GEOLOGIC SETTING OF MAGNETITE DEPOSITS IN THE NJ HIGHLANDS AND NY HUDSON HIGHLANDS

Grenville-province rocks of the New Jersey and Hudson Highlands host hundreds of small magnetite mines. The region was metamorphosed at upper-amphibolite to granulite facies during the Ottawan orogeny at ca. 1.05 Ga. Detailed mapping of three mine sites near Wanaque NJ, Ringwood, NJ and Warwick, NY shows that ore bodies are 1) stratabound and stratiform, 2) hosted by or closely associated with amphibolites that are members of mineralogically variable sequences of supracrustal rocks, and 3) that associated meta-plutonic rocks are not co-eval with ore deposition (Losee basement intrusives and/or post-metamorphic leucogranites. ) Amphibolites contain plagioclase and hornblende ± cpx ± scapolite ± opx ± biotite ± titanite. Cpx-bearing felspathic gneisses vary from quartz- and plagioclase-rich (Roomy) to quartz-poor, kspar-rich (Warwick). Biotite-bearing felsdspathic gneisses are largely garnet-poor and lack sillimanite. Distinct bi-mineralic quartz-diopside gneisses at Roomy and Warwick suggest an exhalative source (ankerite and chert protolith). Metamorphic pyroxenite at Ringwood may have undergone Ca-metasomatism. Trace element abundances in Roomy basalt-derived amphibolites vary from MORB-like to E-MORB-like and suggest a subduction component in the parent magma. Major element and mineral abundances in pyroxene-bearing felsic gneisses at Roomy are consistent with a greywacke protolith, whereas a thin, K-spar gneiss is consistent with an arkosic protolith. At Warwick, K-feldspar-rich gneisses are quartz-poor, suggesting an altered felsic volcanic protolith. K-rich chemistry northwest to Ca-rich chemistry and pockets of graphite southeast are consistent with easterly basinal deepening (Johnson and Skinner, 2003), and an active acid-volcanic depo-center in the west (Peck and Volkert, 2009). U-Pb dating of basement rocks (Aleinikoff and Volkert, 2010) suggests that Ringwood amphibolite, presumably cogenetic with quartzo-feldspathic basement rock, was crystallized prior to ca 1.25 Ga. Subsequent back-arc extension was the tectonic setting and hydrothermal fluid flow was the metal source for near-surface deposition of ore-protolithic sediments, (Volkert, 2007).