New Jersey, USA

Main commodities: Zn
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The Franklin (or Franklin Furnace) zinc deposit is located north of Ogdensburg, Sussex County in north-western New Jersey, USA.   It is approximately 3.5 km NNE of the similar Sterling Hill deposit.

The deposit is located within a 30 to 40 km wide window within the surrounding Palaeozoic sequences exposing Proterozoic metamorphics which have been acutely deformed and metamorphosed during the late Mesproterozoic (~1000 Ma) Grenville orogeny.   This window is known as the Reading Prong in New Jersey and Pennsylvania and the Hudson Highlands in New York State.

The host sequence comprises an older plagioclase rich suite of gneisses, granulites, pegmatites and amphibolites - the Losee Metamorphic Suite - which have been interpreted to represent a metamorphosed felsic and mafic metamorphic pile with associated trondhjemite intrusions.

The structurally overlying suite is composed of quartzo-feldspathic and calcareous gneisses and amphibolites, interpreted to represent detrital and carbonate sediment protoliths.   This suite includes the thick Franklin Marble which hosts both deposits.

The dominant structural fabric is a series of westward verging isoclinal folds with NNE trending axial traces.   Metamorphic temperatures are estimated at 700 to 800°C and pressures of 4 to 5 kbars.   Zircon dating suggests a maximum age of 1550 to 1790 Ma for the protoliths.

The host Franklin Marble was metamorphosed to sillimanite grade in the late Proterozoic and is almost pure, with trace amounts of graphite, silicates (forsterite, diopside, phlogopite, humites) and sulphides (pyrrhotite, pyrite).   The Franklin mineralisation is found in the structurally lower section of the Franklin Marble, being 180 above the contact with the underlying Cork Hill Gneiss member.

The Franklin orebody is the uneroded remnant of a larger deposit. It occurs as an elongate hook-shaped open syncline, the unequal limbs of which form the two main surface exposures, the west and east limbs, which are connected through the nose of a synclinal keel which extends down the 25° NE plunge for more than 1000 m. Toward the northern end of the mine, the plunge of the keel flattens and then reverses slightly to give a basin-like structure. The thicker, higher grade eastern limb has been interpreted to represent a compressed isoclinal antiformal closure. The total outcrop length of the deposit was 750 m, while at depth the total orebody has an extent of approximately 1.5 km.

In detail, the deposit occurs as a deformed and structurally controlled series of lens like bodies of franklinite (ZnFe2O4), willemite (Zn2SiO4) and zincite in a calc-silicate host characterised by calcite, manganoan andradite, rhodonite, bustamite, pyroxenes and amphiboles containing Mn, Zn, Ba feldspars with Pb and Sr, etc.

The mine was exhausted and closed in 1954.

The total mined deposit produced 22 Mt @ 19.6% Zn, 8.7% Mn, 17% Fe.

See also the   Sterling Hill, Franklin Furnace   record.

The most recent source geological information used to prepare this summary was dated: 1996.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
   Selected References:
Hitzman M W, Reynolds N A, Sangster D F, Allen C R, Carman C E  2003 - Classification, genesis, and exploration guides for nonsulfide zinc deposits: in    Econ. Geol.   v98 pp 685-714
Johnson C A, Skinner B J  2003 - Geochemistry of the Furnace Magnetite Bed, Franklin, New Jersey, and the relationship between stratiform iron oxide ores and stratiform zinc-oxide-silicate ores in the New Jersey Highlands: in    Econ. Geol.   v98 pp 837-854
Peck WH, Volkert RA, Mansur AT and Doverspike BA   2009 - Stable Isotope and Petrologic Evidence for the Origin of Regional Marble-Hosted Magnetite Deposits and the Zinc Deposits at Franklin and Sterling Hill, New Jersey Highlands, United States : in    Econ. Geol.   v104 pp 1037-1054

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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