Gold commonly occurs in nature as a single element—gold—but much native
gold has a small amount of some other element, such as silver, copper,
or iron, mixed with it.
Native gold is a shiny, yellow, metallic mineral that does not tarnish,
and it leaves a shiny, golden-yellow streak when you rub it across a
streak plate. If silver is present, the color and streak have a lighter
shade. Pure gold is extremely heavy—its specific gravity is 19.3.
Because it is malleable, this mineral will flatten into a thin sheet
when hammered. It is ductile enough to be drawn out into wires. Gold is
also soft—a pocket knife will scratch it easily. When it is to be used
for ornaments and jewelry, gold is usually mixed with other metal, such
as silver, copper, nickel, or palladium, to make it harder. The amount
of gold that is present is then indicated by _carats_ (or _karats_).
Pure gold is 24 carats. If you have a gold ring that has _14 K_ stamped
inside it, you know that it is made of a mixture of 14 parts gold and 10
parts of other metal.
Gold commonly occurs in nature as plates, scales, or grains. Some of the
grains are large enough to be called _nuggets_. It also is found in a
wire-like shape described as _filiform_, it occurs in a network, called
_reticulate_, and it can have a branching and fern-like shape, described
as _dendritic_. Gold is not often found as individual crystals.
Several other minerals, such as pyrite, chalcopyrite, and mica, are
sometimes mistaken for gold. None of these, however, is malleable and
ductile, and none is nearly as heavy as gold. Pyrite and chalcopyrite
have dark-colored streaks unlike that of gold. Mica cleaves so perfectly
that it can be split into thin, flat sheets, but gold has no cleavage at
all.
The best places to look for gold are in areas near igneous rocks and
along the creeks and rivers that drain these areas. It is thought that
most gold originally was carried up from molten igneous rock by hot
solutions. The solutions moved into cracks and other openings in nearby
rocks and deposited the gold, commonly along with quartz. Later, some of
these gold-bearing rocks weathered away. The gold that the rocks
contained either remained at the spot or was washed into creeks and
rivers. These transported accumulations of loose gold are called _placer
deposits_.
[Illustration: Placer gold in very small quantities has been found in
some of the stream gravels of Texas.]
No really important gold deposit has ever been found in Texas, although
traces and small amounts have been reported in several areas. A little
gold has been found in the Llano uplift area of central Texas. It occurs
in quartz veinlets that cut through some of the Precambrian metamorphic
rocks of Llano, Mason, northeastern Gillespie, and west-central Burnet
counties. Many years ago, a small amount of gold was mined northeast of
Llano in Llano County from the Heath mine. Some gold also has been found
in sands and gravels along streams, such as along Sandy Creek and its
tributaries, in parts of this Llano uplift area.
In the Trans-Pecos country of west Texas, small amounts of gold have
been found in the Van Horn area of Culberson and Hudspeth counties, in
the Quitman Mountains district of Hudspeth County, and in the country
around Shafter in Presidio County. Most of the small quantity of gold
that was mined in west Texas was obtained as a by-product from the
Presidio mine in the Shafter district (described with silver minerals on
p. 90).
Small amounts of gold have been reported from other parts of Texas. Some
of these localities are in Eocene Tertiary sandstones in the Gulf
Coastal Plain, in Cretaceous limestones in Irion, Uvalde, and Williamson
counties, and in sand and gravel in Howard and Taylor counties. None of
these deposits has been found to have any commercial value.