Day Two The Homestake Iron-Formation-Hosted Gold Deposit: An Introduction
Colin J. Paterson, 1990. "Day Two The Homestake Iron-Formation-Hosted Gold Deposit: An Introduction", Metallogeny of Gold in the Black Hills, South Dakota, Colin J. Paterson, Alvis L. Lisenbee, Tommy B. Thompson
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This day's venture involves the Homestake mine and its environs. The morning trip is an underground tour of the Homestake mine to examine the styles of mineralization. In the afternoon we will examine the surface geology of the Lead area in order to provide a general overview of the stratigraphic, metamorphic, and structural features in the Lead area. An attempt will be made to provide a three dimensional perspective of the gold deposit on each of the stops by referencing to the underground mine.
The Homestake mine has been operating continuously since 1876. Over 126 million tonnes of ore have been mined, and during the summer of 1990, the 36 millionth ounce (1116 tonnes) of gold was poured. The Homestake deposit is an iron-formation-hosted gold deposit of epigenetic origin and Early Proterozoic age (see Bachman and Caddey, 1990, this volume). Gold occurs within a folded and sheared iron-formation known as the Homestake Formation, which has an approximate age of 1.97 Ga. The Homestake Formation is typically a carbonate-facies iron formation, dominated by siderite. Locally, pyrrhotite blebs and bands parallel the bedding. Above the garnet isograd (east of a line approximately along the axis of the open cut), the iron formation is manifested as a cummingtonite- grunerite schist. This transition is very erratic, however, and cummingtonite can even be found in biotite zone rocks in the west wall of the Open Cut. Quartz veins and associated arsenopyrite (Fig. 1), chlorite, and pyrrhotite in the Homestake Formation are direct indicators of gold
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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