PAREJA, Guillermo A., and FRIEHAUF, Kurt C. , Dept. of Geological and Environmental Sciences, Stanford University, Stanford CA 94305-2115
The stratigraphically-selective carbonate-hosted massive- sulfide/hematite "manto-type" replacement ores of the Superior District are analogous to those at Gilman and Leadville (CO), Bisbee (AZ), Tintic (UT), and Yauricocha (Peru). 13C-18O isotopic analysis of 125 carbonate rock samples, both adjacent to and distal to ore, but all within the stratigraphic horizons that preferentially host ore, demonstrate significant differences in 18O and minor differences in 13C values between sedimentary-diagenetic carbonates (18O = +20 to +28 , and 13C = -5.5 to +2 ) and hydrothermal carbonates (18O = +10 to +18 , and 13C = -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 garnet skarn and massive sulfide/specularite bodies are restricted to within 1 m from the ore-host rock contact. Similar results are reported at Leadville, Aspen, and Gilman (CO). Primary sedimentary-diagenetic isotopic variation among the carbonate beds (i.e., stratigraphic variation) is also significant, and similar in magnitude to isotopic shifts produced by up to 10-15% hydrothermal carbonate contamination of bulk rock samples within a given bed.
The lack of isotopic variability in matrix carbonate within an ore- bearing stratigraphic horizon, as a function of distance (1 m to > 7 km) from mineralized bodies, and dramatically contrasting values between hydrothermal veins and the matrix they cut suggests that most isotopic shifts in the carbonate host rocks are produced by direct precipitation of carbonate from the hydrothermal fluid rather than by diffusional re-equilibration between sedimentary carbonates and the hydrothermal fluid. For this reason, isotopic values of indiscriminate bulk carbonate samples are strongly affected by the occurrence of hydrothermal carbonate veins, intergranular void filling cement, and/or primary stratigraphic isotopic variation, and may define "false haloes" and yield meaningless calculated fluid:rock ratios.
Any isotopic alteration study in carbonate-host rocks should start with a detailed stratigraphic and characterization study of isotopic variation among the strata. Focus then should be placed on well- defined key beds, analyzing veinlets and pods separately from host- rock matrix.