Summary:
Geological mapping and sampling in the Windy Gulch Segment of the Dawson Project led to the realization that all the rocks hosting the gold mineralization could be better characterized as aluminum-rich (peraluminous) intrusions that not only host the gold but were also the direct sources of the gold mineralization (Keith et al., 2016). As such, the peraluminous intrusions comprise a portion of a staged intrusive sequence. Each stage is associated with a specific magmato-hydrothermal greisen-like hydrothermalite rock that is variously biased for a specific metal suite ranging from copper dominated to gold dominated. When compared to the literature, the gold deposits at Dawson can be broadly described as “intrusion related gold deposits” as in the recent summary of the problem by Pertzel (2013), which is a reconstitution of the phenomena as anticipated by Spurr (1906). The development of the peraluminous portion of the model was discussed by Keith and Swan (1985), Swan and Keith (1986) and at a symposium on the subject chaired by MagmaChem in Denver (Swan and Keith, 1987).
Gold and base metal mineralization occur within an east-northeast trending, south-southwest-dipping fault in amphibolite-grade, Proterozoic-age metamorphic rocks. Strike-length of the entire mapped horizon is about 3.4 mi, of which about the eastern 1.6 mi, which is gold prospective, occurs within the Property.
East of the Windy Point Segment, mineralization is dominated by iron and copper sulphides with minor gold. The mineralized zone is apparently terminated on the east by the Front Range Reverse Thrust Fault, which juxtaposes Proterozoic rocks over Mesozoic rocks; it is terminated on the west by the Marsh Gulch Fault. The mineralized horizon is quite recognizable on the surface as a gossan, with limonite and hematite-stained siliceous rock from about 2 inches to about 10 ft wide and local malachite-azurite fracture-fill.
The mineralized host rock is predominantly a quartz-biotite gneissic aplite with variable amounts of iron-rich garnet and other silicates with local zones enriched in sulphides; it lies stratigraphically above the quartz-biotite-feldspar gneiss (Pbu/Pmu). The horizon has an average thickness of about 9.1 m, with gold occurring predominantly at several stratigraphic positions below the sulphide-bearing zones, but also at the base of the sulphide unit itself. The mineralized parts of the horizon are relatively discrete and range from approximately an inch to 49 ft in true thickness, with a sharp drop-off in metal values away from the contacts (Wilson, 1982).
The sulphide-rich zones are typically 10 to 50 modal percent base-metal sulphides in a fragmental chloritebiotite matrix with local quartz- or anthophyllite-rich zones. Typical gangue mineralogy includes quartz, biotite, phlogopite, garnet, magnetite, amphiboles, sillimanite, cordierite, anthophyllite, gahnite, staurolite, sericite, talc, hematite, limonite, and calcite. Sulphides include pyrite and chalcopyrite with lesser amounts of pyrrhotite, sphalerite, and galena plus rare gold, molybdenite, and bismuth sulphosalts. Later-stage talc and serpentine fill fractures in both the sulphide and auriferous zones.
Historical mineralogical and metallurgical studies of core by US Borax found that native gold occurs as flakes up to 1.4 mm in size. A hole wedge-drilled from GC40 by Uranerz in 1990 (GC40-W2) intersected visible gold particles up to 3 mm in size. Uranerz also had 26 core samples from the Dawson Segment mineralogically analyzed and found gold flakes up to 0.24 mm in size. The gold is typically associated with sillimanite, sericite, or biotite; it inhabits cracks in quartz and garnet, and along grain boundaries, between quartz and sillimanite, sericite, or biotite. Gold can be associated with carbonate and/or siliceous veinlets and rarely occurs as inclusions in pyrite or chalcopyrite (Mettler, 1991). Probing of selected polished sections from drill hole GC05 showed that gold occurs freely as blebs within biotite. Various workers have noted the “nuggety” nature of the gold mineralization. During metallurgical and flotation studies, most gold was found to liberate during processing, with a smaller percentage of gold grains attached to silicate or sulphide gangue (Ganderup and Woods, 1986).