A View through an Epithermal-Mesothermal Precious Metal System in the Northern Black Hills, South Dakota: A Magmatic Origin for the Ore-Forming Fluids
Colin J. Paterson, Nuri Uzunlar, J. Groff, F. J. Longstaffe, 1990. "A View through an Epithermal-Mesothermal Precious Metal System in the Northern Black Hills, South Dakota: A Magmatic Origin for the Ore-Forming Fluids", Metallogeny of Gold in the Black Hills, South Dakota, Colin J. Paterson, Alvis L. Lisenbee, Tommy B. Thompson
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In the northern Black Hills, epithermal to mesothermal Au-Ag-(Pb)-(W) deposits of the sediment-hosted type and the intrusion-hosted type are spatially and temporally associated with an east-west zone of Tertiary (40-60 Ma) alkalic igneous intrusions. Considerable structural relief, together with underground exposure in the Homestake mine, provides a 3-km vertical profile through the Tertiary hydrothermal system.
Gold-silver mineralization occurs throughout the system from thick quartz-pyrite ± galena ± chalcopyrite ± sphalerite ± fluorite ± anhydrite ± biotite ± molybdenite ± cosalite veins in Precambrian schist at depth, to quartz-pyrite-fluorite veinlets and disseminated pyrite in igneous stocks, to silicified arsenian pyrite-marcasite replacement mantos adjacent to vertical fractures in lower Paleozoic sedimentary rocks (calcareous and dolomitic sandstones, limestones) nearer the surface. Stratigraphic reconstruction allows estimation of the range of maximum depths of mineralization from 1.3 to 4 km (0.35-1.1 kbars).
Fluid inclusions in quartz and fluorite in these deposits are diverse. Most fluids have low apparent salinity (>10 equiv wt % NaCl), but saline fluids (up to 63 wt %) and CO2-rich fluids occur deeper in the composite system. Fluid inclusion trapping temperatures range from 400° to 750°C deep in the system to 170° to 240°C at higher levels.
The δ18O and δD values for the fluids are 6.2 to 11.6 and —53 to —75 per mil, respectively. The isotopic and fluid inclusion data together suggest that magmatic water was an important component of the ore-forming fluids. This is in contrast with most epithermal systems which are dominated by meteoric water. The implication is that the alkalic igneous intrusions were the source for most of the gold. The presence of lower δ18O values at shallower levels (about 1-km depth), the abrupt decrease in trapping temperatures, and the gradation in fluid salinities suggest that meteoric waters in the aquifers of the basal Paleozoic sequence may have mixed with the ore fluids. This fluid mixing was a likely cause of gold deposition.
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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