Another hypothesis suggests that gold-bearing

Another hypothesis suggests that gold-bearing
solutions may be expelled from magma as it cools,
precipitating ore materials as they move into cooler
surrounding rocks. This hypothesis is applied particularly
to gold deposits located in or near masses of
granitic rock, which represent solidified magma.
A third hypothesis is applied mainly to gold-bearing
veins in metamorphic rocks that occur in mountain
belts at continental margins. In the mountain-building
process, sedimentary and volcanic rocks may be

deeply buried or thrust under the edge of the continent,
where they are subjected to high temperatures
and pressures resulting in chemical reactions that
change the rocks to new mineral assemblages
(metamorphism). This hypothesis suggests that wateris expelled from the rocks and migrates upward,
precipitating ore materials as pressures and temperatures
decease. The ore metals are thought to originate
from the rocks undergoing active metamorphism

The gold content of rocks is commonly determined by means of a
fire assay, a method known to metalworkers for 3,000 years or
more. In modern practice, a weighed sample of pulverized rock is
melted in a flux consisting of lead oxide, soda, borax, silica, and
flour or potassium nitrate, along with a measured amount of silver
as lead-silver alloy, in a furnace at a temperature of 1000° Celsius
(1800°F). The lead fraction contains the gold and added silver and
settles to cool as a button. The button is then remelted and oxidized
in a bone-ash cupel, which absorbs the lead oxide, leaving
behind a bead consisting of precious metals in the silver collector.
The bead is dissolved in acid and usually analyzed by atomic
absorption spectrometry.

Gold nugget weighing 81.9 troy ounces from the Union Placer
mine near Greenville Plumas County, California (photo courtesy of
Smithsonian Institution).