Proterozoic syngenetic massive sulfide deposits in the Gunnison gold belt, Colorado
— P. A. Drobeck

Proterozoic rocks exposed in the region south of Gunnison, Colorado contain precious-metal and base-metal massive sulfide deposits, which have been referred to in the past as the Gunnison Gold Belt. Work by several authors (Afifi, this guidebook; Drobeck, 1979; Riesmeyer, 1978; Beaty and Zahoney, 1977; Hartley, 1976; Trost, 1975) provides compelling evidence that these deposits were originally formed by subaqueous fumarolic activity during deposition of the local statigraphic successions. A feature that sets these deposits apart from many other districts of syngenetic, sub-aqueous, fumarolic-volcanogenic deposits is the variety of sub-aqueous environments in which the Gunnison deposits occur. Associations include: 1) within a series of basalt and basaltic-andesite flows (Ironcap Mine), 2) in a chert horizon near the contact of komatiitic flowrock and arkosic sediments (Gunnison Mine), 3) within a sequence of rhyolite pyroturbidites (Denver City Mine), 4) along a disconformity within argillites and siltites (Yukon-Alaska Mine), and 5) within a sequence of felsic water laid tuffs and flows (Vulcan Mine) (fig. 1). The lack of a preferred stratigraphic occurrence hampers exploration evaluations within the region.

 

The deposits commonly contain a simple ore assemblage with pyrite and sphalerite comprising 15 to 90 percent of the ore (usually 80-95 percent of the sulfides). Quartz ± chlorite ± calcite ± dolomite gangue usually comprises 20 to 80 percent of the ore. Chalcopyrite and pyrrhotite are usually present as accessories (rarely as much as 10 percent together) and galena is commonly absent, or less than 2 percent. Silver and gold occur in anomalous concentrations in the ores, but only a few samples showed precious metals values of interest as ores (as high as 2 oz/T Ag and .15 oz/T Au at the Ironcap deposit).

 

Lithogeochemical samples from mineralized areas in the belt were analyzed for Cu, Pb, Zn, Ag, Au, Se, and Te using a leach-extraction technique (Clark and Viets, 1979) that measures concentrations of these elements not in silicates. Thus, the concentrations measured are less than a fire or wet chemical assay. This work showed that Cu commonly forms a large dispersion halo, especially in the stratigraphic footwall of the deposits. Pb, Zn, Au, Ag, and Se have narrower halos, being closely restricted to the visibly mineralized horizons (inferred to be the seawater interface at the time of mineralization). Te has different concentration patterns at the different deposits. Cu, Zn, Se, Ag, and Te are the most useful of the elements studied for mineral exploration here.