The copper era. (Clifton, Graham County, Ariz.) 1899-1911, 1910-07-29

of some chalcopyrite; the oxide ore
being such oxidation products of the
sulphide ore as azurite, malachite
cuprite, chrysocolla and other les:
important minerals. The grade oil
the ore is low, about per cent, but
occasionally, pockets OF higher-grade
ore are found.
The gangue usually consists oi
altered porphyry, the feldspar Of
which has been converted into seri
cite; less frequently, the gangue i:
crushed and altered granite and dia
base. The nature of the ore deposits
in so far as it influences the mining
method, will be described in conuec
tion with the mines chosen as ex.
amples.
MINING METHODS.
In few districts can so many differ
ernt methods of mining be seen,
each adapted to the particular char
acter of the ore body to be extract
ed. AS the ore is low in copper, the
utmost economy must be observed in
it's extraction and high standard of
efficiency has to be secured from the
workmen who are principally Mexi-
cans.
The minimum of timbering and fill
ing, compatible with safety, is en-
ployed and every advantage offered
by the nature of the ground is taken
to secure this end. Fortunately the
mines are dry, the workings cool and
we'll ventilated. These are import
ant factors in reducing posts.
OPEN WORK ON METCALF HILL.
The surfrce oxidlzed ores are gain
ed by open-cut and milling methods
and the sulphide ores by various uu-
derground methods. At the Metcalf
mine, which was one of the earliest
locations, bodies of oxidized ore form
a conspicuous outcrop on the hilltop,
and were the source of the high
grade ore first mined. The hill is a
mass of granite porphyry, capped by
the lower members of the sedimen-
tary rock series. Development has
demonstrated the existence of four
parallel gein systems or stockworks
in the granite porphyry, along which
oreshoots of varied magnitude are
found. The vein systems have suf-
fered severe cross faulting, subse-
quent to the formation of the pri-
mary ore, but prior to the surface
enrichment that has occurred. The
surface presents chaotic mass of
blocks of quartzite, shale and gar
netized limestone lying on and in
other places completely imbedded in
the intruded porphyry.
The oreshoots are generally sound
at the functions of the cross faulting
with the vein systems. Although, as
before mentioned, some of the ore
shoots outcropped, the majority of
them are found beneath an overbur-
den of barren rock of varied thick
ness. The horizon on which these
shoots occur is variable and their dis
covery necessitates extensive pros
pecting from levels of not more than
10 or 80 feet apart.
REMOVING OVERBURDEN.
The slope of the hill allows all
work to be done from adit levels,
situated where possible in the barren
ground with crosscuts connecting to
the workings. An oreshoot having
been located sufficiently close to the
surface to warrant working by the
open-cut method, the overburden of
waste is first removed. Should the
rock be hard and break in large
pieces it is, after blasting, loaded by
hand into mine cars and taken to the
waste dump. AS the tracks must be
kept close to the working faces and
be moved easily, 12-pound rails are
used
With softer and finer waste, more
rapid disposal is accomplished by
mill hOling." A raise,6 feet square,
is carried to surface from the level
beneath and a chute covered by a
grizzly of logs, spaced inches apart.
is built or one side of the raise.
Commencing on the surface, the
waste is mined and fall directly into
the chute: the workman standing at
the grizzly beneath breaks with a
sledge all pieces too large to pass
through. The efficiency of this ar
rangement is almost three times that
of loading by hand.
The ore is scripped as completely
as possible from the everything waste;
drill holes are put down only as far
as the top of the ore. An admixture
Of ore and waste is at all times
avoided, necessitating a process of
selective mining to break the ore and
waste separately.
MINING THE ORE.
After the waste has been removed.
the breaking of the ore is commenced
on working faces 15 to 20 feet in
hight and as wide as the ore will per-
nit. On these faces, hole are drill
ed, chambe with 30 per cent. giant
powder and blasted with several kegs
of black powder depending on the