USING STABLE ISOTOPE ANALYSIS TO DETECT BLIND GOLD DEPOSITS, NORTHERN BLACK HILLS, SOUTH DAKOTA

The Lead Mining District in the northern Black Hills of South Dakota contains significant gold deposits formed by Tertiary-aged replacement of Paleozoic sedimentary rocks. Solitario Zinc Corp. has discovered a new target of high-grade gold ore hosted in a carbonate rock unit located about 350 meters above the prospective Deadwood Formation, where geophysical surveys suggest an area that may represent a blind gold target. Surface carbonate rocks show visible signs of hydrothermal alteration over a limited area, potentially representing leakage along a fault/fracture system from significant gold deposits at depth. An oxygen stable isotope depletion of δ¹⁸O, often up to 15 ^o/_oo, is common in carbonates that have been hydrothermally altered. Critically, these values will be depleted over a much larger area (several kilometers laterally and up to 400 meters vertically above ore) than can be detected through visible mineralogy or geochemistry. The novel application of stable isotope methods for exploration beneath carbonate layers in the Black Hills is tested for the first time. We sampled the same horizon within a persistent limestone bed for both altered samples and background samples (to establish the typical range of δ¹⁸O values). Analyses were performed using gas chromatography isotope ratio mass spectrometry (GC-IRMS) at the University of Wyoming to obtain δ¹⁸O values. Results showed an average δ¹⁸O depletion of 3.4 ^o/_oo in whole rock, when compared to background samples. Additionally, there was a δ¹⁸O depletion of 8.5 ^o/_oo when comparing calcite vein samples to background samples. These results are consistent with carbonate reactions to hydrothermal fluids. Maps of δ¹⁸O isotopic values will be compared with geochemical data in both rock and soil to show the extent and range of hydrothermal alteration around the target area and give a better indication of the size of the potential blind ore body.