General discussion

Regional geology

Local geology

Major formations and rock units

The local magnetite deposits

The local magnetite deposits

Magnetite deposits are numerous in the region and especially in the more southern parts of the New Jersey Highlands (Kitchell, 1856; Puffer, 1970, 1980; Puffer et al., 1993; Smith, 1933; Smock, 1874; Cook, 1868; Putnam, 1886; Hotz, 1954; and Buddington, 1966). The discussion below is limited to those magnetite deposits having proximity relations to the zinc orebodies. The early theories of their origins were given by Spencer (1904), Spencer et al. (1908), and Ries and Bowen (1922). The exploitation of these local iron deposits is described in the section entitled “Iron mining at Franklin.”      

The local magnetite deposits were described by Spencer et al. (1908), Bayley (1910), Baker and Buddington (1970), Frondel and Baum (1974), and Stockwell (1951); the writer has drawn heavily on the latter. Early writers referred to these deposits as veins. There are apparently three general magnetite deposits in Franklin; these are the Furnace Magnetite Bed, the intermediate marble-hosted deposits, and the discontinuous string of magnetite deposits in gneiss on and near Balls Hill. This designation of three distinct groups is “forced” for discussion purposes; there is inadequate geologic proof regarding their relations.

The Furnace Magnetite Bed

The Furnace Magnetite Bed lies in the Franklin Marble between the west limb of the Franklin zinc deposit and the underlying Cork Hill Gneiss, and close to the gneiss; the overturned stratigraphic relations have been described above. This relatively thin bed is conformable to the zinc orebody, is present to a depth of at least 1150 feet (350 meters), and is 3 to 8 feet (0.8-2.4 meters) in average thickness (Figures 2-12, 3-4, 8-5, and 9-5). Stockwell (1951) said it extended north to at least coordinate 2420N, a total length of about a mile from the south end of the orebody.

The feature called “the fork of the bed” is the line along which the magnetite bed ceases to be parallel with the Cork Hill Gneiss; the “fork” plunges nearly parallel to the west limb footwall of the zinc orebody near the surface, but departs from it, increasingly, at depth (Figures 8-5 and 9-5). The bed apparently wrapped around the lower keel or the fold in the orebody only slightly and may have been in the vicinity of the Black Hole; evidence is lacking. The bed pinches and swells and has oreshoots up to 15 feet thick pitching to the NE; Spencer et al. (1908) noted that their pitch is the same as that of the keel of the zinc deposit. Much detail on the bed, its shoots, extent, richness, and keel is given by Stockwell (1951), who wrote the unpublished but definitive report on this iron-ore feature. There are areas, especially toward the north, where the bed pinches out in an area where cross-folds are common.   

The Furnace Magnetite Bed is penetrated naturally in the upper mine levels, above the fork of the bed, by pegmatite, and contact skarn minerals (andradite, rhodonite) have been developed locally. Some of these pegmatite penetrations were also invasive to the zinc orebody, possibly carrying magnetite into it. Mine drifts from the Franklin Mine, as well as the surface workings used to exploit the magnetite bed in the 1770-1882 period, also penetrated the bed in the upper levels. Although Stockwell (1951) discussed three separate “orebodies” within the Furnace Magnetite Bed (Figure 8-5), these were but economic-evaluation terms and did not refer to specific workings; they were not exploited.

The Furnace Magnetite Bed consists of magnetite thickly disseminated in calcite. Graphite is present, as are chalcopyrite and pyrite in traces; massive, calcite-free magnetite is also present. Microprobe analyses by the writer indicate that this magnetite is slightly manganoan (0.7-2.5 wt. % MnO) and magnesian (0.1-0.9 wt. % MgO), is essentially zinc-free, and has no detectable aluminum or titanium. This elemental composition serves as a very useful discriminant, signature, or “fingerprint” in distinguishing magnetite ore of the Furnace Magnetite Bed from magnetite ore of the other proximal deposits. The presence of Mn in this bed adds credence to the possibility of it having genetic relations to the orebody. Using isotopic methods, Johnson et al. (1990) found that the oxygen isotopic composition of calcite in the Furnace Magnetite Bed is similar to that of calcite in the Franklin orebody. This further supports the idea that they formed under similar circumstances.

The marble-hosted intermediate deposits

	Figure 8-5. The relations of the Furnace Magnetite Bed in Franklin. This is a longitudinal projection along a north-south plane based on the “Franklin North” coordinate system; the insert is an east-west cross-section on the same coordinate system. The term “orebody” is here applied solely to magnetite and is used only in an economic-evaluation sense; there was no magnetite mining here in the 20th century, and the bed was not commercially exploited in modern times. The proximity of several of these “orebodies” to zinc-mining shafts is coincidental. Illustration adapted from Stockwell (1951).

Immediately south of the Franklin orebody and the above-defined Furnace Magnetite Bed and north of the gneiss-hosted magnetite deposits on Balls Hill, there is a series of marble-hosted magnetite orebodies on strike with the Furnace Magnetite Bed. The three sites are in part presently occupied by a near-vacant area south of the Franklin Mineral Museum, near a water-filter facility, and under a common park adjacent to Franklin Pond (Figures 2-12, 3-4, and 3-5).

It has not been shown that these orebodies (mined as the Longshore Mine, Pike’s Peak Mine, and Furnace Mine, respectively) are continuous or discontinuous parts of the Furnace Magnetite Bed. Numerous 19^th-century investigators thought they were continuous, as did Bayley (1910); Stockwell (1951) held no doubt at all that they are one and the same formation. An 1871 map made for Moses Taylor weakly suggests that just south of the zinc orebody the bed bifurcates into two parallel beds about 50 feet apart, which maintain their separate identity throughout a purported continuous extent southward on Balls Hill (Figure 2-12). Putnam (1886) also indicated that the Pike’s Peak Mine was in two beds, but his report was in error in other respects and may have been incorrect in this regard as well. There were keels found in the bottom of the Furnace Mine and the Pike’s Peak Mine;  these keels were thought to be closely related to that of the zinc orebody (Stockwell, 1951). Near the Pike’s Peak Mine there were 3 ore shoots, one above the other, all pitching NE and dipping 55^o SE.

The only known preserved specimens, labeled as being from these mines, are in the Natural History Bureau of the New Jersey State Museum in Trenton. Microprobe analyses of these magnetite specimens by the writer reveal compositions very similar to those of samples from the Hill Mines (see below) for all mentioned elements, but different from those of samples from the Furnace Magnetite Bed as noted above. However, an added complication results from the sloppy manner in which names have been assigned to these intermediate deposits; this adds a degree of ambiguity to the integrity of assigned provenance. Although samples may be obtained currently from both the Furnace Magnetite Bed and the below-described Balls Hill deposits, the intermediate deposits are buried. More work needs to be done.

The magnetite deposits on Balls Hill

Southwest from the Franklin zinc deposit and its underlying Furnace Magnetite Bed, and from the intermediate deposits described above, and extending nearly a half-mile toward Sterling Hill, is a series of elongate magnetite deposits. These are collectively known as the Hill Mines, and are located on a partially-removed hill, known locally and to Palache (1935) as Balls Hill.

These deposits are said to be mostly in the Cork Hill Gneiss, very close to or at the contact with the Franklin marble; some pegmatite occurs at such contacts. The deposits are on strike with the Furnace Magnetite Bed (Figures 2-12, 2-13, 3-5), but have not been shown to be continuous with it. Microprobe analyses of samples collected in situ by the writer, and other samples from collections, show this magnetite to be characterized by 0.1-1.0 wt. % Al₂O₃, 0.1-0.4 wt. % TiO₂, and an absence of Mn and Mg at levels of detection. The deposits occur as stringers and are associated with much garnet, hedenbergite, and other iron-bearing minerals. Some sparse iron sulfides, chiefly pyrrhotite, chalcopyrite, and pyrite, are also associated, particularly at the Gooseberry Mine, from which Palache (1935) lists numerous other associated minerals, including scapolite, zircon, garnet, and pyroxene. Spencer et al. (1908) showed a lengthy “prospecting ditch” over 300 feet in length, normal to the strike of these magnetite deposits and south of them.                 

The longstanding possibility that all these magnetite deposits are one continuous or nearly contiguous unit was not adopted by Frondel and Baum (1974). The compositional differences mentioned above clearly suggest that the Furnace Magnetite Bed is genetically separate from the other units, and has possible genetic linkages to the zinc orebody. However, the paucity of available information does not preclude the possibility that all the above-described magnetite deposits may be one unit, hosted by the Franklin Marble adjacent to the zinc orebody and south of it, and passing into or near the gneiss, nearly parallel to the contact of these rocks.

Copyright © 1995 by Pete J. Dunn			Website by Herb Yeates
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