Day One - Papers
Auriferous quartz veins cut Lower Proterozoic metasedimentary strata at a number of localities in the Black Hills (Paterson et al., 1988). The majority of these occurrences are in metamorphosed graywaeke, with some occurrences in metamorphosed shale. Many veins occur in metamorphic rocks of relatively low metamoiphic grade (garnet zone or lower) that are far removed from known intrusions of granitic rocks, and a metamorphic origin for the fluids and gold seems probable (e.g., Henley et al., 1976; Kerrich and Fryer, 1979; Paterson, 1986). For other veins, in higher-grade metamorphic rocks that are associated with numerous intrusions of Harney Peak Granite and pegmatites, the origin of the vein-forming fluids and gold is more ambiguous, and both metamorphic and magmatic components must be considered.
The purpose of this review is to summarize results of fluid inclusion studies on some of the gold-quartz veins in metamorphic rocks in the central Black Hills. The ten veins considered in this report range from the lower garnet zone through the sillimanite zone (Fig. 1). Fluid inclusion results are examined with reference to variations in host rock lithology and metamorphic grade, and with respect to proximity of the veins to exposed bodies of granite and pegmatite.
Figures & Tables
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