Kurt C. Friehauf, Dept. of Geological and Environmental Sciences, Stanford University, Stanford CA 94305-2115
Copper veins in the Superior district closely resemble quartz monzonite-hosted "Main stage veins" at Butte (MT), Tintic (UT), Yauricocha (Peru), and Chuquicamata (Chile) in vein filling mineralogy and associated alteration. Carbonate-hosted ores in the Superior district are similar to the carbonate-hosted ores of Bisbee (AZ) and Yauricocha (Peru). The association of Butte, Yauricocha, Chuquicamata, and Bisbee with porphyry copper systems suggests Superior is related to an as-yet undiscovered porphyry copper system.
East-dipping Paleozoic carbonates host the stratabound massive specularite-copper sulfide and Pb-Zn sulfide replacement ("manto") ores where E-W-striking, enargite-bearing veins (e.g. Magma vein) intersect favorable carbonate strata. Favorable strata occur near the base of the Devonian Martin Formation (5 m thick "A-bed" dolostone), the lower part of the Mississippian Escabrosa Limestone (< 60 m thick dolomitic and calcitic "C-bed"), and below and above the shale at the base of the Pennsylvanian Naco Limestone (5 m thick "D-" and E-bed"). "Nonfavorable" limestone and dolostone beds that lie between the favorable horizons show little visible evidence of fluid-rock interaction more than a meter from the feeder veins.
Footwall contacts of mantos are locally discordant (on the order of a meter), but hanging wall contacts are discordant by up to tens of meters. Portions of mantos tend to be either specularite- or sulfide-dominant with sharp (< 25 cm) contacts between zones. Early replacement of carbonates by massive specular hematite with 5-15% disseminated pyrite and chalcopyrite was followed by the formation of massive pyrite-chalcopyrite ± bornite replacement veins and mantos within the specularite body. The time-integrated mineral association zoning is from central bornite+chalcopyrite+pyrite+quartz outward to pyrite-chalcopyrite-quartz to specularite+pyrite+chalcopyrite. Small, isolated massive galena-sphalerite-pyrite-quartz pods within limestones/dolostones occur peripheral to copper orebodies.
Wall-rock alteration adjacent to specularite-copper sulfide mantos is characterized by mm-scale white dolomite or calcite veinlets ± quartz-specularite veinlets with bleached halos within meters of the contact. At one locality, a latite porphyry dike in contact with a manto is pervasively altered to sericite-pyrite and chlorite and is cut by quartz-sericite-pyrite, specularite, and quartz-adularia ± chlorite veins. XRD analyses of siliceous sulfide breccias in the central zone of the C-bed orebody indicate the presence of dickite and zunyite -- minerals typical of hypogene advanced argillic alteration in quartzofeldspathic rocks in other Butte-type systems -- suggesting manto ores at Superior represent the carbonate-hosted analogues of advanced argillic alteration.
