A Stable Isotope Perspective on Sedimentation, Ore Genesis, and Metamorphism in the Southern Grenville Province

New data from Mesoproterozoic marbles in the New Jersey Highlands and the Morin terrane in Quebec allow the assessment of sedimentation, hydrothermal alteration, and subsequent granulite-facies metamorphism in the southern Grenville Province and northern Appalachians. These data also allow assessment of recently published regional ore deposit models. In the Morin terrane oxygen and carbon isotope ratios of marbles overlap published data from the Adirondack Highlands and Lowlands, but with a smaller range and suggesting less water-rock interaction (δ¹³C=-1.9 to 2.5‰ PDB; δ¹⁸O=18.3 to 28.2‰ SMOW). Magnesite deposits from Kilmar in the Morin terrane have isotope and mineralogical characteristics consistent with evaporitic protoliths (δ¹³C=1.7±0.2‰; δ¹⁸O=25.6±0.5‰). Calcite-graphite carbon isotope thermometry in Morin marbles yields temperatures of 755±38 °C, which are attributed to the Ottawan orogeny (1.08-1.05 Ga).

In contrast, the Franklin Marble and other scattered marbles in the New Jersey Highlands have a more narrow range in carbon isotopes (0.3±0.7‰) than most other Grenville terranes, consistent with an origin as a single depositional unit. Although local occurrences of stromatolites, tourmaline, and serendibite are suggestive of shallow water and evaporitic environments, stable isotopes are not as distinct as those at Kilmar or the Balmat deposit in the Adirondacks. Marble-hosted Zn-Fe-Mn ore deposits at Franklin and Sterling Hill likely formed from alteration of the Franklin Marble by water rich-fluids, and so preserve protolith carbon isotopes while showing a range in oxygen isotopes (11.7-17.6‰; Johnson et al. 1990). Genetically related marble-hosted magnetite deposits have similar oxygen isotope ratios, but with δ¹³C values that range from 0.7 to -5.0‰. We interpret ore deposits in the Franklin Marble to have formed from hydrothermal fluids in a back arc basin at ca. 1.3 Ga. Subsequent granulite-facies metamorphism of ore deposits and marble during the Ottawan orogeny is recorded by calcite-graphite carbon isotope fractionations corresponding to temperatures of 767±41°C.