The Homestake Mine, Lead, South Dakota: An Overview
R. L. Bachman, S. W. Caddey, 1990. "The Homestake Mine, Lead, South Dakota: An Overview", Metallogeny of Gold in the Black Hills, South Dakota, Colin J. Paterson, Alvis L. Lisenbee, Tommy B. Thompson
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The Homestake mine in the northern Black Hills, South Dakota is the largest iron-formation-hosted gold deposit known (Fig. 1), and has produced 1,113 tonnes (35.8 million oz.) of gold from 128 million tonnes of ore milled. In 1989 the mine produced 11.9 tonnes of gold. The deposit was discovered in 1876, and the mine has operated continuously to the present day (Fig. 2). Ore is currently mined from depths to 2,438 meters. Gold is the major commodity produced along with a minor silver by-product. The gold/silver ratio averages 5:1; base metal content is negligible.
The Homestake gold deposit is hosted within quartz-veined, sulfide-rich segments of an early Proterozoic, carbonate-facies iron-formation in a sequence of originally calcareous, pelitic, and quartzose rocks (Fig. 3). Strata that contain the Homestake deposit were complexly deformed by a series of tight isoclinal and sheath fold events, and synchronous, extensive ductile and ductile-brittle shearing. Mine area rocks have been subjected to upper greenschist-lower amphibolite facies metamorphism. Early-stage ductile shearing appears to have controlled fluid movement in the system thereby controlling thermal energy and metamorphic processes. Observed prograde metamorphism in the district is therefore characterized as dynamothermal. Intrusion of a 1.72 Ga Harney Peak-type granite in an area northeast of the mine post -dated regional prograde metamorphism, and appears to be contemporaneous with later stages of semi-brittle deformation (Bachman et al., 1990).
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Since the discovery of gold in 1874, the Black Hills has been well-known as a gold metallogenic province. In fact it is one of the richest areas in the world, having produced approximately 354 kg/km2 (31,750 oz/mile2). The premier mine in this province, theHomestake mine, is one of the oldest and longest -operating in the world, having been in production since 1876. Furthermore, the variety of gold deposit types in such a small area is unique. These include Au-U quartz pebble conglomerate deposits of early Proterozoic age, iron-formation-hosted and quartz vein gold deposits of middle Proterozoic age, paleoplacer Au in basal conglomerates of Cambrian age, epithermal igneous-hosted and sediment-hosted Au-Ag deposits of early Tertiary age, and recent gold placer deposits (see summary of gold deposits in Paterson et al., 1988; reprinted in this volume).
Although the history of mining here is a long one, the origins of the Homestake and other gold deposits in the Black Hills are yet to be fully explained. This is not a result of lack of interest or investigation. Significant studies regarding these deposits were conducted as long ago as 1904 by Irving, and subsequently by Connolly (1927) and Connolly and O'Harra (1929), and by Noble (1950) and Noble and Harder (1948). Then as now, there were opposing schools of thought regarding the origins of the various deposit types. For example, for the Tertiary sediment-hosted replacement deposits, Irving (1904) favored ore deposition from meteoric waters heated by the Tertiary igneous intrusions, whereas Connolly (1927) was a proponent of the magmatic-hydrothermal origin for the sediment-hosted replacement gold-silver deposits. Simultaneously, it was recognized that there were important structural and stratigraphic controls on ore localization, and that the mechanics of the sill and laccolith emplacement influenced the continuity and distribution of ores.
There remain many important questions to be answered regarding the origin and distribution of the gold deposits in the Black Hills. We summarize here some of the more important ones for your consideration during this field conference.
Is the Homestake deposit epigenetic (Noble, 1950; Slaughter, 1968; Bachman and Caddey*; Kath and Redden) or syngenetic but later remobilized (Rye and Rye, 1974; Rogers)? There is no consensus here, even among geologists working directly or indirectly with the Homestake Mining Company. There is general agreement however, that the mineralization is Proterozoic in age (Bachman and Caddey), and not Tertiary as reported inadvertently in the introduction to the