GENESIS OF NON-STRATAFORM MAGNETITE ORES NEAR RINGWOOD, NEW JERSEY HIGHLANDS

High-grade metamorphosed units of the New Jersey Highlands (Grenville Province) host several hundred small historic magnetite mines. Strataform deposits hosted by supracrustal rocks that were deposited in a back-arc basin have been interpreted to be meta-iron formations. However, the origin of granitoid-hosted ores is more controversial. Detailed mapping of one such deposit near Ringwood, New Jersey documented five lithologic units. Magnetite ore occurs within the meta-igneous basement rock of the Losee Suite, which is composed predominantly of felsic to intermediate, medium to coarse-grained, weakly foliated quartz-feldspar gneisses. However, ore pits are associated with localized zones of strongly foliated “tiger-striped” amphibolitic rock. Overlying supracrustal strata include laterally and stratigraphically variable pyroxene-bearing units: massive, very coarse-grained, monomineralic diopsidite immediately overlies the magnetite pits; felsic to intermediate Opx-bearing gneiss (±Cpx-Hb-Bt-Pl) occurs to the west of the diopsidite; mineralogically and texturally variable Opx-free Cpx-gneiss occurs to the east of the diopsidite, as well as stratigraphically above the Opx-gneiss and diopsidite. Strataform diopsidite at other magnetite mines in the area are interpreted to represent metamorphosed carbonate-facies iron formation. The pyroxene-bearing strata are overlain by well-foliated Bt-gneiss of metasedimentary origin. The spatial relationships between the basement-hosted magnetite pits and overlying diopsidite could represent a feeder pipe (magnetite-amphibolite association) overlain by seafloor exhalative carbonate-facies BIF (diopsidite) which grades downslope into Fe-enriched clastic sedimentary strata (Opx-gneiss). Chemical composition of Pxn-gneiss units, and the mafic minerals that they contain, may provide data to test this hypothesis further.