The rock the prospectors saw near Virginia City was a rock
(hydrothermally altered) that had turned white and red from hot
waters that had washed through the rocks about 14 million years
ago. This photograph from the road over the Virginia Range to
Virginia City, the Geiger Grade, shows such hydrothermally
altered rocks. This modern map of the geology of
the area around Virginia City, used by permission of the NBMG
(Purkey, B.W. and Garside, L.J., Geologic
and Natural History Tours in the Reno Area, NBMG Special
Publication 19, University of Nevada, 1995), shows the heavily
altered rocks in a large stippling.
The hot waters that wash through the rocks can carry precious
metals dissolved out of the rocks in the surrounding area. The
hot waters lose these precious metals from the solution when
conditions of the rocks through which the waters are moving
change. The temperature can drop, or the rock type can promote
the precipitation of the metal either as a compound (often with
sulfur) or by reducing the metal in solution to the free metal.
As discussed in the Chemistry section of Ore Processing, silver was generally
combined with sulfur in the Virginia City ores. In this way,
precious metals that were scattered throughout the rocks were
concentrated in an ore deposit. The deposit at Virginia City was
called the Comstock Lode.
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