Franklin, New Jersey Essentials, Part 1: Regional and Local GeologyLast Updated: 20th Jun 2013
By Stephen Fritz
The Franklin district is located within the New Jersey Highlands physiographic province, illustrated below.
Attribution: JimIrwin at en.wikipedia
Precambrian GeologyThe New Jersey Highlands are one of several outliers of Grenville-aged metamorphic rocks exposed in the eastern Appalachian Mountains.
As described by Volkert and Drake, it is underlain by Middle Proterozoic gneisses and marble that were metamorphosed to upper amphibolite to hornblende granulite facies and were intruded by granitic rocks. These rocks record oceanic-continental collision (the Losee metamorphic suite), continental uplift and erosion and then marine sedimentation (quartzofeldspathic gneisses, quartzite, calcsilicate gneiss and marble) followed by Grenville-aged syntectonic and post tectonic granitic igneous intrusions.
During the middle Proterozoic period, the region resembled the modern Pacific Ocean - South American plate boundary (preserved as the oceanic Losee metamorphic suite) and subsequently (during collision) was the site of continental followed by marine sedimentation derived from continental and island arc source rocks. The Franklin and Wildcat Marbles record sedimentation in shallow marine environments. These oceanic igneous and overlying sedimentary rocks were deformed and intruded by the Grenville-aged granitic rocks. The Grenville orogeny ended approximately 950 million years ago and was followed by a long period of erosion and uplift. By the late Precambrian, the Franklin district was exposed at the surface.
|Grenville Metasedimentary Rocks (cont.)|
Losee Metamorphic Suite (Basement Rocks) and Intrusive Rocks
In the latest Precambrian or earliest Cambrian, the Franklin district was covered by rising seas, first depositing the Hardyston quartzite and then a series of shallow marine limestones and dolomites. By the middle Ordovician, the region subsided rapidly, becoming a deep sea between the continent and an approaching island arc. When this island arc collided with North America, the first of the Appalachian deformations -- the Taconic Orogeny -- deformed New Jersey. Sole detachment thrust faults in the basement -- Grenville-aged metamorphic rocks -- and imbricate fan thrusts cutting upwards through the overlying Cambro-Ordovician sedimentary rocks affected the entire region. In the Franklin District, this resulted in a block of Precambrian rocks being thrust upwards into contact with younger Paleozoic sedimentary rocks. Later, high angle faults cut across the thrust faults. Also, there was igneous activity in the region. The southern end of the Franklin Mine is cut by the post-Ordovician Great Dike, a diabase (locally called “camptonite”) with biotite phenocrysts. Diabase is the most common post-Ordovician Ordovician igneous rock, however, there is a large nepheline syenite intrusion -- the Beemerville Complex -- located to the northwest.
Yet another orogenic event -- most likely the late Pennsylvanian-aged Alleghenian Orogeny -- subsequently deformed the region. Many of the older Taconic-aged faults were reactivated and additional folding occurred.
There may, however, have been additional orogenic activity in the region. While evidence of deformation during the middle- to late- Devonian Acadian Orogeny is lacking, there may have been thermal effects (such as minor retrograde metamorphism and hydrothermal activity) associated with that event.
These Paleozoic events are often difficult to date conclusively. The extensive hydrothermal activity and alteration -- particularly in the Franklin Mine -- may have been due to numerous events over a long period of time.
Mesozoic and Cenozoic Geology
Rifting during the late Triassic and early Jurassic is recorded by rocks of the Newark Basin within the Piedmont Geologic Province. This event did not affect the Franklin District.
In the Cenozoic, the southern part of New Jersey is underlain primarily by marine deposits. These are absent in the Highlands. During the Pleistocene, northern New Jersey experienced continental glaciation. Salisbury (1902) provides excellent descriptions of New Jersey glacial deposits. In the Highlands, they consist of till and stratified deposits.
The till of the Western Highlands is composed largely of gneissic and granitic material although some material was carried up from the sandstone and limestone Kittatinny Valley on to the Highlands to the east. There are also Salisbury (1902) provides an excellent description of local tills excerpted below:
Till near Franklin
“The limestone bowlders in the Central Highlands are of two kinds, a white crystalline variety and a blue, compact dolomite. The latter is the more widely distributed, and its bowlders are well worn and striated where protected from weathering. The crystalline variety is the less widely distributed. The most easterly point where it was observed was south of Morris pond, and the most southerly about three miles north by west of Woodport. The distribution of white limestone is essentially the same as that of zinc ore in the drift, and the two probably came from the same place, the vicinity of Franklin Furnace.”
Kames (stratified deposits) near Ogdensburg
“At Ogdensburg a huge triangular embankment of stratified drift crosses the valley, its base resting against the east bluff (Plates XXXIII and LVII). Its crest has an altitude of about 660 feet, and is 100 feet above the valley to the south. The free end of the embankment does not now reach the western side of the valley, and the evidence seems to indicate that it never did, or at least that it never did at the level of the present crest. This embankment of drift is believed to have been deposited in a huge crevasse in ice which had lost its motion before the deposition took place. The material is plainly stratified, and on the whole is well assorted, although large bowlders occur at all depths.”
Lacustrine deposits of glacial Lake Sparta
“From a point just north of Sparta to Ogdensburg, the Wallkill flows through a marshy alluvial plain, which locally has a width of half a mile or more. Along this part of its course the river has been silting up its valley in post-glacial times. A few small kames and kame terraces occur at several points along the sides of the valley, some distance above the alluvial plain, the best developed being on the east side of the valley a mile and a half south of Ogdensburg.”
A detailed cross section of the northern New Jersey is available here.
Formal boundaries of the Franklin-Sterling Hill area as provided in Dunn (1995), page 71:
East: The base of the Hamburgh and Sparta mountains.
West: The eastern edge of the Wildcat Band of marble to the place where it plunges beneath the unconformity. The west boundary north of that point is defined by the Wallkill River.
South: Brooks Flat Road in Ogdensburg, just a bit south of Sterling Hill.
North: A line east-west from the intersection of Route 23 and Route 517 at the Hamburg-Franklin border (formerly known as Hardistonville, and the type locality for the Hardyston Quartzite).
The district is underlain by rocks of the Precambrian metasedimentary sequence (the Franklin and Wildcat Band Marbles and Cork Hill Gneiss) and later Grenville-age intrusive rocks (Byram Intrusive Suite). To the northwest, The Hardyston Quartzite and overlying Cambro-Ordovician limestones rest unconformably upon the Precambrian rocks. There is also an important downthrown fault block -- bounded by the East and Zero Faults -- that truncates portions of the Franklin Marble and gneisses. The Zero Fault is economically significant because it also truncates a portion of the Sterling Hill deposit as illustrated in cross section F-F’ in the Palache (1935) plate reproduced above.
The Franklin Marble, as described by Vlokert and Drake (1999), is primarily “white to light-gray, medium- to coarsely crystalline, massive to moderately layered, calcitic to locally dolomitic rock. Principal accessory minerals in the Franklin area are graphite, phlogopite, chondrodite, and clinopyroxene.” This describes the local Franklin Marble, except in close proximity to the ore deposits, where there is commonly a zone lacking graphite surrounding the ore bodies.
Geologic Notes on the Franklin Mine
There are several important features associated with the Franklin Mine:
- Furnace Magnetite Band
- Hydrothermal Alteration
Furnace Magnetite Band
The Furnace Magnetite Bed lies between the Franklin ore body and the footwall gneiss. It consists of magnetite disseminated in calcite and is conformable with the ore body and footwall (Cork Hill) gneiss over much of its length. At depth, it departs from both the gneiss and ore body.
Nearly vertical melanocratic dikes -- in the diabase family -- cut the southern end of the Franklin deposit. These dikes have not been intensively studied, but are post-Ordovician aged with biotite phenocrysts.
Pegmatites -- both sodic and potassic -- occur in the ore body. They have no significance to the genesis of the ore, but are significant to the collector both for the associated igneous and hydrothermal minerals and the reported recrystallization effects on the ore and calcium silicate units. The largest and best-formed rhodonite crystals from Franklin were reportedly encountered in proximity to pegmatites.
Hydrothermal veins and overprints later, and of lower temperature and pressure, than the Precambrian Grenville Orogeny metamorphism are pervasive in the Franklin deposit. These veins may be simple or complex and are hosts to some of the rarest minerals encountered. The borate- and arsenate- bearing minerals are most commonly encountered in these veins. There are two other assemblages of particular interest. First, the Buckwheat Dolomite was a vein system in the southern end of the deposit replaced primarily with gray dolomite; this vein contains a great variety of microscopic minerals as described by Peters, Koestler, Peters and Grube (1983). Second, the so-called Parker Shaft lead-silicate minerals, actually encountered within the calcium silicate units in the ore body in the Palmer shaft pillars. Originally encountered on the dumps of the Parker Shaft in the 1890’s, the minerals were not noted in situ until near the end of mining at Franklin when the shaft pillars were being removed. In addition to the lead silicate minerals, willemite, hancockite, andradite/grossular, vesuvianite, datolite, cahnite, thomsonite, xonotlite, hodgkinsonite, manganaxinite, andradite, rhodonite, prehnite, cuspidine, pectolite, and a great many others are also encountered.
Geologic Notes on the Sterling Hill Mine
There are several important features associated with the Sterling Hill Mine:
- Zero Fault
- Rubble Breccia
- Mud Zone
- Sterling Depression
The Zero Fault is a major structure that forms the western side of a downthrown fault block. At the surface, the fault block exposes Cambro-Ordovician limestones. At depth, it truncates the eastern portion of the ore body. Although the New Jersey Zinc Company went to considerable effort to locate the “missing” section of the ore body, it was never found.
The rubble breccia was encountered from the 700 foot level of the mine to the 1850 level, where further exposures were lacking. The breccia is a karst feature consisting of angular fragments of Franklin Marble and ore minerals, cemented by a limestone that resembles the local Cambro-Ordovician limestones. Fossils in the breccia cement indicate it is of Paleozoic age.
The Mud Zone is a deeply weathered zone extending from the surface to 680 feet depth. It is black-to-brown zincian mud; samples contained up to 35 wt. % Zn, but it was impractical to mine. The mud was studied by Metsger et al. (1958) and found to be comprised of hemimorphite, goethite, kaolinite, and nontronite. The Passaic and Noble Pits were mineable areas near the surface, early exploited for hemimorphite.
The Sterling Depression is a large saprolite zone east of the ore body. It extends over 350 meters deep and may be associated with glacial Lake Sparta.
 Richard A. Volkert, Robert E. Zartman, Paulus B. Moore, U–Pb zircon geochronology of Mesoproterozoic postorogenic rocks and implications for post-Ottawan magmatism and metallogenesis, New Jersey Highlands and contiguous areas, USA, Precambrian Research, Volume 139, Issues 1-2, August 2005, pp 1-19
 Volkert, Richard A. and Drake, Avery Ala Jr., Geochemistry and Stratigraphic Relations of Middle Proterozoic Rocks of the New Jersey Highlands, U.S. Geological Survey Professional Paper 1565–C, 1999
 Herman, Gregory C. & Monteverde, Donald H., Tectonic framework of Paleozoic rocks of northwestern New Jersey: bedrock structure and balanced cross sections of the Valley and Ridge Province and southwest Highlands area in, Paleozoic Geology of the Kittatinny Valley and Southwest Highlands Area, New Jersey, Field Guide and Proceedings of the Sixth Annual Meeting of the Geological Association of New Jersey, 1989
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