h they are associated; and other
groups have such associations as to indicate much less intense
conditions of temperature and pressure. Depth is only one factor
determining intensity of conditions, but it affords a convenient way to
indicate them; so mineral deposits associated with igneous rocks are
sometimes classified by economic geologists on the basis of deep,
intermediate, and shallow depths of formation.

There are a considerable number of minerals which are formed in all
three of these zones, although in differing proportions. There are
comparatively few which are uniformly characteristic of a single zone.
On the whole, it is possible to contrast satisfactorily mineral deposits
representing very intense metamorphic conditions, usually associated
with formation at great depth, with those formed at or near the surface;
but there are many deposits with intermediate characteristics which it
is difficult to place satisfactorily.

The accessible deposits of the deep zone are associated with plutonic
igneous rocks which have been deeply eroded, and not with surface lavas.
They are characterized by minerals of gold, tin, iron, titanium, zinc,
and copper, and sometimes of tungsten and molybdenum, in a gangue of
quartz, which contains also minerals such as garnet, corundum,
amphibole, pyroxene, tourmaline, spinel, and mica. The deep-zone
minerals are not unlike the pegmatite minerals in their grouping and
associations.

Deposits formed at shallow depths are related to extrusive rocks and to
intrusives near the surface. Erosion has not been deep. Mercury, silver
and gold (tellurides, native metals, and silver sulphides), antimony,
lead, and zinc minerals are characteristic, together with alunite,
adularia, and barite. Metallic copper also is not infrequent. Very often
the gangue material is more largely calcite than quartz, whereas calcite
is not present in the deep zone.[5]

The trend of evidence in recent years has favored the conclusion that
the principal ores associated with igneous rocks have not developed at
very great depths. Even within our narrow range of observation there is
a difference in favor of the shallower depths, and the greatest depths
we can observe are after all but trivial on the scale of the earth.

A survey of the ore deposits of Utah has suggested the generalization
that ores are more commonly related to intrusive stocks than to the
forms known as laccoliths, and that within and about intrusiv