artz-diabase
sill. Locally they penetrate the sill. Native silver and various silver
sulphides, arsenides, and antimonides are associated with minerals of
cobalt, nickel, bismuth, lead, and zinc, in a gangue of calcite and some
quartz. The ore is of very high grade. The ore minerals are believed to
have been deposited by hot solutions emanating from deep magmatic
sources after the intrusion of the diabase. The present oxidized zone is
very shallow, but may have been deeper before being stripped off by
glaciation; it is characterized by native silver and arsenates of nickel
and cobalt in the form of the green "nickel bloom" and the pink "cobalt
bloom." The silver minerals are distinctly later in origin than the
cobalt and nickel in the unoxidized zone, as evidenced by the relations
of the mineral individuals when seen under the microscope. This fact,
together with the abundance of native silver in the oxide zone, has
suggested downward concentration of the silver by surface waters; but
recent studies have indicated the probability that some of the silver at
least was deposited by the later ascending solutions of magmatic origin.

In the Tintic district of central Utah, Paleozoic limestones have been
intruded by monzonite (an acid granitic or porphyritic igneous rock),
and covered by surface flows, the flows for the most part having been
removed by subsequent erosion. The sediments have been much folded and
faulted, and the ore bodies occur as fissure veins which locally widen
into chimneys or pipes in fracture zones, accompanied by much
replacement of limestone. There is a rough zonal arrangement of the ore
minerals around the intrusive, gold and copper minerals (chiefly
enargite and chalcopyrite) being more prominent near the intrusive, and
argentiferous galena and zinc blende richer at greater distances. Silver
constitutes the principal value. The gangue is mainly fine-grained
quartz or jasperoid, and barite. The water table is at unusually great
depths (2,400 feet) and there is a correspondingly deep oxidized zone,
which is characterized by lead and zinc oxide minerals much as at
Leadville (p. 219).

The Comstock Lode at Virginia City, Nevada, on the east slope of the
Sierra Nevadas, was one of the most famous bonanza deposits of gold and
silver in the world. While the richer ore has all been extracted,
lower-grade material is still being mined and the fissure is still being
followed, in the hope of some day striking