Abstract

Major element concentrations of Cu, Fe, Zn, and Ag in tetrahedrite and delta 34 S values in pyrite and tetrahedrite were determined for the carbonate-bearing lower Chester vein within the Precambrian St. Regis and Revett Formations. Selected tetrahedrite samples were also analyzed by electron microprobe for Ag, Fe, Zn, Cu, As, Sb, Hg, Ni, and Co. Cu + Zn increases with depth on the west end of the vein and Cu values in the east end are lower and more variable relative to the west end. Tentative local elemental variations include increasing Cu and Ag from east to west between adjacent raises and decreasing Cu upward in most raises sampled. Fe usually varies inversely with Cu and Ag in tetrahedrite.Sulfur isotope compositions of 24 tetrahedrite and 13 pyrite separates were determined (tetrahedrite delta 34 S mean = 4.4, range -- 2.9 to 5.9; pyrite delta 34 S mean = 3.1, range = 1.0 to 5.0). Pyrite is depleted in 34 S upward at the rate of 1.0 per mil/70 m. Within the raises, delta 34 S of tetrahedrite varies directly with weight percent Cu. Tetrahedrite is enriched in 34 S compared with pardgenetically earlier but coexisting pyrite.Elemental and isotopic variations in tetrahedrite are apparently due to a combination of the changing availability of Fe during replacement of earlier formed siderite and pyrite and the changing physicochemical conditions within the solution during migration through the fracture system. The 34 S depletion in pyrite with elevation may be due to steadily increasing pH and/or f (super O 2 ) upward during mineralization or precipitation from a finite solution reservoir of sulfur. The enrichment of 34 S in tetrahedrite relative to pyrite suggests that an Fe-rich solution changed with time to a Cu-Zn-Ag-Pb-rich solution or that two different solutions were involved in the sulfide mineralization.

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