(photos by James St. John; photo stitch by Mary Ellen St. John)

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.

Shown above is the open pit portion of the Homestake Mine. Near the center are steeply-dipping, light-colored bands of rock. These are Tertiary-aged rhyolitic igneous dikes - planar to irregularly-planar igneous intrusions that cut across country rocks. They can be thin or extremely thick & they can be vertical or obliquely-oriented.