e is mainly quartz.

At Juneau, Alaska, great dikes of albite-diorite intrude greenstones and
schists, and low-grade gold ores occur in shattered portions of the
diorite. These ores were mined on a great scale at the Treadwell Mine.

Another famous low-grade deposit is the Homestake Mine in the Black
Hills of South Dakota, where pre-Cambrian slates and schists of
sedimentary origin are impregnated with gold, associated with quartz,
dolomite, calcite, pyrite, and other minerals. The origin is supposed to
have some connection with intrusives into the schists; but the relations
of the ores to intrusives, both in age and in place, present many
puzzling questions which make conclusion as to origin very difficult.

In the Cripple Creek district of Colorado, a volcanic neck two or three
miles in diameter breaks through pre-Cambrian granites, gneisses, and
schists. The volcanic rocks consist mainly of tuffs and breccias cut by
basic dikes. The ore bodies are in fissures and sheeted zones,
principally in the granitic rocks, but associated with these dikes. The
ore is mainly gold telluride, in a gangue of quartz together with pyrite
and a variety of minerals characteristic of hot-water solutions. Also
the wall rocks have the characteristic hot-water alterations. There is
slight enrichment near the surface.

At Goldfield, Nevada, native gold is found in surface igneous flows of a
dacite type, which have undergone extensive hydrothermal alterations
characterized by the development of alunite (a potassium-aluminum
sulphate), quartz, and pyrite. The ore fills fissures to some extent,
but is mainly a replacement of the wall rock. Association with typical
hot-water minerals and hydrothermal alterations of the wall rock are
again believed to indicate the origin of the ores through ascending hot
solutions from a deep source.

One of the interesting features of this occurrence is the abundance of
alunite. Sulphate minerals are commonly formed by oxidizing solutions.
The abundant presence, therefore, of a sulphate mineral with minerals of
a primary deep-seated source has led to much discussion of origin. The
hypothesis was developed that these minerals result from the interaction
of deep-seated sulphide-bearing solutions with surface oxidizing
solutions.[35] It may be noted that in recent years other sulphate
minerals have been occasionally regarded as primary, including gypsum,
anhydrite, barite, and others. It has been suggested that