Controls On Tertiary Epithermal Gold-Silver Mineralization, Northern Black Hills, South Dakota
Jack A. Redden, 1990. "Controls On Tertiary Epithermal Gold-Silver Mineralization, Northern Black Hills, South Dakota", Metallogeny of Gold in the Black Hills, South Dakota, Colin J. Paterson, Alvis L. Lisenbee, Tommy B. Thompson
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Tertiary gold-silver deposits in the Lead-Deadwood area have been exploited almost as long as the Precambrian Homestake gold deposit. The deposits were first described by Irving (1904) who was able to visit and map many of the mines. Recent papers by Shapiro and Gries (1970), Norton (1989), Paterson et al. (1988), and Redden and French (1989) summarize and update data on the deposits. The major mines of the late 1800's-early 1900's were developed along relatively small, high-grade replacement deposits of dolomitic sandstone beds at one or more levels in the Cambrian Deadwood Formation. Ore extended laterally from obvious vertical feeder fractures or joints which were commonly referred to as "verticals". Some deposits were also influenced by the emplacement of igneous sills in the Deadwood Formation that acted as barriers or traps for hydrothermal solutions. Because mineralization was closely related to the vertical joints and the favorable host beds were of limited thickness, the Tertiary deposits were of limited size. Nevertheless, approximately 90,000 kg (3 million oz) of gold were recovered from such deposits by the time production essentially ceased about three decades ago.
With the major increase in gold price in the late 1970's and early 1980's, and the development of low cost open pit mining and heap leach processing, exploration was undertaken by several companies to determine whether there were low-grade, large, disseminated-type deposits which were amenable to low cost mining and leaching operations. At present four companies are operating open pit mines in this type of mineralization.
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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