in a gangue of baryta (heavy spar), with calcite (lime spar)
and some quartz. A clay selvage usually separates the vein material from
the wall, this feature being well developed in places on the foot wall
side, as though open crevices had occurred and the finely divided
material which was carried by the percolating waters had found a resting
place when an open space was reached, the absence of any current
permitting the material to settle.
The clay may have been derived in part from the decomposition of the
overhanging wall, the fine silt settling by gravity on the foot wall
side of the vein. In places a soft, clayey gouge constitutes the entire
vein filling, suggesting that the clay selvage and gouge are also partly
due to the attrition of the walls. Galena occurs sparingly in small
disseminated crystals, but the occurrence is so infrequent as to be
scarcely worth mentioning. In width the crevice varies from a thin seam
to over 4 feet. A banded structure is not uncommon in the vein.
The rocks inclosing the vein differ in various parts of the mine. A much
decomposed rock, containing iron in the form of carbonate, occurs
frequently, while a chloritic, more or less schistose, sometimes
massive, rock, also plays an important part in this connection. The dike
of dark basic rock, resembling diorite, previously referred to, is
exposed at numerous places throughout the workings, often in contact
with the vein, or close to it. Since its formation the Kelsey vein has
been subjected to severe torsion, which has resulted in abrupt fracture
and displacement. To me it seemed very probable that the vein was the
result of chemical precipitation, and no doubt, to some extent, the
replacement of country rock along the line of a fissure or fault plane;
that subsequent to the filling in of the vein the region was subjected
to further violent disturbances, which fractured the rocks along an east
and west course, and causing the turning of a large mass of rock
formation lying south of this fault to the west. The vein being included
in the general movement, was deflected from its natural course north and
south. I came to these conclusions from close observations taken along
the surface of the ground on the course of the vein, and in all
accessible underground workings.
Most of the ore extracted from these workings has been high grade,
usually running over $200 per ton, small lots often assaying several
thousand ounces. The property, at the time of my visit last spring, was
under the management of Dr. Endlich, E.M. This gentleman was making
every effort to systematically open and recover a vein that had been as
systematically and outrageously gouged. The workings were in bad
condition and at some points were positively dangerous. The mine was
gradually assuming an improved appearance and promised to yield better
returns than ever before. A good mill has been erected at the foot of
the mountain, in the San Gabriel Cañon, where a large stream of water
flows during the entire year. An office, boarding house, stables,
corrals, etc., had been built for the accommodation of men and animals.
In addition to this I found a complete assay office and chemical
laboratory, and here Dr. Endlich was experimenting with the rich cobalt
and nickel ores. As a result of his labors in this direction he
exhibited several bars of cobalt speiss containing a very high value in
silver. The assorted ore contains from 7 to 15 per cent in cobalt, 2 to
3 per cent nickel, and from 1,000 to 1,400 ounces silver per ton.
Dr. Endlich thus describes his methods: “The ore is crushed through a
twenty-mesh sieve, mixed with sufficient litharge to produce an 8 per
cent charge, and enough borax is added to take up the gangue (quartz,
heavy spar, carbonate of lime, magnesia, and iron). Carbonate of soda
and flour are mixed with the charge. If the percentage of arsenic in the
ore is sufficiently high to produce speiss none is added; otherwise some
metallic arsenic is mixed in. Some sulphides in the ore and reduced
sulphur from the heavy spar are utilized to produce mattes. The mixture
is melted in large Dixon crucibles; the slag poured off, and the
metallic product allowed to cool. The bars obtained are composed of
lead, silver, cobalt, nickel, arsenic, and sulphur, principally; the
lead being in the form of sulphide, the cobalt and nickel in the form of
arsenides. The bars contained from 4,500 to 7,000 ounces silver per ton.
The slag contained a trace of silver, and averaged about 0.75 per cent
cobalt, which can be worked over by arsenizing, if desired, and the
cobalt obtained in the resulting speiss.”
At this writing about 560 pounds of ore has been treated in this way and
the product shipped to Balbach’s works in Newark, N. J., for refining.