Tertiary Epithermal To Mesothermal Porphyry-Related Au-Ag Mineralization In The Homestake Mine, Lead, South Dakota: Mineral And Metal Zoning
Uzunlar Nuri, Colin J. Paterson, Alvis L. Lisenbee, 1990. "Tertiary Epithermal To Mesothermal Porphyry-Related Au-Ag Mineralization In The Homestake Mine, Lead, South Dakota: Mineral And Metal Zoning", 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 at Lead has produced gold since 1876 from a Precambrian iron-formation-hosted Au deposit (Bachman and Caddey, 1990, this volume). However, a distinct and separate epithermal to mesothermal Au- Ag-sulfide vein mineralization occurs in the mine and is spatially associated with a Tertiary rhyolitic dike complex. The mineral and metal associations, vein textures, and the crosscutting relationship with Tertiary rhyolite dikes distinguish these Tertiary veins from the Precambrian quartz veins and iron-formation-hosted gold deposits. Underground exposures to a depth of 3 km (9000 feet) allow documentation of vertical and lateral zoning of the Tertiary mineralization.
The purpose of this paper is to report mineralogic assemblages of the Tertiary epithermal-mesothermal veins, mineral and metal zoning, and timing of the veins based on crosscutting relationship and mineral assemblages.
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Metallogeny of Gold in the Black Hills, South Dakota
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