the
open cut in a northeast direction with southeast dips. Other seams, and
one especially prominent seen in the end of the cut, run nearly east
and west with a high northerly to vertical dip. Turquoise occurs in
films, seams, nuggets, and irregular masses in the matrix. The
prominent seam mentioned was from 2 to 6 inches thick, and carried pale
turquoise and matrix in masses of nearly equal thickness. Much of the
turquoise in this seam is too soft and of too poor color to be of great
value; part, however, would yield good matrix gems.
The
openings farthest south consist of irregularly shaped open work with a
small amount of tunneling. The main cut is nearly 50 feet deep on one
side and probably 100 feet across. This cut is situated on the contact
of the typical quartz porphyry on the northeast and the altered country
granite on the southwest. The rock is jointed in different directions,
some of the seams running northeasterly. The turquoise occurs in films
and veinlets in many of the joints, and some of a fine bright blue
color nearly a quarter of an inch thick was seen in the open cut.
Part
of the turquoise from these mines is of the same high grade as much of
that from the Burro Mountains. The guaranteed cut gems of this company
are marked with a cross (x) on the lower side.
Porterfield Turquoise mine.—The
Porterfield Turquoise mine is on the west side of St. Louis Canyon, on
each side of the mouth of a small gully. The work at this mine has not
been extensive and consists of several tunnels and shafts with small
open cuts and prospect pits. Two of the deeper shafts are between 40
and 50 feet deep, and the longest tunnel is nearly 170 feet in length.
Turquoise of the best quality was found most plentifully during 1906 in
irregular open work near the bottom of the gully.
The
country rock at the mine consists of different types of granite
porphyry. A prominent type has large crystals or phenocrysts of red
orthoclase through a quartz and biotite matrix, and approaches a
granite in texture. Another type is a finer spotted porphyry, composed
of orthoclase with some plagioclase and quartz phenocrysts in a
groundmass of feldspar and quartz. Biotite is locally present in
quantity in small six-sided crystals generally badly altered to
chlorite. Another type is a quartz-feldspar granite, somewhat
porphyritic, in which biotite and similar minerals are lacking. In the
majority of these rocks the feldspars have been partially kaolinized,
while abundant silicification has taken place. The latter is
represented by much secondary quartz binding the particles of rock more
closely together and in seams cutting the rock in various directions.
Turquoise
has been found in prospects in a belt about 100 yards wide and over 200
yards long in a direction east of north and west of south. This area is
marked by numerous joints, of which many prominent ones strike
northeast, though others cut across this direction at various angles.
The turquoise occurs in seams, veinlets, lenses, and groups of nodules
or nuggets in lens-shaped masses. Among the minerals associated with
turquoise the most common is a white, koalin-like clay which coats the
nuggets and in places forms an important part of the filling of seams.
Among other minerals observed associated with turquoise and in
turquoise-bearing seams were hematite, quartz, limonite, hyalite opal,
chalcedony, and greenish, waxy halloysite, which in places assumes the
form of nuggets like turquoise. The turquoise occurs in seams running
in various
