Where most byproduct gold has come from

Most byproduct gold has come from porphyry
deposits, which are so large that even though they
contain only a small amount of gold per ton of ore, so
much rock is mined that a substantial amount of gold
is recovered. The largest single source of byproduct
gold in the United States is the porphyry deposit at
Bingham Canyon, Utah, which has produced about 18
million troy ounces of gold since 1906.
Geologists examine all factors controlling the origin
and emplacement of mineral deposits, including those
containing gold. Igneous and metamorphic rocks are
studied in the field and in the laboratory to gain an
understanding of how they came to their present
location, how they crystallized to solid rock, and
how mineral-bearing solutions formed within them.
Studies of rock structures, such as folds, faults,
fractures, and joints, and of the effects of heat and
pressure on rocks suggest why and where fractures
occurred and where veins might be found. Studies of
weathering processes and transportation of rock debris
by water enable geologists to predict the most likely
places for placer deposits to form.
The occurrence of gold is not capricious; its presence
in various rocks and its occurrence under differing
environmental conditions follow natural laws. As
geologists increase their knowledge of the mineralizing
processes, they improve their ability to find gold.