High-temperature, carbonate-hosted massive replacement ores of the Superior District are analogous to those at Gilman and Leadville (CO), Bisbee (AZ), Tintic (UT), and Yauricocha (Perú). Isotopic analysis of 125 carbonate rock samples within the stratigraphic horizons that preferentially host ore demonstrate significant differences in del18O and minor differences in del13C values between sedimentary-diagenetic carbonates (del18O = 20 to 28‰, and del13C = -5.5 to 2.0‰) and hydrothermal carbonates (del18O = 10 to 18‰, and del13C = -4 to -1‰). Samples of host-rock carbonate matrix within millimeters of sulfide-carbonate veinlets are isotopically indistinguishable from sedimentary-diagenetic carbonate. Alteration effects around massive sulfide Cu-Au ores are restricted to within 1 m from the ore-host rock contact, similar to results reported at Leadville, Aspen, and Gilman (CO). Primary sedimentary-diagenetic isotopic variation within and between unaltered carbonate stratigraphic units is similar in magnitude to isotopic shifts produced by the presence of 10-15% hydrothermal carbonate contamination of bulk rock samples.
Chemically distinct overgrowths of hydrothermal Mn-Fe-rich dolomite on sedimentary-diagenetic dolomite rhombs and the sharply contrasting isotopic values between hydrothermal veins and the carbonate matrix they cut indicate that most isotopic shifts in carbonate host rocks are produced by direct precipitation of carbonate from the hydrothermal fluid rather than by diffusional equilibration between sedimentary carbonates and the hydrothermal fluid. For this reason, isotopic compositions of bulk carbonate samples are strongly affected by primary stratigraphic isotopic variation and the abundance of hydrothermal carbonate veins. The lack of correlation between the isotopic composition of matrix carbonate within an ore-bearing stratigraphic horizon and distance from mineralized bodies (1 m to > 7 km), and the small magnitude of this variation relative to sedimentary variation suggests that bulk sampling of carbonate rocks in an exploration project may produce highly ambiguous results.
In order to differentiate sources of isotopic variation, isotopic alteration studies in carbonate host rocks should include detailed characterization of isotopic variation among unaltered strata. Focus then should be placed on well-defined key stratigraphic units, analyzing veinlets and pods separately from host-rock matrix.