696 MINERAL RESOURCES, 1913—PART II.
some
of the workings. The prehistoric workings were widely s tered and
served as guides to the modern workings. Mr. Hamstadt states they were
chiefly pitlike holes rarely over 15 feet deep and from 10 to 25 feet
across.
Probably
the most important workings were those 250 yards northwest of the
camp, in the hillside near the summit of the low gap. An open cut 100
feet long in a N. 60° W. direction, 30 feet wide, and 15 to 20 feet
deep, two shafts about 40 feet deep, at least 300 feet of tunnels, and
stopes were made at this place. Tunnels were driven in from the
hillside to the south and connected with the bot torn of the open cut
by shafts or stopes. Other tunnels were made from each end of the open
cut. Among other large workings were deep shafts near the camp, open
cuts and tunnels north and also west of the camp on both sides of the
draw. Some of these openings were large, consisting of open cuts 20 to
40 feet long and 10 to 25 feet deep and tunnels aggregating several
hundred feet in length.
Varied
types of rock are exposed in the turquoise region. Along the road to
Valley Wells coarse porphyritic biotite granite, inclosing masses of
diorite outcrops for a distance of several miles. In the hills south of
the mine is a metamorphic series containing hard quartzite, biotite
schist, mica gneiss, etc. The turquoise-bearing area is bounded by
biotite schist on the west also, and this in turn is overlain by a
mesa-forming basalt flow. The turquoise deposits are in coarse
porphyritic granite and a porphyry which is probably the monzonite
type, slightly quartzose, occurring in the form of dikes. One of these
dikes about 250 feet wide cuts the granile with a northwest strike on
the north and within a few feet of the mine buildings.
Both
the coarse granite and the porphyry have been so fractured and
decomposed near the turquoise deposits that it is difficult to
distinguish between them. Decomposition has resulted in sericitiza-tion
and kaolinization of the feldspars of the rocks with a deposition of a
quantity of limonite iron stains and probably also some secondary
silica. Extensive fracturing or jointing opened many channels for the
passage of the water or solutions which caused the decomposition. Later
solutions carrying the elements of turquoise and passing through the
same channels deposited turquoise in seams and veinlets and in nodular
masses imbedded in kaolin or sericite in larger veins. Some of the
turquoise is in light-colored matrix, but in other places the matrix is
heavily stained with limonite. The abundance of limonite stains in the
altered rocks, with which the turquoise is associated, indicates that a
quantity of pyrite or other iron sulphides have been decomposed during
the weathering. Limonite pseudomorphs and rusty cavities left by
weathered pyrite were observed in the fracture zones near the turquoise
veins. Veins of chrysocolla and limonite stains were found in a
prospect nearly 300 yards east of the camp buildings in another small
body of porphyry.
The
majority of the gem turquoise found at East Camp was in nodules or
nuggets. These ranged in size from small specimens to those over an
inch across, as a rule the larger pieces being of poorer
g
rade. Some large
nuggest of high-grade turquoise are reported to have been found,
however. George F. Kunz * mentions one specimen
i Gems, jewelers' materials, and ornamental stones of Caliiornla: California State Min. Bur. Bull. 37, p. 163, 1905
