24

The copper-bearing deposits contain gold and in general the gold value
determines the grade of the ore. More thorough replacement and a greater
concentration of chalcopyrite also cause variation in the value of the ore.
Very little is known about the uniformity of individual shoots of copper-
gold ore, but from surface assays it appears that the gold does not segre-
gate to form richer shoots as might be expected but occurs sporadically
throughout the whole deposit. The net result of the gold seems to be to
raise the general standard of the whole.

Several points that may be of general interest were suggested by study
of ore specimens in polished section. The better parts of the Dunwell vein
contain tetrahedrite, whereas the leaner parts lack this mineral. Although
the conclusion that the ore-shoot is due to the local presence of a richer
mineral is obvious and may seem of no importance, it may in certain
circumstances have a practical value. In sulphide bodies where the lead-
zinc-silver and gold content is fairly uniform and the ore contains essen-
tially galena and sphalerite, the local presence of tetrahedrite will indicate
a richer ore-shoot. Rough examination of polished specimens will quietly
determine the presence of such a mineral and consequently the presence of
an ore-shoot. Assays will indicate ore-shoots, but ore examination will
not only do this but will also show the cause. Ore-shoots of this type are
quite different from those caused by a wider ore-body or by a higher ratio
of ore mineral to gangue.

Another type of ore-shoot was suggested by ore specimens from the
L and L Glacier Creek mines. Here primary vein matter has been frac-
tured and the fractures filled with richer primary minerals. The extent of
fracturing determines the size and shape of the ore-shoot. Ore examina-
tion in this case also indicates the cause of, as well as the presence of, the
ore-shoot.

In deposits of the silver-lead-zine type the order of mineral deposition
as illustrated by polished specimens from twenty-five different mineral
deposits showed little variation. Pyrite and arsenopyrite were the first
metallic minerals to form and were followed by chalcopyrite, sphalerite,
and pyrrhotite. Pyrite and arsenopyrite were in general contemporaneous.
Chalcopyrite, sphalerite, and pyrrhotite were usually contemporaneous, but
any one of the three in certain specimens might appear to be slightly
earlier or later than the other two. Galena, tetrahedrite, and silver sulpho-
salts were formed next. These minerals are in general intergrown and were
perhaps formed at the same time. In a few specimens the silver sulpho-
salts may be later than the other two. Later than any of the previously
mentioned minerals is Jamesonite and particularly an unidentified lead
sulph-antimonide.

Some of the copper-bearing deposits contain hematite and magnetite,
as well as the usual pyrite, arsenopyrite, and chalcopyrite. In these the
chalcopyrite appears to be later than the other minerals mentioned.

In ores containing galena, sphalerite, chalcopyrite, tetrahedrite, pyrite,
with or without other minerals, pyrite, chalcopyrite, and sphalerite were
deposited prior to galena and tetrahedrite. The galena is distinctly later
than, and appears to replace, the sphalerite. Tetrahedrite is present at the