Abstract

The Number One orebody is the largest and most sulfide-rich ore zone (at least 200,000 tons containing 8.4 wt % Cu) at the Ruby Creek copper deposit, in the southwestern Brooks Range of Alaska. Pyrite and copper-bearing sulfide minerals are concentrated within the matrix of a dolostone breccia body, which is enclosed by phyllite and calcic marble of Middle to Late Devonian age. The Number One orebody has three mineralogical zones that grade into each other: (1) an outer zone, widest toward the hanging wall, containing mostly pyrite with minor amounts of chalcopyrite and traces of carrollitc and sphalerite; (2) an intermediate zone containing major chalcopyrite and pyrite, minor tennantite-tetrahedrite, bornitc, carrollitc, and sphalerite, and traces of galena; and (3) a core zone containing major bornitc, chalcopyrite, pyrite, and chalcocite, minor carrollitc, digenite, and sphalerite, and traces of galena, covellite, and the germanium-bearing sulfides renierite and germanitc. Small clots of anthracitelike organic material (anthraxolite) are found throughout the ore. Much of the pyrite is fine grained and was deposited before the other sulfides, being increasingly replaced by Cu-bearing sulfides from the outer zone to the core. Some of this pyrite recrystallized into coarser grains having cobaltiferous rims, and these grains were generally not replaced. The sulfide minerals are commonly pseudomorphous after lath-shaped crystals within the dolomite clasts; no unreplaced examples were found, though the crystals closely resemble those of marcasite. The abundant fine-grained, porous pyrite also may represent replacement of marcasite. The origin and timing of brecciation and ore deposition remain unknown, though dolomitization and ore deposition appear to have occurred in preexisting carbonate breccia. Close mineralogical and geologic similarities are noted with the Kipushi Cu-Zn-Pb deposit in Zaire and with several other carbonate-hosted copper-rich organic-bearing deposits. The common concentration of the rare metals Co, Ge, and Ga in these structurally and mineralogically complex ores should increase their economic attractiveness.

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