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Abstract

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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