20
VEINS

Most of the veins are 5 feet or less wide, but are fairly uniform in
width and continue along the surface for several hundreds of feet. A few
of the veins are known to be at least 1,000 feet long, but only in a few
cases has the full length been shown. Most of the high-grade silver-lead-
zinc ores occur in veins 5 feet or less in width. Veins wider than 5 feet
are fairly common and are on the whole much lower in grade. Veins of
the silver-lead-zine type wider than 5 feet may contain sporadic shoots
of shipping ore, but as a rule they consist either of milling ore or else of
material too lean for profitable mining. Those less than 5 feet wide con-
tain as a rule ore that can be shipped profitably without concentration or
else are too lean for mining. The narrow veins can not as a rule supply
a sufficient tonnage for milling operations. As very narrow veins must
of necessity be very rich if they are to be mined at a profit it follows that
most of the shipping ore will be from veins 2 to 5 feet wide. When mill-
ing operations become established the greater tonnages will be derived
in all likelihood from the wider veins.

The veins in which copper and gold are the main valuable metals
show less variation in richness for different widths. A copper-bearing vein
12 feet wide may be just as rich, locally at least, as one 8 inches wide.
The usual gold-silver-lead-zinc vein does not contain $40 per ton in all
values if it is more than 5 feet wide, yet many of the narrow veins of the
type a few inches wide may be worth over a hundred dollars per ton.

The veins do not exhibit any striking differences where crossing rock
formations of different types. The few in granitic rocks are narrow.
Some of those in argillites are quite wide and continuous, but small veins
also occur. Those in volcanic rocks are also of various sizes. The rich-
ness of the veins does not depend on the nature of the enclosing rock, as
both rich and lean veins occur in all the various rock types. Neither the
richness nor the size depends on the rock type. The nature of the coun-
try rock may exert some influence on the nature of the minerals in the
veins. Barite and jasper are common gangue minerals in veins in vol-
canic rock, but are very scarce in veins in the sediments. Some of the
constituents of these two minerals were probably derived by the ore solu-
tion from the volcanic rocks in which the veins lie.

REPLACEMENTS

Most of the mineral deposits in the area are in vein form, but some
of the larger are replacement bodies. Most of the replacement deposits
lie in stratified rocks and their forms are determined by the presence of
beds of different permeability or solubility. A few lie in schistose rocks
where their form is controlled by the extent and intensity of shearing.
Those in limestone are irregular in shape and the limestone is completely
or nearly completely replaced where ore occurs. The deposits in argillite
on the other hand are beds of argillite impregnated with mineral, but
which forms only a very small proportion of the argillite beds. Replace-
ments in tuffaceous rocks appear to be of a more bunchy nature than in.