FRIEHAUF, Kurt. C., Dept. of Geological and Environmental Sciences, Stanford University, Stanford CA 94305-2115, kurt@pangea.Stanford.edu.
click here to jump to more detailed report with photos
Carbonate-hosted massive replacement Cu-Au ores of the Bingham, Superior, and Bisbee districts show a similarity in their opaque mineralogy, mineral zoning, ore textures, alteration of feldspathic rocks, and metal grades, suggesting a common genetic model for these late-stage ores related to porphyry copper plutons. The differences in major non-opaque gangue mineralogy (Bingham - talc+carbonate, Superior - quartz or none, Bisbee - carbonate+quartz+chlorite) may be due to differences in host rock composition (limestone, dolomite, and argillaceous limestone, respectively) and/or the distance from porphyry.
Replacement bodies are zoned from core to margin: 1) low-grade massive pyrite, 2) high-grade Cu-Fe-sulfide (bornite-chalcocite, bornite-chalcopyrite, pyrite- chalcopyrite), 3) specular hematite±pyrite-chalcopyrite. Sn- and W-bearing minerals are common accessories. Characteristic ore textures include mm-scale rhythmic mineral layering (hematite // pyrite-chalcopyrite // hematite // ...) and sieve-textured pyrite with anhydrite inclusions. Feldspathic rocks adjacent to the orebodies at Superior and Bisbee were altered to kaolinite- or sericite-bearing assemblages, with local pyrophyllite or zunyite. Typical metal grades are 2.5-6 % Cu, 1 ppm Au, and 5-30 ppm Ag.
The mineral zonation in these deposits probably reflects pH gradients between oxidized, acidic ore fluids and carbonate wall rocks. Preservation of anhydrite inclusions within pyrite suggests these ores may have contained abundant anhydrite during early stages of their formation (due to reaction of oxidized sulfur-bearing fluids with calcic host rocks) which has since been removed by later hydrothermal solutions or by deeply-circulating post- mineralization fluids.
The occurrence of carbonate-hosted massive sulfide/specular hematite replacement Cu-Au ores (e.g. Tintic, UT; and Yauricocha, Perú) could be interpreted as evidence for undiscovered, related porphyry copper deposits nearby (within 1-2 km).
Key words (choose 5): Bingham, Superior, Bisbee, copper, gold
abstract number 50178