Auriferous sulfidic chlorite schist from the Precambrian of South Dakota, USA. (public display, South Dakota School of Mines and Technology Museum of Geology, Rapid City, South Dakota, USA)

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. 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.

The rock shown above is a auriferous sulfidic chlorite schist. The dark greenish-gray material is chamosite chlorite, which makes up the bulk of the rock. The metallic-lustered areas are the sulfide minerals pyrrhotite (Fe1-xS - imperfect iron monosulfide) and arsenopyrite (FeAsS - iron arsenic sulfide). Gold in the above specimen mostly occurs as smaller-than-0.5 millimeter-sized masses associated with sulfide minerals and chlorite.