STABLE ISOTOPE GEOCHEMISTRY OF THE FRANKLIN MARBLE (GRENVILLE PROVINCE, NEW JERSEY)

Thirty-one samples of the granulite-facies Franklin Marble were examined petrographically and analyzed for stable isotope ratios. These samples are from a 40 km long, northeast-southwest traverse of the New Jersey Highlands, an outlier of the Grenville Province. Calcite (or dolomite) crystals, 0.25-30 mm in size, compose over 95% of most samples. Euhedral, reflective graphite, 0.5-5.0 mm diameter, is ~1% of most samples. Diopside, quartz, fluorite, chondrodite and white mica compose the remainder of these rocks. Oxygen isotope ratios (delta values) vary throughout the study area, ranging from 16.8 to 24.5‰ (SMOW) and averaging 20.4 ± 2.4‰. In contrast, carbon isotope ratios are homogeneous, ranging from -1.5 to 1.0‰ (PDB), averaging 0.3 ± 0.5‰. These values are similar to other Grenville marbles, and marine carbonates worldwide. A subset of eight samples, taken across layering in the Limecrest (Southdown) Quarry, have variable oxygen isotope ratios that do not correlate with carbon isotopes. No simple correlation between oxygen isotopes and proximity to intrusive rocks can be found in the quarry or elsewhere in the marble belt. Oxygen isotope variation in the Franklin Marble is most likely caused by localized hydrothermal alteration of marble, preserving protolith carbon isotope ratios. It is hoped that calcite-graphite geothermometry can help refine published P-T estimates (ca. 680-750ºC) and that variability in metamorphic conditions can be detected, if present.

The Franklin Marble hosts the world-class Franklin and Sterling zinc deposits, both of which have carbon isotope ratios similar to barren marble. Oxygen isotope ratios of the Sterling deposit range from ~23‰ to ~12‰ (Johnson et al., Ec Geol 1990). Preliminary data from the Franklin deposit suggests low oxygen isotope ratios as well. In contrast, the smaller, skarn-hosted Sulfur Hill iron deposit has similar oxygen isotope ratios to the Sterling deposit, but more mantle-like carbon isotope ratios (-3 to -4‰).