GEOCHEMISTRY OF PARAGNEISSES ASSOCIATED WITH MAGNETITE MINES IN THE NEW JERSEY AND HUDSON (NY) HIGHLANDS

The Grenville-province supracrustal rocks of the contiguous New Jersey and Hudson Highlands host hundreds of small magnetite mines and the major zinc-oxide deposits of Franklin and Sterling Hill. The region was metamorphosed at upper-amphibolite to granulite facies during the Ottawan orogeny at ca. 1.0Ga. Detailed property mapping near Wanaque, NJ and Warwick, NY reveal that magnetite ores are stratabound and stratiform and must have been deposited prior to the Ottowan orogeny. Spatially associated gneisses consisting of only diopside and quartz are interpreted to be metamorphosed ankerite-chert banded iron formations, consistent with syngenetic ore genesis. Regionally the magnetite orebodies are associated with compositionally variable amphibolites and pyroxene-bearing felsic gneisses. Mineralogically similar amphibolites (Hb+Pl±Cpx±Qtz±Ap) yield scattered geochemical compositions. Those that plot as N-MORB on trace element discrimination diagrams (Pearce and Cann, 1973; Meschede, 1986) are interpreted as metabasalts, although alkali enrichment indicates some alteration. Other amphibolites are enriched in Fe, Ti, P and REE, all of which are abundant in magnetite ore, suggesting an ore component in this amphibolite protolith. A third group of amphibolites yield high CaO, and an overall composition consistent with a calcareous shale protolith. Pyroxene-bearing felsic gneisses and K-spar-bearing gneiss have major element profiles that are inconsistent with an igneous protolith. Pyroxene-bearing felsic gneisses are consistent with greywacke, whereas the K-spar gneiss is consistent with an arkosic protolith based on the SiO₂/Al₂O₃ vs K₂O/Na₂O discrimination diagram (Tran et al. 2003). High Ba and Sr concentrations in these rocks suggest a continental sediment source. The N-MORB signature of some amphibolites, the immature character of sediments, the presence of marbles, and the presence of exhalative ore are consistent with deposition in a shallow back arc basin (Volkert, 2007). The high compositional variation of strata that are spatially associated with magnetite ore bodies can be explained by deposition of a mix of volcanic-arc-derived siliciclastics, limestone, volcanic ash and flows, exhalative precipitates, and hydrothermal metasomatism.