THE EDISON IRON MINE, NEW JERSEY: A MIDDLE PROTEROZOIC METARHYOLITE HOSTED APPALACHIAN MAGNETITE DEPOSIT

There are probably several genetic types of middle Proterozoic magnetite deposits throughout the Appalachians. Most have been described as precipitates from hydrothermal solutions derived from metamorphic processes, early Ottawan granites, or late- to post-Ottawan granites, or as metasediments. Granites and calc-alkaline metavolcanic gneisses host most deposits but some, including the historic Edison iron mine of New Jersey are hosted by quartz-potassium-feldspar gneiss interpreted as metasediment (Buddington, 1966) or more specifically as meta-arkose (Drake, 1984). The quartz-potassium-feldspar gneiss is interbedded with metasediments including marble, quartzite, metagraywacke, and metapelites. However, our new geochemical data indicate that the protolith of the quartz-potassium-feldspar gneiss of the Edison mine area is A-type rhyolite, not arkose. Supporting evidence includes the absence of an underlying potassic source provenance for arkosic sediment, and potassium + sodium / calcium + magnesium ratios that exceed the range of arkoses except for those deposited in extremely arid environments. In addition, the Edison deposit is similar to some gneiss-hosted magnetite deposits at Cornwall, NY that Johnson (1997) has determined do not show sulfur isotopic evidence consistent with a sedimentary environment. Although we propose a volcanic protolith, we are not suggesting the iron concentrations are volcanogenic or syngenetic. Instead most evidence supports high temperature hydrothermal activity. The TiO₂ content of the Edison magnetite (~1.3%) exceeds that of typical banded iron formation (<0.1%) but is about the same as large magnetite crystals found in common co-existing metamorphic pegmatites. Magnetite-ilmenite geothermometery indicates precipitation at 710 to 750 Cº. In addition to iron the ore is highly enriched in hydrothermally mobile phosphorus, and barium. Therefore, the most likely source of iron enrichment is a hydrothermal phase extracted from either surrounding rocks during subsequent granulite facies Ottawan metamorphism, or from nearby early Ottawan (Byram) granites. However, post-orogenic magmatic sources are not close to the Edison deposit and are not consistent with magnetite banding that is conformable to the foliation of the host rock.