MAJOR FORMATIONS Chapter 8. Regional and local geology of the Franklin-Sterling Hill area

Major formations and rock units

Detailed descriptions of all the regionally significant major rock units are given by Hague et al. (1956) and Baker and Buddington (1970). Portions of the geologic maps from both of these studies are presented in figures 8-2 and 8-3.




	Figure 8-2. Geologic map of part of the Franklin area. Black circles with “Z” overprints indicate the location of the zinc orebodies. Vertical margin is true north and is 2.85 miles (4.5 kilometers) for scale. Map taken from Baker and Buddington (1970).		Figure 8-3. Geologic map of the area adjacent to the Franklin (1) and Sterling Hill (2) orebodies, which are shown as heavy-ruled, black, numbered, hook-shaped symbols in the lower half of the map. The thin gray line passing through the Sterling Hill orebody is the 41^o05’ 00” north latitude line; that passing through the lower part of Franklin Pond is 74^o35’ 00” west longitude and is true north. Map taken from Hague et al. (1956).

The summary rock descriptions and stratigraphic column given below are only of those units which are in rough proximity to the zinc deposits at Franklin and Sterling Hill; geologic ages are given in boldface type. Such information is drawn largely from Spencer et al. (1908), Hague et al. (1956), Baker and Buddington (1970), and Frondel and Baum (1974).

Cambrian-Ordovician

Kittatinny formation - Blue limestone, mostly dolomitic, with locally shaly and sandy areas. Thickness is 2500-3000 feet (762-914 meters).

Lower Cambrian

Hardyston formation - Quartzite with locally conglomeritic domains. Thickness is about 30 feet (9 meters) locally.

Precambrian

Pochuck Gneiss series - Interlayered gneisses, predominantly microcline-, biotite-, hornblende-, or pyroxene-rich, with local concentrations of graphite or garnet. Thickness is over 2000 feet (610 meters).

Wildcat Marble - Coarsely crystalline marble, containing blocks of gneiss and pegmatite; still unstudied. Thickness is 300 feet (91 meters).

Cork Hill Gneiss - Very similar to the Pochuck Gneiss. Thickness is 800-1000 feet (244-305 meters).

Franklin Marble - Coarsely-crystalline marble, locally dolomitic and graphitic throughout. Thickness is 1100-1500 feet (335-457 meters).

Median Gneiss - Biotite gneiss. Thickness is 50-300 feet (15-91 meters).

Franklin Marble - Description given above. Thickness unknown, may be over 500 feet (152 meters)?

Byram Gneiss - Similar to Pochuck Gneiss. Thickness over 2000 feet (610 meters).

Below are noted some explanatory comments for some of the local rock units.

Kittatinny Limestone




	Figure 8-1. Geologic sketch map of the Franklin-Sterling Hill area, adopted from Pinger (1950). N-arrow is true north.

The Kittatinny Limestone is of Cambrian- Ordovician age and historically has been referred to as the Wallkill Limestone and the “blue limestone;” it was formally named by Kümmel (1900). It overlies the Cambrian age Hardyston Quartzite (Figure 8-1). It is bluish in part, is composed mostly of dolomite, and was described by Kümmel (1940) and others. Its extension westward toward Phillipsburg was described by Ludlum (1940). A summary is given by Baker and Buddington (1970), and a more recent discussion is given by Dalton (1989). Eighteen chemical analyses of Sussex County occurrences were tabulated by Wolfe (1948). The Kittatinny Limestone contains hydrocarbons locally and numerous accessory minerals including fluorite, sphalerite, arsenopyrite, galena, quartz, albite, and others. The closely related Leithsville Formation was discussed by Brewer (1993).

Hardyston Quartzite

The Hardyston Quartzite is of Cambrian age and occurs in contact with the erosional surface of the Precambrian rocks, marking the great unconformity (Figure 8-1). It had been early referred to as the Potsdam Sandstone, then was called the Hardistonville, then the Hardiston, and was given its present name by Kümmel (1900). It is composed of conglomerates, quartzites, sandstones, and sandy or shaly dolomite. Where it is absent locally, pure dolomite has formed on the unconformity. Conglomerates with source material similar to the Hardyston formation were found as cave-fillings and fissure fillings 170 feet (52 meters) below the Precambrian erosional surface in the Franklin Marble, thus indicating severe erosion and weathering in the Precambrian. These were found as a result of drillings done near the Franklin Iron Company Quarry within the Franklin Marble (Baum, 1986a). The wider extent of the Hardyston Quartzite has been described by Ludlum (1940) and Aaron (1969).

Cork Hill Gneiss

The Cork Hill Gneiss is a Precambrian, heterogeneous gneiss series indistinguishable from other local biotite-, hornblende-, and pyroxene-gneisses such as the Pochuck Gneiss except where it is stratigraphically identifiable between the Wildcat Marble and the Franklin Marble. New Jersey Zinc Company geologists (Hague et al., 1956) used the name Cork Hill Gneiss to refer specifically to the gneiss between these marble units (Figure 8-1). The contact with the Franklin Marble is overturned in the Franklin-Sterling Hill area, but is normal in the Lime Crest Quarry a bit northwest of Sparta. The west limb of the Franklin deposit is underlain by approximately 30 feet (9 meters) of the Franklin Marble within which occurs the Furnace Magnetite Bed; this marble is in turn underlain by the stratigraphically younger Cork Hill Gneiss. The Cork Hill Gneiss was also known as “miscellaneous gneiss” to Baker and Buddington (1970). Ries and Bowen (1922) noted that they saw no clean gneiss-marble contacts; much pegmatite occurred at the interface between the two formations.

Wildcat Marble

The Wildcat Marble is a thin band of Precambrian marble, persistent, but variable in width, and younger than the Cork Hill Gneiss (Figure 8-1). It is found to the west of the zinc deposits and is visually indistinguishable from the Franklin Marble.

Median Gneiss

This Precambrian gneiss is named the Median Gneiss because it occurs within the Franklin Marble; few outcrops are known (Figure 8-1). It varies in thickness from 50 to 300 feet (15-91 meters). It is largely composed of biotite- and quartz-gneisses, with local hornblende- and microcline-gneisses; some garnets are REE-bearing. Locally there are quartz-feldspar rich areas (pegmatite?). The median gneiss, not always recognizable as a gneiss, crops out 1200-1500 feet (366-457 meters) east of the orebody.

Pegmatite

Pegmatite, both conformable and discordant as shown by Wolff (1898), is common throughout the Precambrian rocks of the area. Those with orebody relations are discussed in detail under the geology of the deposits. The pegmatite in the surrounding rocks was mapped by Spencer et al. (1908), Hague et al.(1956), and others.

Franklin Marble

Early work

The Precambrian Franklin Marble has intimate contact relations with the zinc orebodies; both ore deposits are wholly enclosed within it, the Sterling Hill deposit approximately a mile, and the Franklin deposit three and a half miles, from the marble’s southern local limit in the Sparta area (Figure 8-1). Accordingly, it is deserving of a more expansive discussion.

The Franklin Marble was noted early and is discussed in varying degrees in almost all of the papers on the geology and origins of the deposit, which are cited where pertinent; the literature is voluminous. There is an isolated extension of the Franklin Marble in Pennsylvania, first noted by Bascom (Miller, 1912), and later studied by numerous others, including Crawford and Valley (1990) who provide numerous references to other studies. This outlier is not discussed further here.

The earliest studies of the local marble were by Vanuxem and Keating (1822), Pierce (1822), and Nuttall (1822). Later in the 19^th century, there ensued a number of studies focused specifically on the Franklin Marble, including those of Cook (1861) and others.

At the end of the century, there was a pivotal series of intensive studies by Nason (1890a with map, 1891a, 1891b, 1894b, 1894c; see Nason also under Dana, 1891), Williams (1894), and Kemp and Hollick (1894). There was much dispute at the time as to whether the white and blue limestones were each distinctly unique, or if the white was a metamorphosed part of the blue as argued by Nason (1890a). The confusion is understandable for there were areas where contacts were subject to differing interpretations. The argument for a Cambrian age, first put forward by Rogers (1836), was well and vigorously argued by Nason, but the matter was put to rest by a definitive and excellent study by Wolff (1895) and Wolff and Brooks (1898, with map), who superbly set out the relations accepted today: the Franklin Marble as Precambrian (Azoic in the early literature) and the Kittatinny Limestone as Cambrian in age. Wolff and Brooks also provided the best summary of the theories to date. Although Nason’s tenet of a Cambrian age was refuted, his work was recognized as a colossal contribution, great and seminal in nature. In 1906, Kümmel published much chemical data obtained by R. B. Gage on the Franklin Marble; many other data were summarized by Spencer et al. (1908). The modern specific delineations and structure, and discussions of “black rock” inclusions and other rock inclusions, are given by Hague et al. (1956) and Baker and Buddington (1970).

Nomenclature

The nomenclature needs some clarification. The Franklin Marble was called “white limestone” or “Franklin limestone” in all of the 19^th century literature, and this terminology was adopted in the Franklin Furnace Folio by Spencer et al. (1908) and by Palache (1935). The term “white limestone,” abbreviated as (wl), was also used consistently by all the geologists who prepared the geologic maps for the New Jersey Zinc Company. The early use of this designation was, in part, to differentiate it from the overlying Paleozoic “blue limestone,” now called the Kittatinny Limestone.

Description and mineralogy

The Franklin Marble is coarsely crystallized. Locally, it extends northeasterly and is over 20 miles in length, from south of Ogdensburg, to Big Island in Orange County, New York, and varies from 2 to 0.5 miles in width, but seldom over a half-mile in the Franklin-Sterling Hill area. The texture and chemical composition vary substantially, but the color varies only trivially, in weak hues of gray, yellow or pink; it is in general white throughout. The texture is generally coarse-grained, but medium-grained to fine-grained material is known; pressure twinning is common, and large rhombs are found locally. Metsger (1977, 1980) noted that “calcite grain diameters range from less than a centimeter to as much as sixty centimeters. In general, the sparsely mineralized calcite has a coarser texture than that which is heavily mineralized. It also appears that the calcite grains are flattened in planes parallel with the mineral banding in the marble.”

Compositionally, the Franklin Marble varies from nearly pure calcite to magnesian calcite, and locally to nearly pure dolomite; siliceous impurities are abundant only locally. Sixty-six chemical analyses of Sussex County occurrences were tabulated by Wolfe (1948), and over 1000 chemical analyses provided the basis for a statistical study by Tennant and White (1959).

The Franklin Marble is graphitic throughout its extent, except as noted below; isotopic data were given by Valley and O’Neil (1981) and Johnson (1990). Most of this graphite occurs in thin bands, some of which are deformed; they are usually conformable with other bands in the marble. Kümmel (1908) discussed the presence of graphite in other regional rocks and Crawford and Valley (1990) and others discussed it in the Franklin Marble in Pennsylvania.

Irregular bands and irregular blocks of gneissic rocks are common in the Franklin Marble; they were described by Spurr and Lewis (1925) and Hague et al. (1956). Metsger (1977, 1980) reported “anomalously high traces of zinc” at marble-gneiss contacts. Metsger also noted that the contacts between the gneiss and the marble are, within the marble, “commonly characterized by increasingly abundant coarse crystals of pyroxene, garnet, spinel, biotite, apatite, and feldspar as the gneiss is approached. Grain diameters reach as much as 3-6 centimeters.”

At its contacts with the Franklin orebody, the marble is graphite-free in a border zone 5-6 feet (1.5-1.8 meters) in thickness; this zone is coincident with an asymmetrical δ¹⁸O depletion halo (Johnson, 1990). Metsger (1977, 1980) noted that there are also graphite-deficient halos around certain pegmatite bodies. Frondel and Baum (1974) reported that “the banding in the hanging wall marble is conformable to the west limb of the Franklin orebody. It is in general parallel to the persistent bands of gneiss and of quartzite within the marble and, in the upper levels of the deposit, to the contact with the footwall Cork Hill Gneiss.” The banding in the marble near the keel is also conformable to that within the zinc deposit, at least in the contact areas. Other nearby bands in the marble, defined in part by graphite, silicates, or magnetite, also are conformable to the zinc orebody. The same relations obtain at Sterling Hill and additional studies of Fe, Mn, Zn, and Mg concentrations in the marble there were made by Metsger et al. (1954b) and Buis (1987).

Barlow (1981) studied the calcite-dolomite chemistry of the Franklin Marble, and Trim (1983) studied the petrology of quartz-feldspar bodies within it. Johnson (1990) reported δ¹⁸O isotopic compositions of 20-25 per mil for the Franklin Marble distal from the Sterling Hill orebody; these values are typical of those obtained for marine limestones of comparable age.

Kearns (1977) and Kearns et al. (1980) studied minerals from the northern part of the marble in Orange County, New York, with a specific focus on the F/(F + Cl + OH) ratios, and suggested that the presence of abundant fluorine in the marble increased the upper stability-limit for a tremolite assemblage. No such detailed studies have been made in the Franklin Marble in the vicinity of the orebodies. Tracy (1991) discussed the occurrences of barian micas, both in the marble at the Lime Crest Quarry in Sparta and at Sterling Hill, as noted by Reilly (1983). Germine (1982, 1986) studied some accessory minerals, with an eye to “asbestos” concentrations, and Wopenka and Pasteris (1993) provided Raman spectroscopy data.

Accessory minerals are locally abundant in the Franklin Marble, providing lithologic information in part, but also providing a wealth of information on the trace-element and minor-element composition of the marble formation as a whole. Palache (1935) reported tremolite, diopside, scapolite, tourmaline [now known to be uvite], chondrodite, and norbergite as common accessory minerals. Hague et al. (1956) reported about 25 sulfides, silicates, and oxides, with graphite, in insoluble residues from the Franklin Marble near Sterling Hill; the most common, in order of abundance were graphite, tremolite, mica, pyrite, pyrrhotite, quartz, and talc.

Many species have been found in superb mineral specimens in the local marble quarries, with crystal sizes well over 1 cm in some cases, and are much sought after to this day. These include, in part, chondrodite, norbergite, pyrite, uvite, tremolite, pyrrhotite, fluoborite, arsenopyrite, corundum, margarite, fluorite, phlogopite, spinel, rutile, sphalerite, and titanite. Many rare borates have been found in the Bodnar Quarries in the Franklin Marble north of the immediate area and probably exist unrecognized locally.

The mineral assemblages of the Franklin Marble are little studied; numerous replacement textures are also in need of study. Some species occurring in fine crystals, such as uvite, pyrite, and phlogopite, tend to occur as isolated euhedra with few other associated minerals. Many crystals, particularly of the silicate minerals, are often dull-edged, rounded, and subhedral (Rutstein, 1982). Of the numerous assemblages of note within the Franklin Marble, a few of the many treasured by the collector are spinel-chondrodite-calcite, norbergite-pyrrhotite-phlogopite- graphite, and the uncommon margarite-corundum- anorthite assemblage (Dunn and Frondel, 1990). A sampling of Franklin Marble mineral occurrences found in the Franklin Quarry was given by Betancourt (1986), and a sulfide-bearing Paleozoic vein was described from the Lime Crest Quarry by Cummings (1993).

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