Geology Of The Annie Creek Sediment- And Porphyry-Hosted Gold Deposit
James F. Lessard, Thomas A. Loomis, 1990. "Geology Of The Annie Creek Sediment- And Porphyry-Hosted Gold Deposit", Metallogeny of Gold in the Black Hills, South Dakota, Colin J. Paterson, Alvis L. Lisenbee, Tommy B. Thompson
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The Annie Creek mine is a Au-Ag hydrothermal replacement deposit hosted within the Cambrian Deadwood Formation and in intermediate and alkalic Tertiary (40-60 Ma) intrusives. The deposit is located in a well-defined belt of Tertiary intrusives in the northern Black Hills. The reader is referred to Redden and Lisenbee (1990, this volume) for discussion on regional geology.
The Annie Creek deposit is located approximately 8 km southwest of Lead, in Lawrence County, South Dakota (Fig. 1). The deposit is on the western edge of the Portland mining district which is part of the larger Bald Mountain mining district. Production from the Portland mining district between 1877 and 1959 is estimated at a minimum of 24,335 kg (785,000 oz) gold and 65,100 kg (2,100,000 oz) silver; the Annie Creek mine produced approximately 837 kg (27,000 oz) gold between 1906 and 1916 (Shapiro and Gries, 1970).
In 1983, Wharf Resources initiated production from the Annie Creek deposit and produced 1953 kg (63,000 oz) gold by the end of 1989. Approximately 775 kg (25,000 oz) gold remain to be mined before the pit area is backfilled in the mid-1990's.
Mining is concurrent at the adjacent Foley Ridge mine, where similar sediment- and porphyry-hosted gold deposits occur (Fig. 2). The ore at both locations is mined by open pit methods, and gold is produced by heap leach/carbon adsorption recovery. Together, Wharrs Annie Creek and Foley Ridge mines have produced over 7750 kg (250,000 oz) 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