Adirondack & Mid-Atlantic (Reading Prong) Iron Belt - Benson, Lyon Mountain, Dover, Rittenhouse Gap


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The Adirondack and Mid-Atlantic (or Reading Prong) belts contain two groups of important magnetite-apatite deposits, some with associated copper and REE mineralisation, which have been significant historical iron producers in the states of New York, New Jersey and Pennsylvania, in the USA. These deposits are hosted by Proterozoic and Palaeozoic rocks and are members of the iron-oxide copper-gold family of deposits. They include the Benson, Lyon Mountain, Jayville, Sanford Lake and Mineville, in the Adirondack belt, separated by a 175 km wide belt of Palaeozoic cover from Rittenhouse Gap, Dover, Scrub Oaks and other mines to the south in the Reading Prong area.

There is evidence of at least five major compressional orogenies in the northeastern United States during the Phanerozoic. The earlier high grade metamorphic event associated with late Mesoproterozoic the Grenville orogeny occurred in two stages, when arc-derived sedimentary and volcanic rocks were metamorphosed to upper amphibolite and granulite facies during the i). 1300 to 1200 Ma Elzevirian Orogeny, and then again during the ii). 1100 to 1050 Ma Ottawan Orogeny (Grauch and Aleinikoff, 1985), separated by the deposition of a rift sequence of supracrustal gneisses of sedimentary and volcanic origin and marble (Volkert and Drake, 1999). Anorogenic granites and granitic pegmatites were intruded within region, both between these late Mesoproterozoic orogenies, and later (Drake et al., 1991, Drake and Volkert, 1991).

Rift-related clastic sedimentary and bimodal volcanic rocks of Neoproterozoic age unconformably overly the Mesoproterozoic rocks (Drake, 1999, Volkert et al., 1999). Rifting resumed at ~575 Ma, primarily south of the Adirondack region, producing a sequence of bimodal volcanic and clastic sedimentary rocks. Iron oxide mineralisation is thought to have been deposited both during the Mesoproterozoic and Neoproterozoic events.

Palaeozoic sedimentary rocks unconformably overlie the Proterozoic section, over a 175 km wide width separating the Adirondack region from the Reading Prong belt to the south. Compression during the Ordovician Taconic Orogeny, Devonian Acadian Orogeny and Permian Alleghany Orogeny subjected the sedimentary rocks to lower greenschist facies metamorphism, and the development of a fold and thrust belt range of mountains (Appalachian range). Little igneous activity and no iron oxide mineralisation is known to be associated with the Palaeozoic compressional orogenies.

Late Triassic and early Jurassic rifting led to the opening of the Atlantic Ocean, intrusion of voluminous dolerite sills, and formation of restricted red bed rift basins. Metasomatism of carbonate rocks near the dolerite intrusions formed several large magnetite skarns (eg., Cornwall, Pennsylvania). The eastern margin of the North American continent has remained passive since the Jurassic with minor isostatic uplift associated with erosion of the highlands.

The 1055 to 1035 Ma (U-Pb dates; Chiarenzelli and McLelland,1991, and zircons; McLelland, et al., 2001) Lyon Mountain Granite Gneiss hosts most of the major magnetite deposits in Adirondack region. There are several phases of the Lyon Mountain Granite Gneiss, based primarily on the differences in their feldspar mineralogy. McLelland, et al., (2001) report the relative abundances of these phases in the Adirondacks to be quartz-microperthite leucogranite (45%), quartz-albite leucogranite (20%), quartz-microcline leucogranite (10%) and microantiperthite granite (25%). In addition to quartz and feldspar these rocks all contain minor titanite, aegerine-augite and disseminated magnetite. The Lyon Mountain Granite lacks strong deformation, indicating intrusion after peak metamorphism.

Magnetite deposits throughout the region occur as lenses and veins within the Lyon Mountain Granite, with associated apatite, minor amounts of ilmenite, and locally, trace pyrite. All of the magnetite deposits in the region are associated with alkali exchange hydrothermal alteration and some of the apatite is enriched in REE (McKeown and Klemic, 1956; Lindberg and Ingram, 1964).

The key deposits include:

Lyon Mountain, Clinton County District, New York

    The total recorded production from the Clinton County district was <20 Mt of ore (Postel, 1952), most of which occurred in pyroxene-rich micro-perthite phases of the Lyon Mountain Granite and/or associated with pyroxene skarn alteration, although sodic phases of the granite host almost 25% of the ores in the district (Postel, 1952). There is no clear spatial regional trend segregating deposits in potassic rocks from those in sodic rocks, although deposits in sodic rocks generally occur in the centre, surrounded by ores in potassic rocks where several deposits cluster. The Chateaugay or Lyon Mountain Mine was the biggest single producer in Clinton County, yielding 15 Mt of magnetite ore.
    Ore in the Lyon Mountain Mine occurred in pyroxene-bearing, pink, microperthite-rich phases of the Lyon Mountain Granite (Postel, 1952). "Miarolitic cavities" up to a metre in diameter are described by Gallagher (1937) in the Lyon Mountain Granite, lined with pegmatitic orthoclase, microperthite, hastingsite and quartz, overgrown by magnetite and albite. As the gneissic foliation and pegmatites in the district are clearly folded, Newland (1908) interpreted the concordance of massive magnetite mineralisation with gneissic foliation to indicate mineralisation predating metamorphism. The magnetite in ore lenses contain trace amounts of ilmenite, along with highly variable apatite contents and extremely scarce pyrite (Gallagher, 1937). In addition, quartz, feldspars, aegerine-augite and hastingsite are also found as minor gangue minerals in magnetite bodies. McLelland, et al. (2001) report data on quartz-magnetite oxygen isotope pairs that indicate equilibrium at temperatures between 600 and 700°C and δ18O values consistent with an evolved basinal brine source for the oreforming fluids.
    Martite ores in the Ausable group of mines south of the Lyon Mountain Mine are hosted by sodic plagioclase-rich (albite-oligoclase) phases of the Lyon Mountain Granite, while nearby magnetite ores are primarily found in pink microperthite-rich, ferromagnesian mineral-poor phases of the same granite, associated with variable amounts of purple fluorite (Postel, 1952).

Mineville District, New York

    The Mineville district is the largest producer in the Adirondacks, with ore in relatively sodic phases of the Lyon Mountain Granite (the "syenite" of Kemp, 1908). The host rocks comprise sodic plagioclase, microperthite, microcline and quartz, with minor augite, titanite and magnetite, commonly assaying up 6% Na
2O (McKeown and Klemic, 1956). The magnetite ore lenses, which pinch and swell, but are conformable with the metamorphic foliation, are accompanied by variable amounts of REE-bearing fluorapatite. McKeown and Klemic (1956) reported monazite and bastnaesite associated with hematite filling fractures within, and rimming apatite, giving fourteen apatite separates from magnetite ores an average of 0.032% U, 0.15% Th and 11.14% REE oxides. Kemp's (1908) description suggests REE-bearing apatite is particularly abundant in "Red Ore". Traces of pyrite and tourmaline are as gangue.

Benson Mines, New York

    Historic production statistics indicate a total production to 1965 of 18.6 Mt of ore (Crump and Beutner, 1968), although Friehauf et al., 2002 suggest the 3.5 km long pit infers significant subsequent production before the mine closed in 1976. The Benson Mines deposit differs from most other magnetite deposits in the Adirondack region in its close association with metasedimentary rocks in addition to granitic gneisses. Garnet-quartz-K feldspar gneiss, sillimanite-quartz-microcline gneiss, and pyroxene-hornblende-quartz-orthoclase gneiss all host ore (Crump and Beutner, 1968). A significant amount of specular hematite accompanies the magnetite, in addition to traces of molybdenite, fluorite, apatite, sphene, rutile and ilmenite. Minor copper, occurring as bornite, chalcopyrite, chalcocite, covellite, azurite, malachite and native copper, is also recorded at Benson Mines (Crump and Beutner, 1968). Overgrowths of poikiloblastic garnet and sillimanite suggest that at least some iron mineralisation predated metamorphism (Hagni, et al., 1969) .

Jayville Mine, New York

    Magnetite at this deposit is accopanied by biotite "sköls" that are strongly altered to earthy chlorite cut by calcite-hematite- pyrite-jasper veinlets (Leonard and Buddington, 1964). Abundant limonite throughout the deposit suggests the gneiss contained large amounts of sulphide prior to supergene oxidation. Abundant vonsenite ((Fe
+2)2Fe+3BO5), and elevated Mn, Ba and F also characterise the Jayville deposit. The origin of the calcite veins has not been explained.

Rittenhouse Gap, Pennsylvania

These are the largest and most studied of the Proterozoic deposits in Pennsylvania, and comprise en echelon magnetite veins cutting granitic gneiss. The foliated host granitic gneiss is largely composed of sodic plagioclase and quartz with only trace amounts of biotite and K feldspar. Magnetite veins with sharp margins contain quartz, biotite and riebeckite (partially altered to brown sericite along cleavage planes), whereas disseminated ores comprise magnetite-quartz-plagioclase±biotite. Intergrown, granoblastic, magnetite-quartz-feldspar, with the magnetite interstitial to quartz-feldspar grains, suggests equilibrium during high-grade metamorphism of the ores. Coarse-grained quartz-feldspar-magnetite-allanite "pegmatites" are also found in the district, but do not constitute ore. Flow-banded, aphanitic felsite dykes with sparse feldspar phenocrysts, and metabasalt dykes crosscut magnetite veins. These dykes contain high Nb, Ta, Zr, Hf, Be, Ga, Sn and W concentrations characteristic of volcanic rocks related to the Neoproterozoic event, supporting a Mesoproterozoic age of mineralisation. Pyrite, molybdenite and fluorite-bearing veinlets with trace sphalerite cut felsite dykes, and do not appear to be related to the magnetite mineralisation.

Dover District, New Jersey

This is the largest iron oxide district in the Mid-Atlantic (Reading Prong) belt, having produced over 26 Mt of magnetite ore. The ore occurs as high-grade magnetite veins that pinch and swell cutting the highly sodic Losee Gneiss. Magnetite, locally has minor associated ilmenite, and highly variable amounts of apatite. Pyrite, chalcopyrite and pyrrhotite occur with magnetite, although usually totalling <2%. Leucocratic alaskite, essentially comprising pure K feldspar and quartz, is spatially associated with ore (Sims and Buddington, 1958). These potassic alteration zones lie within a larger volume dominated by strongly sodic leucocratic rocks such as albite-oligoclase gneiss. Granitic pegmatites are also spatially associated with some deposits within the district, although crosscutting relationships suggest they pre-date iron mineralisation (Sims and Buddington, 1958). The Scrub Oaks deposit contained significant REE mineralisation in magnetite veins that cut sodic-altered metavolcanic gneisses. The coarse-grained magnetite at Scrub Oaks was associated with lesser apatite, rutile, tourmaline and biotite (Sims, 1953; Williams, 1965), while "pegmatites" are spatially associated with some magnetite deposits. REE concentrations are locally up to 3% oxides in coarse-grained magnetite, partially altered to hematite (Klemic et al., 1959).

Andover/Sulfur Hill District, New Jersey

The Andover/Sulfur Hill district produced less than half a million tonnes of iron ore (Sims and Leonard, 1952), but is representative of hematite-rich deposits in the Mid-Atlantic Iron Belt. Ore in the Andover deposit occurred primarily as massive black hematite (martite) hoted within highly potassic metasedimentary rocks. The ores are rich in manganese (up to 0.45% MnO), lack sulphides, and consist primarily of hematite, magnetite and quartz with minor chlorite and sericite alteration of the wall rocks (Volkert and Puffer, 2001). Host rocks have only been subjected to greenschist facies metamorphic grades, leading Volkert to conclude that, unlike the magnetite deposits at Dover and Rittenhouse Gap, they are Neoproterozoic and postdate high-grade metamorphism. The Sulfur Hill deposit ore is altered to a skarn assemblage of andradite garnet, salite pyroxene, scapolite, stilpnomelane/biotite, apatite and fluorite (Neumann, 1952; Sims and Leonard, 1952) associated with pyrite, pyrrhotite, sphalerite, galena, chalcopyrite and minor molybdenite (Neumann, 1952).

These summaries are taken direct from Friehauf, 2002 - Comparison of the geology of Proterozoic iron oxide deposits in the Adirondack and Mid-Atlantic Belt of Pennsylvania, New Jersey and New York (see citation below).

The most recent source geological information used to prepare this summary was dated: 2002.     Record last updated: 23/8/2013
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
 References to this deposit in the PGC Literature Collection:
Friehauf K C, Smith R C and Volkert R A,   2002 - Comparison of the Geology of Proterozoic Iron Oxide Deposits in the Adirondack and Mid-Atlantic Belt of Pennsylvania, New Jersey and New York: in Porter T M (Ed.), 2002 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.2 pp. 247-252

 References to this deposit in PGC Publications: Want any of our books ? Pricelist
Friehauf K, Smith R, Volkert R A, 2002 - Comparison of the Geology of Proterozoic Iron Oxide Deposits in the Adirondack and Mid-Atlantic Belt of Pennsylvania, New Jersey and New York,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v2  pp 247-252
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