CONTROLS ON PREFERRED LOCALIZATION OF GOLD ORE IN SYNCLINES WITHIN THE HOMESTAKE MINE, LEAD, SOUTH DAKOTA, USA

The Homestake iron-formation-hosted deposit produced more than 40 million ounces of gold between 1876 and 2001, and is regarded as a giant. The controls on this deposit are still in question. The Proterozoic host sequence is tightly to isoclinally folded in a structure that generally plunges southeasterly. Several hypotheses have been proposed over the years that gold deposition was structurally controlled. Mineralization is spatially associated with quartz veins and shear zones, but the relationship of ore bodies to the fold structures is intriguing. In-mine exploration focused on synclines, and almost all of the gold ore was mined from the synclinal hinges (odd-numbered ledges) of the Homestake Formation; the anticlinal hinges (even-numbered ledges) were rarely mined, and by implication were less favored for ore deposition. This study focuses on the structural characteristics of ledges 6 through 9, including the volume percent of quartz veining, variability of thickness of the Homestake Formation, fold geometry, and gold grade distribution. To accomplish this, 200 scale and 50 scale maps were used to compare and contrast the differences between the synclinal and anticlinal ledges. Drill logs were used to determine variation of thicknesses of the Homestake Formation.