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, 1989. "A View through an Epithermal-Mesothermal Precious Metal System in the Northern Black Hills, South Dakota: A Magmatic Origin for the Ore-Forming Fluids", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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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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When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.
There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits.
Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in