CARBON AND OXYGEN ISOTOPE EVIDENCE FOR THE ORIGIN OF LATE-KINEMATIC CARBONATE VEINS IN PROTEROZOIC METABASALTIC ROCKS, EASTERN BLACK HILLS, SD

Carbonate minerals are relatively scarce among the Precambrian metamorphic units in the Black Hills. Thin bands of impure dolomites occur in some lower-grade regions and isolated marbles appear as enclaves in the Harney Peak Granite. Other than those concentrations, calcite, dolomite, and more iron-rich carbonates are known in carbonate-facies iron formation and veins systems.

A curious association exists between carbonate-bearing veins and early Proterozoic gabbroic sills and basaltic flows. The origin of the veins is closely linked to the latest phase of deformation and metamorphism observed in the Black Hills.

Samples of vein material were analyzed for their carbon and oxygen isotopic signatures. Sampling locations include the extensive vein networks from the Pactola Dam exposure of metabasalt and an outcrop of higher (amphibolite)-grade metamorphic amphibolite, interpreted as metabasalt, near Keystone. Low-grade metadolostone (?) near Nemo and high-grade marble from the Needles Highway, were also analyzed for comparison. Analyses were made to test the hypothesis that the veins were of hydrothermal, open-system orign. Thin-section studies do not support the derivation for voluminous carbonate from the metamorphic breakdown of mafic volcanic material.

Plots of δ13C versus δ18O values strongly suggest that the Pactola veins are of hydrothermal origin. Very low δ13C values also infer a source of reduced organic material and are also consistent with magmatic influence. Metasedimentary-“S-type” granitic components for the vein fluids are good matches for the δ18O values between 10 and 12‰ (SMOW). In concert with other data, the stable isotope compositions support the concept of widespread fluid infiltration along shear zones and concurrent with granite intrusion.