MAGNETITE OREBODIES IN THE NEW JERSEY AND HUDSON (NEW YORK) HIGHLANDS: METAMORPHOSED BANDED IRON FORMATIONS

The New Jersey Highlands and contiguous Hudson (New York) Highlands, Appalachian inliers of the Grenville Province, host hundreds of small, worked-out magnetite mines. Detailed mapping of three small deposits near Wanaque, NJ, Ringwood, NJ and Warwick, NY reveal that that magnetite ores are hosted by supracrustal rocks of varied composition and at Wanaque and Warwick, ores are stratiform and stratabound. Geochemistry reveals that amphibolites in proximity to the ore are metabasalts with a back-arc basin REE signature; alkaline metavolcanic rocks are highly enriched in LREE, suggestive of provenance in a rift environment, and immature clastic metasedimentary rocks are derived from a volcanic arc and/or from continental sediments. This evidence is consistent with a back-arc basin depositional setting, (Volkert ,1999). At Wanaque and at Warwick, bi-mineralic quartz-clinopyroxene gneiss lies in close proximity to magnetite ore, and is concordant with the strataform trend of the deposits. The quartz-clinopyroxene gneiss consists of alternating cm-scale layers of quartz and clinopyroxene with compositions straddling the diopside-hedenbergite boundary (i.e., FeO=MgO). Aluminum is very low in this unit (0.88% as Al₂O₃). Layers are continuous over meters, and contacts between layers are sharp and non-gradational. No other unit with the study areas displays such distinct layering, and so is likely a primary layering rather than metamorphic segregations. Quartz-clinopyroxene gneiss is spatially associated with massive strataform diopsidite, and in some cases the transition is gradational along strike. Massive diopsidite is spatially associated with magnetite ore at all three study localities. Typical IF is defined as “a chemical sediment, typically thin bedded or laminated, whose principal chemical characteristic is an anomalously high content of iron, commonly but not necessarily containing layers of chert” (Klein, 2005). Thus the quartz-clinopyroxene gneiss and spatially related magnetite are consistent with the definition of IF, and the clinopyroxene-dominated assemblages are compositionally consistent with carbonate-facies IF metamorphosed under granulite conditions:Quartz + Ankerite = Clinopyroxene + Carbon Dioxide