Kurt C. Friehauf
Dept. of Geological and Environmental Sciences
Stanford CA 94305-2115
The presence of pre-mineral porphyry dikes, early pebble breccias, early skarn, later-stage high- sulfidation state massive copper sulfide-specularite replacement ores in carbonate rocks, and enargite- bearing Cu-Au veins suggest the manto ores of the Superior district are similar to the porphyry-related carbonate-hosted ores of Tintic (UT), Bisbee (AZ), Yauricocha (Peru), Morococha (Peru), and Cananea (Sonora) and the carbonate-hosted analogues of high-sulfidation base metal lode veins such as at Butte (MT), Bor (Yugoslavia), Recsk (Hungary), Srednegorje (Bulgaria), Chuquicamata (Chile), Lepanto (Philippines), and Nena (Papua New Guinea).
East-dipping Paleozoic carbonates in the Superior District (26 Mt 4.7% Cu, 1 ppm Au, 45 ppm Ag, Matt Knight, Pers. Comm., 1994) host garnet , amphibole, and talc skarn and 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 lithologies. 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"). Hydrothermal fluids did not react visibly with "non-favorable" carbonate strata.
Garnet-amphibole-pyrite-(sphalerite) skarn , followed by rhythmically-layered sphalerite-magnetite-talc bodies, pre-date specularite-copper sulfide manto formation. Garnet-bearing skarn occurs predominantly as minor east-striking veins (i.e. the same fracture set occupied by copper ores and pre- mineral porphyry dikes) with small mantos flaring out in favorable members of the "D-bed". Amphibole-bearing skarn consists of an early, forest green, fine-grained (2-3 mm grains) variety growing on and cutting compositionally-zoned garnet grains which is in turn cut by light green coarse-grained (10-15 mm) amphibole veins. No copper minerals precipitated during skarn formation. Garnet is weakly altered to soft, tan-colored clay and locally specularite where cut by pyrite-bornite ± specularite- calcite veinlets. Skarn amphiboles have been altered to talc. Rhythmically-banded talc-magnetite skarn replaces dolomitic units and locally contains talc- altered coarse-grained amphibole veins. Massive galena-pyrite-sphalerite-quartz mantos (< 1m thick) and replacement veins lacking associated talc alteration occur as separate bodies from the main specularite-copper-sulfide mantos. No magnetite or specularite and only traces of chalcopyrite precipitated with galena-bearing massive sulfide.
Massive specularite-sulfide mantos (5-60 m thick, < 90 m wide, and < 300 m down the dip) post- date skarn and consist of coarse-grained specular hematite, pyrite, chalcopyrite, bornite, minor chalcocite, and < 5% quartz. Rock types tend to be either specularite- or sulfide-dominant (> 80:< 20) with sharp (< 25 cm) chalcopyrite-rich contacts between types. Sulfide-dominant (2-4 mm granular pyrite-chalcopyrite (85:15) ± bornite) bodies occur as coalescing elongate pods (typically 6- 25 m) within a "sea" of specularite-dominant (80- 95% 1-5 mm specularite + 5-10% 2-5 mm pyrite + < 10% chalcopyrite) rock that generally extends to the sharp (< 10 cm wide) contact with wall-rock carbonate. Specularite-dominant rock predominates in shallower levels, along the footwall, and along the northern margin of the manto, but sulfide pods predominate at intermediate levels. WNW-elongate sulfide pods (i.e. similar orientation to veins in the district) widen at some stratigraphic levels within the specularite-dominant zone of the C-bed orebody, possibly reflecting stratigraphic control on a late sulfidizing fluid flow into favorable replacement horizons within earlier specularite. NW and NNE- striking, irregular bornite-chalcopyrite and bornite- pyrite replacement veins and bornite-matrix pyrite- bornite- + pyrite-chalcopyrite-fragment breccias are the locus of high-grade Cu-Au ore within the sulfide pods. Bornite veins commonly have rhythmically-banded bornite-pyrite or nebulous bornite- chalcopyrite/chalcopyrite-pyrite selvages. Paragenetic relations suggest an early stage of specularite + minor pyrite-chalcopyrite replacement of carbonate followed by formation of sulfide- dominant zones by sulfidation of specularite to replacement veins and masses of pyrite, chalcopyrite and bornite. Even, nearly continuous 0.1 - 0.5 cm specular hematite rinds on small (< 30 cm thick) massive pyrite < < chalcopyrite mantos in limestone, suggest specularite also precipitated as a peripheral mineral zone during introduction of Cu- Fe sulfides.
Wall-rock alteration peripheral to specularite-copper sulfide mantos is characterized by mm-scale white dolomite or calcite veinlets ± specularite veinlets with bleached halos, and white talc spots (2-40 mm diam.) in dolomite within meters of the contact. Small (< 1 m) pyrite-chalcopyrite mantos do not visibly alter limestone. A 20-foot thick, quartz-eye- poor, hornblende-rich "latite" porphyry dike, where cut by massive sulfide/specularite mantos, is pervasively altered to sericite-pyrite and chlorite and cut by quartz-sericite-pyrite, specularite, and quartz- adularia ± chlorite veins. Hornblende sites were locally replaced by specularite.
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