Abstract

The Sundance gold deposit is hosted by Tertiary intrusion breccias emplaced into Archean granites and Tertiary alkaline volcanic rocks of the Bear Lodge domal complex. The host breccia is matrix supported and contains clasts of syenitic, trachytic, and latitic lithologies as well as clasts from the pre-Tertiary rocks including Deadwood Quartzite, Archean granite, and schists and amphibolites of unknown age.

Four alteration phases and associated alteration products characterize the deposit. Desilication was the first phase, producing a porous vuggy texture. This texture is the result of hydrothermal solutions containing sufficient hydrofluoric acid to dissolve free quartz and high silica-bearing minerals and clasts. Fluorite resulted from the second alteration phase and postdates desilication, occurring as replacement of minerals and clasts and as small crystals in vugs. Euhedral orthoclase characterizes the third alteration phase, as veins and veinlets and occupying vugs and fractures left by desilication but not filled by fluorite, and in veins and veinlets. The distribution of this secondary orthoclase through the deposit is irregular and may reflect proximity to discrete feeder structures. Clays are a product of the fourth alteration phase and form an irregular cap to the deposit that is 20 to 50 feet thick.

Deep hypogene oxidation postdates the four alteration phases and may be coincident with ore deposition. Iron oxides locally replace pyrite but mostly appear to reflect deposition of primary ferrihydroxides. Microprobe and SEM work indicate gold occurs intimately with iron oxides in submicron to micron sized grains.