General discussion


Regional geology


Local geology


Major formations and rock units


The local magnetite deposits

Local geology

General relations

The oldest surface rock units in the Franklin-Sterling Hill area are Precambrian rocks which are interlayered, are of both metasedimentary and metavolcanic origins, and which were severely metamorphosed and folded in the Precambrian. These are accompanied by igneous rocks which are both concordant and discordant. These early rocks are overlain by Cambrian and younger rocks, which lie unconformably upon them or are faulted down against them. These, in turn, are overlain by glacial deposits, contemporary sediments, and weathering products. The main structural features of the region, including the belts of major rock formations, major faults, foliation of the Precambrian rocks, and the long axes of the intrusive bodies, all trend NNE - SSW. The area is seismically inactive in terms of major events; Isacks and Oliver (1964) give a discussion of local activity.

The Precambrian

The earliest Precambrian rocks are metasediments or metavolcanics which occur within the Byram Gneiss east of McAfee. They were, without interruption, followed in the deposition sequence by a thick mass of limestone, now known as the Franklin Marble. This was followed without interruption by sediments and possible volcanics which formed the Cork Hill Gneiss. West of this is found a limestone band now known as the Wildcat Marble. Deposition continued and formed the rocks of the Pochuck Mountain Gneiss.

Metamorphism: extent, age, temperatures, and pressures      

There then ensued a long period of intense folding of the regional Precambrian rocks without additional sedimentation. This period of Precambrian folding was associated with a regional, intense, and long lasting period of metamorphism. There were a number of metamorphic events (Grauch and Aleinikoff, 1985). All the extant local rocks were metamorphosed to the sillimanite grade (upper amphibolite to granulite facies). The large and concordant rock masses, such as the Byram Gneiss, Losee Gneiss, and other gneisses were formed at this time. After this period of intense folding, many of the discordant pegmatites were intruded; they are likely related to postmetamorphic Precambrian granitic intrusives. Detailed discussions of the metamorphism are provided by Dallmeyer (1972, 1974).  

The age of the metamorphism of the orebodies has been the subject of many investigations. Long et al. (1959) and Long and Kulp (1962), using K-Ar radiometric methods, provided ages for Sterling Hill phlogopite (905 million years) and Franklin hendricksite (810 million years), but suggested these values might have been lowered by later metamorphism from a true value of 1150 million years; see Johnson et al. (1990) for recalculations of these values and other data. Frondel (1970) reported an age of 955 (±30) million years by dating of uraninite from Sterling Hill and gave comparable, but less precise, values for zircon and thorite from the Cork Hill Gneiss. Frondel (1970) also reported the age of the orebody skarn (calcium silicates) at 900 (±45) million years. Dallmeyer et al. (1975) reported the age of the regional rocks to be 1.06 billion years, and Frondel and Baum (1974) reported an approximate age of 950 million years. Additional work at Sterling Hill by Johnson et al. (1986) found ages for biotites above, below, and very near the orebody to be 884 (±35), 883 (±35), and 908 (±36) million years; Johnson (1990) attributed these dates to the period of cross-layer fracturing at Sterling Hill. An age of 1.1 billion years was reported for galena within a marble “dike” crosscutting older pyroxene granulite gneiss from the core of the Sterling Hill orebody. Such “dikes” were formed when “mobile carbonate flowed into fissures in the more brittle rocks” (Metsger, 1977, 1980).

Carvalho (1978) and Carvalho and Sclar (1979, 1988) found a minimum, peak metamorphic temperature of 760o C based on gahnite-franklinite intergrowths at Sterling Hill. This temperature is in approximate agreement with the finding of Frondel and Klein (1965) of a rough estimation of the minimum peak temperature of 650-700o C for the initial homogeneous oxides of the orebody. In further support, studies of Precambrian gneisses in the nearby Hudson Highlands found peak metamorphic temperatures of 700-775o C (Dallmeyer and Dodd, 1971; Young, 1971). Leavens (1988, 1990) has offered some discussion in connection with this subject. Takahashi and Myers (1963) suggested that if the deposits formed under a total pressure of 700-1300 atm., the temperature of formation would be between 557 and 827o C, with partial pressures of oxygen estimated to be between 10-8.2 and 10-37.5 atm., and the upper limit for the partial pressure of sulfur at 0.2 atm.

Kearns et al. (1980) studied the Franklin Marble and estimated a temperature of 836 (±40)o C at 4-7 kilobars. However, Valley et al. (1982) recalculated this to 685o C at 8 kilobars. Hewins and Yersak (1978), using geothermometric techniques applied to a variety of assemblages, concluded that the deposits experienced temperatures of at least 700o C and pressures on the order of 5 kilobars. Johnson (1990) calculated pressures of 4.1-4.9 kilobars and suggested the rocks had been buried to a depth of at least 11-16 km.

Subsequent to this period of burial and metamorphism the local rocks were uplifted and cooled (Dallmeyer et al., 1975). The rate of cooling of the orebodies was estimated to be on the order of 1.6o C per million years (Carvalho and Sclar, 1988) to 3o C per million years (Johnson, 1990). The rate of uplift was probably variable according to Dallmeyer et al. (1975) who suggested that after metamorphism the regional rocks (about 1060 million years before the present [b.p.] at 15 km depth and 700-750o C) were uplifted at an approximate rate of 0.01 mm/year (to about 900 million years b.p. at a depth of 14.2 km at 525o C), and then uplifted at an approximate rate of 0.03 mm/year (to 790 million years b.p. at a depth of 10.3 km at 325o C). He estimated the final period of uplift at an approximate rate of 0.07 mm/year with final exhumation of these rocks at the surface at about 650 million years b.p.

The great unconformity and the Cambrian and later rocks

Subsequent to uplift, the area was exposed to a very lengthy period (well over 100 million years) of intense erosion in the late Precambrian and early Cambrian. Here was created the basis for the great unconformity. The Franklin deposit was partially exposed, perhaps in large part but at least at the northern end of the west limb, during this period, resulting in some formation of secondary zinc and iron minerals such as hemimorphite and goethite. The Sterling Hill deposit was likely exposed as well, but direct proof is lacking. The erosion surface was quite irregular in the area, and the period of erosion was very long. There is some resultant Karst topography locally.

The Precambrian rocks were then overlain by the Hardyston Quartzite of the early Cambrian. Detrital franklinite, found near the Franklin Mine in the lower part of the Hardyston Quartzite, definitively attests to the formation of the zinc deposits in the Precambrian (Baum, 1986a). The Hardyston Quartzite was in turn overlain by the Kittatinny Limestone (mostly dolomite), deposited in Cambrian-Ordovician time. These rocks, which crop out mostly to the west of the ore deposits, exhibit broad open folding, which presumably took place in the late Paleozoic and did not markedly affect the orebodies. These rocks are also locally faulted. The whole area was subjected once again to uplift and erosion in the late Mesozoic and in the Tertiary; this period of erosion resulted in the bedrock basis for the present topography. A description of a particularly good local exposure of these younger rocks is given by Johnson (1952).

Large to small dikes branch vertically and are assumed to be intruded along minor faults or joints, resulting in part from the Paleozoic folding noted above. They were first described from the Franklin Mine as diabase by Emerson (1882) and are discussed in more detail below. Kearns (1977) provided additional data on some of the rock units, especially those with relations to the Franklin Marble.

Surficial geology

The local area was glaciated during the Pleistocene; the glaciers moved nearly N-S, and glacial deposits and recent alluvium cover much of the area. The local ponds are mostly caused by glacial outwash debris, except the few which are man-made. The effects of local glaciation were first described by Jackson (1851), who noted a 4-foot pothole in Franklin. Boulder trains of ore, first noted by Alger (1845), were carried south from the Franklin Mine, and are found to this day in Search’s gravel pit on Cork Hill Road. Some of these are cobble-sized and rounded; others are very large and have some planar surfaces. 

Such localized glacial deposits of ore minerals caused some early confusion about the extent of the deposits. Much of it was straightforward confusion and was cleared up as the science of geology provided explanations. However, in early times not all were convinced. Kitchell (1855) reports his conversation with a man who had applied his “mineral-rod” to the search for extensions of the deposits and had “traced the franklinite and zinc ores of Sterling Hill for miles from their present location.” The man was sure of his findings, in part, because of the numerous glacial erratics on the hills, and he observed that “a good many scientific geologists have been around here, who think they know everything about minerals and so on, and they say that these are loose pieces that have travelled from the mine at Sterling Hill, but it ain’t so; it’s agin’ nater [nature] for stones to swim or run up hill.”

Blake (1894a) commented on boulders of ore strewn for miles to the south, to elevations of 500-600 feet on the flanks of some mountains, and remarked that such boulders were being shipped as ore from a great accumulation of them south of Sterling Hill. He also reported a mass of zinc ore, some thousands of tons, lying on granite near Sparta; it was being worked for profit.

  Figure 8-4. Geologic map of the Franklin mining district; this is the “Special Map.” Illustration from Palache (1935).  

Regional glacial effects were discussed in great detail by Salisbury in Spencer et al. (1908), and subsequently by Hague et al. (1956). Local glacial effects were further investigated by Herpers (1961), Stanford and Harper (1985), and Stanford (1993), who studied the Ogdensburg-Culvers Gap moraine. This moraine crosses the Wallkill Valley south of Franklin at the northern end of Ogdensburg; it was utilized at different times for road-metal, mine-fill, and physical support of railroad lines.

The present topography is dominated by northeast-trending ridges. Between these are valleys, both broad and narrow, formed by stream erosion and modified by glacial erosion and deposition features, but morphologically controlled in large part by the underlying structure of the bedrock. In the general area of the Franklin Furnace Folio, altitudes vary from 500-1400 feet (152-427 meters), but they only vary from ~ 530-833 feet (162-254 meters) within the area designated by the “Special Map” (Figure 8-4).


Hague et al. (1956) reported that the principal structural feature locally is “an overturned isoclinal syncline which contains some smaller local folds and is flanked on either side by overturned anticlines with cores of intrusive rock; the Franklin Marble and Wildcat Marble in the vicinity of Franklin and Odgensburg are overturned.” These same writers gave detailed discussions of local lineation, foliation, the major and minor folds, cross-folds, joints, faults, deformation mechanics, and the local stratigraphy. Other specific structural features are discussed below in relation to the orebodies. Metsger (1977, 1980) reported the regional structural pattern as that of a group of northeast-trending, overturned, isoclinal folds, plunging 10-30o northeast. Faults are normal in general, dip from 70o to vertical, and strike northeast with the regional Precambrian trends.



Copyright © 1995 by Pete J. Dunn
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