STABLE ISOTOPE CONSTRAINTS ON THE ORIGIN OF MESOPROTEROZOIC MARBLE-HOSTED ZINC AND IRON DEPOSITS, NEW JERSEY HIGHLANDS

Granulite facies marble-hosted Zn-Fe-Mn deposits of Mesoproterozoic age at Franklin and Sterling Hill in the northwestern New Jersey Highlands are thought to have formed from hydrothermally altered protoliths on the continental margin side of a back arc. Oxygen isotope data from Sterling Hill (11.7 to 17.6‰ SMOW; Johnson et al. Econ Geol 1990) and Franklin (11.9 to 16.6‰, this study) are consistent with alteration of marble protoliths by water-rich fluids, but the Zn deposits at both locations lack a distinctive carbon isotope signature and have δ¹³C from -1 to 1‰ (PDB), identical to unmineralized Franklin Marble. These restricted carbon isotope ratios argue against significant smithsonite in the protolith of the deposits, as metamorphic decarbonation reactions would have resulted in more scatter in δ¹³C and values that are different from the Franklin Marble. In contrast to the Zn-Fe-Mn deposits, widely distributed, genetically related marble-hosted magnetite deposits have δ¹⁸O values that range from 11.4 to 24.7‰ and δ¹³C values that range from -5.0 to 0.7‰ (from 18 deposits). The stable isotope data from marble-hosted magnetite deposits are consistent with water-rock interaction between the Franklin Marble protolith and a mixture of igneous fluids and seawater. This is in agreement with the proposed tectonic model in which the Franklin Marble and precursor metals were deposited in a back arc basin at ca. 1.3 Ga (Volkert and Aleinikoff, 2007). Hydrothermal seafloor vents in the back arc likely provided Fe+Mn in magnetite deposits hosted by spatially associated bimodal volcanic gneisses and Franklin Marble. Felsic volcanic rocks have an A-type granite affinity, similar to VMS-related rhyolites from continental back-arcs. They represent crustal melts formed in a shallow magma chamber that supplied heat to the seafloor hydrothermal system. Granulite-facies metamorphism and conversion of Zn+Fe+Mn to the present high-grade ore assemblages occurred during the Ottawan orogeny.