Ore Paragenesis And Fluids In The Gilt Edge Deposit, South Dakota
J. Groff, C. J. Paterson, 1990. "Ore Paragenesis And Fluids In The Gilt Edge Deposit, South Dakota", Metallogeny of Gold in the Black Hills, South Dakota, Colin J. Paterson, Alvis L. Lisenbee, Tommy B. Thompson
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The Gilt Edge Au-Ag porphyry-related deposit is located in the northern Black Hills of South Dakota, approximately 8 km southeast of Lead (Fig. 1). The deposit is part of a larger east-west trending belt of Tertiary (38.8-60.5 Ma) alkalic intrusions (Shapiro, 1971). Intrusions at the Gilt Edge deposit include hornblende trachyte, trachyte, and quartz trachyte porphyry (MacLeod, 1986). Fracture zones trending NW-SE, NE-SW, and N-S traverse the deposit and may represent pre-existing structures in the Precambrian basement that were reactivated during the Tertiary igneous activity in the Gilt Edge area (MacLeod and Barron, 1990, this volume).
Gold mineralization is contained in breccias, along lithologic contacts, and in fracture zones. Intermittent mining from the late 1800's to the 1940's produced 2675 kg (86,000 oz) of gold (Norton, 1974) from breccias and fracture zones in trachyte and quartz trachyte porphyry. Active mining of the deposit resumed in 1988, with current reserves of 49.0 million tonnes at 1.28 g/t (0.041 oz/t) gold (Barron, 1990).
The origin of ore-forming fluids in the Tertiary hydrothermal systems in the Black Hills has been proposed to be either largely meteoric water (Irving, 1904) or magmatic fluids derived from the intrusions (Connolly, 1927). Paterson et al. (1989) suggested that the ore-forming fluids contained a large component of magmatic fluid, but mixing with meteoric waters above the Precambrian unconformity is suggested by isotopic and fluid inclusion data. The source of gold at the Gilt Edge deposit has been proposed to be the quartz trachyte stocks (Mukherjee, 1968), or magmatic
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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