Geologic Setting of Proterozoic Iron, Zinc, and Graphite Deposits, New Jersey Highlands
Authors: Volkert Richard A. 
Published: August 2024

The New Jersey Highlands are underlain principally by Mesoproterozoic rocks that were metamorphosed at upper amphibolite to granulite facies between 1080 and1030 Ma during the Ottowan Orogeny. The oldest rocks are inferred to be metamorphosed ca. 1300 Ma dacite, tonalite, and trondhjemite and associated charnockitic plutonic and metavolcanic rocks of the Losee Metamorphic Suite. These originated in a continental margin magmatic arc setting dominated by calc-alkaline and tholeiitic magmatism. Rocks of the Losee record a compressional event in the north-central Appalachians that was likely coeval with the 1300 to 1200 Ma Elzevirian orogeny in the northern Appalachians and Grenville Province.

The Losee is unconformably overlain by a sequence of supracrustal rocks older than 1100 Ma that were deposited in an extensional tectonic setting. These include quartzofeldspathic gneisses, metaquartzite, calc-silicate rocks, marble, and amphibolite. The earliest recognized part of this succession consists of metamorphosed arkosic and quartzose sandstones that were deposited in a continental rift basin. These are spatially associated with a bimodal assemblage of metarhyolite and metabasalt. Overlying sanddominated clastic rocks (quartzofeldspathic and siliceous calc-silicate gneisses) and marble are interpreted as a platformal, shallow-marine succession based on lithologic associations, stromatolite occurrences in marble, and stable isotope data. This succession grades into overlying supracrustal rocks that reflect an arc-derived sediment source, and that contain a higher proportion of mafic volcanic rocks, including locally pillowed metabasalts of MORB affinity.

Losee and supracrustal rocks were intruded by voluminous, widespread synorogenic A-type granitoids of the Vernon Supersuite that consist of the 1116 ± 41 Ma hastingsite and biotite-bearing Byram Intrusive Suite and the 1095 ± 9 Ma hedenbergite-bearing Lake Hopatcong Intrusive Suite. The postorogenic 1020 ± 4 Ma Mount Eve Granite and associated 1004 ± 3 Ma pegmatites, which straddle the boundary between New Jersey and New York, provide a minimum age for the Ottawan orogeny in the Highlands. Discordant postorogenic pegmatites were emplaced at 998 to 989 Ma and 965 Ma.

Mesoproterozoic rocks of the Highlands host economic deposits of iron, zinc, and graphite. Deposits of high Ti and low Ti magnetite ± hematite are widely distributed throughout the region and occur within virtually every Mesoproterozoic rock type. Small U and REE deposits are associated with a few of the Fe deposits. Neoproterozoic greenschist-facies felsic volcanic and terrestrial sedimentary rift basin rocks host small hematite deposits that have characteristics of iron-formation. Two large marble-hosted zinc (zincite, willemite, franklinite) deposits occur in the northwestern Highlands at Franklin and Sterling Hill, and marble-hosted sphalerite occurs at the Raub mine in the southwest Highlands. Zinc deposits at the Franklin and Raub mines directly overlie Mn-bearing Fe oxide deposits, implying a genetic relationship. Graphite deposits are confined to the eastern Highlands where they are hosted by sulfidic biotite-quartz-feldspar gneiss and metaquartzite. Available geologic evidence suggests that metals in the Zn and most of the Mesoproterozoic Fe deposits were introduced prior to ca. 1080 to 1030 Ma granulite facies metamorphism.