Sterling Hill, Franklin Furnace

New Jersey, USA

Main commodities: Zn
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The Sterling Hill & Franklin (or Franklin Furnace) zinc silicate/oxide deposits are respectively located to the west and north of Ogdensburg, Sussex County in north-western New Jersey, USA. Franklin is approximately 3.5 km NNE of Steling Hill.

Both deposits are similar, but are unlike almost any other deposit in the world.   Both are high grade with a unique mineralogical composition.   The ore minerals are willemite (Zn2SiO4), franklinite (ZnFe2O4) and zincite (ZnO).   Of the 333 minerals recognised in the two mines, 33 have not been found anywhere else.

The deposits are 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 is almost pure, with trace amounts of graphite, silicates (forsterite, diopside, phlogopite, humites) and sulphides (pyrrhotite, pyrite).   Sterling Hill and Franklin Furnace are both found in the structurally lower section of the Franklin Marble, being 180 and 1 m respectively above the contact with the underlying Cork Hill Gneiss member.

At Sterling Hill the orebody was approximately 800 m long in outcrop and occurs on the two limbs and keel zone of an intricately folded, steeply plunging isoclinal syncline in which the ore is wrapped around a central core of graphitic marble and an annular cylinder of mica, feldspar, hornblende, pyroxene and garnet gneiss. In detail, the orebody is a complex series of attenuated isoclinal folds which modify the limbs of the larger, but similarly attenuated, isoclinal syncline. Structures in both the ore and the infolded wallrock demonstrate extreme plastic deformation during folding. The structural complexity of the orebody is much greater than that of the surrounding region. In general, the deposit is conformable with the banding in the marble with the two main limbs, the longer east and the shorter west limb, both of which dip east at 55°.

The orebody was composed of a series of chemically and mineralogically well defined and laterally continuous lithologies from cms to metres in thickness which have been folded into the tight NE plunging isoclinal synform.   The stratigraphic sequence in the ore zone from bottom to top comprises:   i). a willemite + franklinite + zincite + calcite unit,   ii). a willemite + franklinite + calcite unit,   iii). a calcite unit composed of calcite ± pyroxene ± garnet ± other calc-silicate minerals, and   iv). a horizon of angular fragments of biotite and hornblende gneiss encased in marble.

The Sterling Hill mine closed in 1986. Production was as follows:

Sterling Hill:  11 Mt @ 20% Zn, with 16% Fe, 8% Mn. - the total mineralised zone including sub-ore would have totalled around 48 Mt.
Franklin Furnace:  22 Mt @ 19.6% Zn, 8.7% Mn, 17% Fe.

For detail consult the reference(s) listed below.   See also the   Franklin   record.

The most recent source geological information used to prepare this summary was dated: 1989.    
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, Rye D M, Skinner B J  1990 - Petrology and stable isotope geochemistry of the metamorphosed Zinc-Iron-Manganese deposit at Sterling Hill, New Jersey: in    Econ. Geol.   v85 pp 1133-1161
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
Valentino A J, Carvalho A V, Sclar C B  1990 - Franklinite-Magnetite-Pyrophanite intergrowths in the Sterling Hill Zinc deposit, New Jersey: in    Econ. Geol.   v85 pp 1941-1946

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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