HOMESTAKE GOLD ORE
The largest gold mine in the Americas was the
long-lived Homestake Mine in the town of Lead (pronounced "Leed"),
South Dakota, USA. Located in the Lead Window of the northern Black Hills
Uplift in western South Dakota, the Homestake Mine produced about 40 million
ounces of gold. The gold at Homestake is almost exclusively confined to
the Homestake Formation, a Paleoproterozoic (~1.9-2.0 billion years)
sedimentary unit that originally consisted of interbedded Mg-rich siderite iron
formation and marlstones.
The Homestake Formation has been strongly deformed
& multiply metamorphosed, and many of the original rocks were converted to
greenschists (cummingtonite schists). The gold has been interpreted as
having been originally deposited with the iron formation sediments by seafloor
volcanogenic exahalative processes. Slight metamorphic gold mobilization
and tight structural folding has resulted in the formation of auriferous
greenschist pods along fold axes.
Auriferous greenschist (8.5 cm across) from the Homestake Mine, town of
Lead, northern Black Hills, western South Dakota, USA. Two small masses
of native gold (Au) are visible near the bottom margin.
Auriferous greenschist (closeup of above rock; field of view ~3.0 cm across)
from the Homestake Mine, town of Lead, northern Black Hills, western South
Dakota, USA. Some small blebs of native gold (Au) are visible.
Homestake Mine gold ore (above & below) - numerous visible gold blebs in
high-grade gold ore sample from the Homestake Mine, town of Lead, northern
Black Hills, western South Dakota, USA. Above: ~3.5 cm
across. Below: ~1.25 cm across.
Specimen owned by Robert Klingensmith.
Info. mostly synthesized from:
Guilbert
& Park (1986) - The Geology of Ore Deposits. New York.
W.H. Freeman and Company. 985 pp.