Mesoproterozoic marble in the New Jersey Highlands hosts small magnetite deposits that occur in discrete groups along linear trends and are concentrated mainly in the western part of the region. Magnetite also forms a layer structurally beneath the Zn-Fe-Mn orebody at the Franklin mine. Marble host rocks are interlayered at Fe and Zn-Fe-Mn deposits with bimodal metavolcanic rocks (amphibolite and rhyolitic gneiss) that were deposited synchronously in a back-arc basin developed on the eastern margin of Laurentia between 1.3 and 1.25 Ga. Magnetite in the deposits ranges from massive to disseminated and is characterized by low Ti, variable Mn, and locally high S. Concentrations of As, V, Sb, and Zn are enriched in magnetite compared to marble host rocks and associated metavolcanic rocks. Carbon and oxygen isotope ratios of the marbles are consistent with water-rock interaction between the marble protolith and a mixture of igneous fluids and seawater. Regional geology, petrology, and isotope data indicate that Fe was introduced into the carbonate protolith as low-temperature Fe oxides and hydroxides via hydrothermal fluids discharged along basinal fracture zones on the sea floor. Related marble-hosted Zn-Fe-Mn deposits at Franklin and Sterling Hill contain the assemblage willemite + zincite + franklinite. Stable isotope compositions of the Zn deposits are consistent with alteration of host rocks by water-rich fluids, preserving protolith carbon isotope ratios. Stable isotope modeling suggests that Zn silicate + oxides or hydroxides represent an appropriate protolith for the high-grade Zn ores.

Alteration of metavolcanic rocks at the Fe deposits, combined with the isotope data and other petrologic and geochemical evidence, document the presence of an extensive hydrothermal system of Grenville age centered in the western Highlands. Metals precipitating from this hydrothermal system were responsible for Fe in the marble-hosted magnetite deposits and Zn in the deposits at Franklin and Sterling Hill from the same or a related hydrothermal system. Underplating of the back arc by mafic magma provided the heat to melt the lower crust and drive the hydrothermal system, resulting in carbonate-hosted Fe and Zn ore deposits and associated bimodal volcanism.

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