19

Bear and Marmot Rivers valleys approximate 160 square miles. In
this area twenty properties have produced ore. Development in Marmot
River valley has not been nearly so extensive as in Bear River valley.
The ratio of the value of production to the costs of prospecting and
development is much higher than that in Bear River valley. The mineral
pects on Bear river are larger but lower in grade than those on Mar-
mot river.

GENERAL CHARACTER OF THE MINERAL DEPOSITS

The mineral deposits are of two main types: (1) veins formed by
open space filling and by replacement; (2) replacement deposits. Grada-
tions exist between the veins formed by open space filling and those
formed by replacement, and between the veins and replacement deposits.
The veins, as contrasted with the replacement deposits, are tabular in
form, continue for long distances, consist in general of gangue or metallic
minerals or both, and are distinct from the country rock. The replace-
ment deposits, on the other hand, are mostly not so markedly tabular in
form, do not continue so far, consist of country rock and the replacing
minerals, and the ordinary gangue minerals of the veins are not usually
present in any quantity. The veins when forming were guided by frac-
tures which may pass from one rock type to another, whereas the replace-
ment deposits usually owed their outlines to bedding planes, planes of
schistosity, or the outlines of easily replaced strata.

The metals that have been obtained from the ores of the area are
gold, silver, lead, zinc, copper, and, on one property, molybdenum. Most
of the deposits contain intimately intergrown gold, silver, lead, and zinc-
bearing minerals. In most of these the lead and zine proportions are
approximately equal, but in a few the zinc content is negligible, and in a
few others lead minerals are rare. The gold content seems to be about the
same in all the silver-lead-zine ores, that is the gold value per ton is
usually no greater in high-grade than in low-grade ores of this type. The
gold is contained chiefly in pyrite and sphalerite, and also in arsenopyrite
where this mineral is present. The silver content in some of these deposits
is about one ounce per unit of lead and is associated with galena. In
others the silver content is high and is carried in silver-rich tetrahedrite
and to a lesser extent in silver minerals. The richness of the deposits in
silver depends chiefly on the percentage of lead and the quantity of
tetrahedrite.

Several deposits are valuable for their copper and gold content. In
these deposits the copper is present as chalcopyrite and the gold is either
associated with this mineral or with pyrite and arsenopyrite. In a few
deposits the arsenopyrite contains most of the gold, but in most this
relationship has not been established. The arsenopyrite when present is
not uniformly distributed through the deposits. If the gold is associated
with this mineral, the gold values will also be sporadic and this is known
to be true on several deposits of this type. Where arsenopyrite is not
present, gold is less plentiful but is more uniformly distributed and seems
to be carried mainly in the pyrite and chalcopyrite.