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Most of the lode deposits are described as fissure veins, meaning
fractures, usually faults, into which vein material has been
introduced.
In general the mineralogy of the veins is simple. The major constituent
is quartz, which is usually milky white. Commonly it contains
drusy cavities, and some quartz veinlets have a median cavity into
which small crystals project. Calcite commonly accompanies quartz
in minor amounts. Other nonmetallic gangue minerals include sericite
and, rarely, albite.
Pyrite is the most widespread metallic gangue mineral. It occurs in
quartz, in wallrock inclusions, and in the wallrock next to the veins;
it forms individual crystals and veinlets. Other metallic sulfide minerals
which may be present in subordinate amounts are arsenopyrite
galena, and sphalerite. Still less common are chalcopyrite, pyrrhotite,
and molybdenite. The amount of sulfides in most quartz veins is less
than 3 percent but may be as much as 5 percent.
Gold occurs almost exclusively in the veins. It occurs as free metal
in quartz and in the sulfide minerals, mainly pyrite. Gold tellurides
have been reported from some veins.
Many veins contain wallrock fragments; indeed, in some of the
most productive veins wallrock fragments are abundant; veins in
slate are commonly ribboned with included fragments, and the quartz
is bluish, due to comminuted slate (Ferguson, 1914, p. 59-71).
The quartz-filled fractures range from veinlets a fraction of an inch
in width to great masses 10 feet or more thick; however, most commonly
they are 1 to 5 feet thick. The quartz bodies are more or less
lenticular, both horizontally and vertically, so that the veins pinch
and swell and may disappear completely. In places the main body of
quartz may feather out into several thin nonpersistent stringers. The
veins seldom follow a single simple fracture; rather, they occupy
complex structures which branch and rejoin, thus enclosing bodies
of country rock called horses. Locally veins send off subparallel subsidiary
branches that may rapidly terminate or extend long distances.
Commonly a fracture zone contains two or more parallel veins or a
series of shorter discontinuous veins arranged en echelon. The course
of the veins is generally relatively straight, although in detail the
strike is variable, and the dip may vary within several degrees. Most
veins are steeply dipping, although some are known to be inclined
at relatively low angles. The contacts of many veins with the wallrocks
are slickensided owing to postmineralization movement, resulting
in clearly defined vein boundaries. In other veins only one contact
is faulted while the other is frozen to the wall and sends out veinlets
and stringers of quartz into the wallrock. In rare instances neither
wall shows postmineralization movement.
The strike length of most of the veins is no more than a few hundred
feet. Several of the most productive veins, however, have been
followed for 2,000 feet or more. The most persistent of these veins are
in the southeastern part of the Klamath Mountains province in the
French Gulch-Deadwood district. The vertical depths to which veins
extend are much the same as their horizontal dimensions. Most of
them have been followed to less than 500 feet, and a few, to over
1,000 feet; the mining depth, of course, is dependent on mining economics
rather than on the real persistence of the vein. Ferguson
(1914, p. 37) pointed out that the minimum vertical range through
which the gold-quartz veins were deposited in the French Gulch district
was 3,400 feet, as determined by the difference between the altitude
of the highest outcropping veins in the area and the lowest
underground point reached by mining. However, no individual vein
is known to be this persistent.
The auriferous veins are not uniformly productive throughout their
extent. The enriched, economically productive parts of a vein are
called ore shoots. Most of the ore shoots have lengths of a few hundred
feet and persist in depth for similar distances, although in some
of the more productive mines, shoots have been followed in depth for
several times their strike length. The pitch (or rake) of the shoots
is generally steep but is seldom the same as the dip of the vein and
rarely as low as 45°. Commonly the ore shoots are the widest part of
the veins.
For many of the mines in the Klamath Mountains, the controls on
localization of the ore shoots are poorly known. From observations
made by Ferguson (1914), Shenon (1933a), Youngberg (1947), and
Albers (1965), the most obvious controls have been structural. Most
commonly the shoots occur at the intersection of veins. These vein
intersections may be the junction of subparallel veins or at branches
of the same vein, the intersection of a main vein with subsidiary veins,
or along the junction of veins with opposing dips. Changes in strike
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