BENSON MINE, Star Lake, St. Lawrence County, NY

The Benson Mines is a Grenville-age, gneiss-hosted, low Ti-Fe (oxide) deposit located in the Adirondack Mountains, Star Lake, St. Lawrence County, New York. It was mined in an open pit that was about 4 kilometers long and 244 meters wide. Information about the minerals from this deposit is very scarce (Hagni 1968; Hagni et al. 1969; Lupulescu et al. 2010) and did not catch the attention of many of mineral collectors. Presented here, however, are compiled and original data referring to the mining history, geology, and the mineralogy of the Benson Mines, in an effort to highlight the beauty and diversity of the minerals and the environments in which they formed during the complex geological history that shaped this interesting mineral deposit. While the mine does not contain an exceedingly large number of mineral species, some very exquisite and unusual mineral specimens have been collected over the years from the site.  Many of these samples now exist in private or institutional collections like Canadian Museum of Nature and the New York State Museum.

HISTORY
The Benson Mines had a pattern of boom and bust in its mining history that spanned over 160 years since its initial discovery to the final closing date. In this way its history is very similar to almost all the iron deposits in the Adirondacks. The brief succession of historical stages in the development of this mine was presented by two staff engineers from the Jones & Laughlin Steel Corporation (Crump and Beutner 1968) and their paper provides the basis for the discussion on the history of the Benson Mines.

In 1810, when engineers were doing a site survey for a military road to connect Albany with Ogdensburg, they found that a local magnetic field disturbed the needles of their instruments (Crump and Beutner 1968). That was the first mention of a possible underground accumulation of a magnetic mineral at the surveyed site. Latter on, Emmons (1839) wrote in his annual report “it is known as Chamont bed, and was explored to some extent, twenty years ago ... a portion of this hill is occupied by the ore mixed with flint, varying in proportions, from 50 to 80 per cent of iron. It is black and resembles the Palmer ore. It is described by Dr. Ambler from Rossie ... as a complet pepper and mixed salt”. In this way, Dr. Ambler pointed for the first time to the main texture of the ore, grains of magnetite in quartz. Emmons also mentioned the host rock as a “gneiss rock”, the orientation of the deposit as “nearly east and west, and the richest portion is on the southern declivity”, and made the first observation on the quality of the ore: “nothing appears injurious, and the probability is, that it will prove valuable”.

A lumber company that worked in that part of the Adirondacks extended a road in the vicinity to the deposit in 1889. That is the moment of the first attempt to open the deposit. This opportunity occurred because the ore from the neighboring Jayville iron deposit was “...not very rich but it does for Bessemer metal” (Smock 1889) and was mined at a very low profit margin. Thus, the Magnetic Iron Company that operated the Jayville deposit moved their heavy mining equipment to the Little River (the actual site of the Benson Mines) with hopes for a larger and richer iron deposit. But, the competition from the newly found large and low-cost iron deposits in Minnesota and Michigan forced the owners to abandon the property. Between 1895 and 1899, the mine output was small, approximately 112,211 tons of low-grade ore.  The mine reopened for a short period in 1900 and closed that same year and remained closed until 1907 when the Benson Mines Company formed and started the open pit mining operations and produced, intermittently up through 1918, a total of 69,590 tons.  The mine remained closed from 1919 to 1941.

The Jones and Laughlin Ore Company leased the mineral properties in 1941 and a year later a processing plant was built on site by the Defense Plant Corporation.  The plant was later sold back to the Jones and Laughlin Ore Co. And thus, mining and ore processing proceeded at a larger scale, and the company started to ship the ore to Pittsburg, the headquarters of the parent corporation. In 1952, Jones and Laughlin Ore Co. merged with the parent corporation to form the New York Division of Jones & Laughlin Steel Corporation that continued the mining activity under this name until 1965 (Crump and Beutner 1968).

ORE CHARACTERIZATION
The Benson Mines iron deposit is located in the metamorphosed sedimentary (metasedimentary) rocks of the Adirondack Highlands. This metasedimentary sequence contains gneisses with different proportions of garnet, pyroxene, microcline, albite, sillimanite, and quartz. These rocks formed by metamorphism at T = 600° to 650°C and high pressure (P~6-8 kb) (Buddington 1963). Hornblende-bearing and alaskitic granites occur in the area surrounding the Benson Mines.

The ore occurs as a folded layer or lenses in the host gneiss. Based on the iron oxide species, two sub-types of ore were distinguished: hematite-rich and magnetite-rich ores. The hematite ore occurs in the sillimanite-microcline-quartz gneiss; the magnetite ore is situated stratigraphically below the hematite ore in the garnet-amphibole-biotite-quartz and pyroxene-amphibole-biotite-feldspar gneiss. There is a spatial correlation between the ore and associated silicate minerals; hematite is concentrated in the biotite-rich layers while the quartz-feldspar-rich layers contain none, or very few, ore minerals (Hagni et al. 1969). Magnetite and hematite both are found disseminated throughout the host gneiss; very rare veins of iron oxide are known in the surrounding rocks. The iron ore, essentially consisting of magnetite and hematite, was divided into magnetic (magnetite) and non-magnetic (hematite) varieties, based on how the ore could be mined, milled, and recovered efficiently by magnetic or gravimetric concentration methods. Starting in 1943 until the end of the mine life, the open pit produced 18,600,813 tons of siliceous (5.08 - 7.17 % SiO2) low-grade magnetic ore containing 62.58 - 64.39 % FeO and 5,705,704 tons of siliceous (4.91 – 6.08 % SiO2) low-grade non-magnetic ore with 61.06 - 61.89 % FeO (Crump and Beutner 1968).

The ore body is associated with, or crosscut by, small (meter scale) pegmatite dikes that, in places, contain iron oxides. Late hydrothermal veins with complex mineralogy occur mainly in the surrounding rocks and low-grade ore.

MINERALOGY
Minerals found at the Benson Mines where studied in samples collected from the ore body, pegmatite dikes, and hydrothermal veins. They were examined first by naked eye in hand specimens or using a binocular microscope, and if there were doubts for their proper identification analytical techniques such as SEM-EDS or electron microprobe were used. The mineral species identified from the Benson Mines are listed in Table 1, where the minerals are grouped according to their origin as metamorphic, pegmatitic, hydrothermal, or weathering minerals.

Table 1. Minerals from the Benson mines iron deposit


Metamorphic Stage

Pegmatitic
Stage

Hydrothermal
Stage

Weathering Stage

Almandine
Fe3Al2(SiO4)3

Allanite-(Ce)
(CaCe)2(AlFe)3(SiO4)(Si2O7)O(OH)

Bornite
 Cu5FeS4

Brochantite
Cu4(SO4)(OH)6

*Anatase, TiO2

Chrysoberyl, BeAl2O4

Barite, BaSO4

Cu-Fe sulfates

Chrysoberyl, BeAl2O4

Fluorapatite, Ca5(PO4)3F

Calcite, CaCO3

Goethite
 FeO(OH)

Corundum
 Al2O3

Isokite
 CaMg(PO4)F

Chabazite-(Ca), CaAl2Si4O12.6H2O

 

Dumortierite, Al7(BO3)(SiO4)3O3

K-feldspar
KAlSi3O8

Chalcocite
Cu2S

 

Hematite
Fe2O3

Magnetite
Fe3O4

Chamosite, (Fe2+MgFe3+)5Al(Si3Al)O10(OH,O)8

 

Ilmenite, FeTiO3

Molybdenite, MoS2

Chalcopyrite, CuFeS2

 

Magnetite
Fe3O4

Muscovite KAl2(Si3Al)O10(OH,F)2

Covellite
CuS

 

Quartz
SiO2

Phlogopite KMg3Si3AlO10(OH,F)2

Dolomite
CaMg(CO3)2

 

Albite
NaAlSiO8

Quartz
SiO2

Dumortierite Al7(BO3)(SiO4)3O3

 

*Rutile, TiO2

Scheelite, CaWO4

Fluorapatite, Ca5(PO4)3F

 

Sillimanite
 Al2SiO5

Schorl NaFe3Al6(BO3)3Si6O18(OH)3(OH)

Fluorite
 CaF2

 

Spinel
MgAl2O4

Scorzelite-Lazulite
(FeMg)Al2(PO4)2(OH)2

Hematite
Fe2O3

 

Titanite, CaTiO4

Sillimanite, Al2SiO5

Hydrocarbon, C7.2H11.2O0.15

 

 

Uraninite, UO2

Microcline, KAlSi3O8

 

 

Vivianite
Fe3(PO4)2.8H2O

Monazite-(Ce) (Ce,La,Nd,Th)PO4

 

 

Wagnerite, (MgFe)2(PO4)F

Prehnite, Ca2Al2Si3O10(OH)2

 

 

 

Pumpellyite-(Mg) Ca2MgAl2(SiO4)(Si2O7)(OH)2.H2O

 

 

 

Pyrite, FeS2

 

 

 

Pyrophillite, Al2Si4O10(OH)2

 

 

 

Quartz, SiO2

 

 

 

Siderite, FeCO3

 

 

 

Sphalerite, ZnS

 

 

 

Stilbite NaCa2Al5Si13O36.14H2O

 

 

 

Strontianite, SrCO4

 

ORIGIN
The Benson Mines iron deposit, like most of the magnetite ores from the Adirondacks, is a low Ti, Fe-oxide deposit. Over the years, many hypotheses regarding the origin of these deposits have been put forth, but they can be grouped into three major categories: (a) Metamorphism of an iron-rich sedimentary protolith (Cumings 1906; Nason 1922; Linney 1943; Hagni et al. 1969; Palmer 1970); (b) Hydrothermal migration of iron from mafic silicates during metamorphism (Miller 1919; Hagner et al. 1963; Hagner 1966; Hagner and Collins 1967); and (c) Post-metamorphic hydrothermal replacement (Newland 1908; Colony 1923; Alling 1925; Leonard and Buddington 1964; Crump and Beutner 1968).

The ore from the Benson Mines formed by the metamorphism at the granulite facies (medium to high temperature and pressure) conditions of an aluminum-, silica-, iron-, and potassium-rich sediment. The reasons for favoring this hypothesis are the following: (a) the ore is conformable and extends for more than 3 kilometers within the same horizon of the metasedimentary sequence (Palmer 1970; Crump and Beutner 1968); (b) There is very little or no evidence for replacement seen in the ore (Palmer 1970); (c) Microscopic textures “clearly indicate that at least part of the iron ore grains must have been present at the time of development of those silicates” (Hagni et al 1969); (d) The unusual high-aluminum and potassium content of the ore (Palmer 1970; Crump and Beutner 1968); (e) Experimental work of Palmer (1970) indicating that the silicate and ore minerals formed together and were stable (equilibrated) at circa 640° ± 50°C. Because the ore could have been contaminated by the intrusion of the late pegmatite dikes or fluxed by fluids that generated hydrothermal veins, the real nature of the protolith is very hard to asses, but aluminous shales, greywacke or alkali-rich claystone (Engel and Engel 1953), and laterite could all be taken into consideration as potential protoliths.

Visiting the Site
The Benson mine officially closed in 1965, and now the quarry is a beautiful lake. Some minerals can be collected from the waste piles along the lake shore. The site can be accessible to the public if they ask permission from Mr. George Peerson (a very kind person) from Star Lake. It is easy to find him, he has a carpenter shop and the village and everybody knows him.

Star Lake is located approximately 3.5 hours north of Albany.

REFERENCES

Alling, H. L. 1925. Genesis of the Adirondack magnetites. Economic Geology, 20:335, 63.

Buddington, A. F. 1966. The Precambrian magnetite deposits of New York and New Jersey. Economic Geology, 61: 484 – 510.

Colony, R. J. 1923. The magnetite iron deposits of southeastern New York. New York State Museum bulletin, 249-250.

Crump, M. R. and E. L. Beutner. 1968. The Benson Mines ore deposit, Saint Lawrence County, New York. Ore Deposits of the United States, 1933-1967, ed. J. D. Ridge, 49-71. The Graton Sales Volume.

Cummings, R. J. 1906. The Mineville magnetites. Eng. And Min. Journal, 82. 26-26.

Engel, A. E., Engel, C. G. 1953. Grenville series in the northwest Adirondack Mountains, New York. U.S. Geol. Surv. Prof. Paper 376.

Emmons, E. 1839. Geology of New York, Part 2, Survey of the Second Geological District.  Albany, New York, W.and A. White and J. Visscher.

Hagner, A. F. 1966. The Precambrian magnetite deposits of New York and New Jersey (discussion). Economic Geology, 6. 1291-1294.

Hagner, A. F., L. G. Collins and C. V. Clemency. 1963, Host rock as a source of magnetite ore. Scott Mine, Sterling Lake, NY. Economic Geology 58: 730-68.

Hagner, A. F., and Collins, L.G. 1967. Magnetite ore formed during regional metamorphism, Ausable magnetite district, New York. Economic Geology, 62, 1034-1071.

Hagni, R. D. 1968. Titanium occurrence and distribution in the magnetite-hematite deposit at Benson mines, New York. Economic Geology, 63. 151-155.

Hagni, R. D., Masiello, R. A. And Tumialan, P. H. 1969. Metamorphic aspects of the magnetite-hematite deposit at Benson mines, New York. Economic Geology, 64. 183-190.

Leonard, B. F. and A. F. Buddington. 1964. Ore deposits of the St. Lawrence County magnetite district, northwest Adirondacks, New York. Geological Survey Professional paper 37.

Linney, R. J. 1943. A century and a half of development behind the Adirondack iron mining industry. Mining and Metallurgy 24: 480-87.

Lupulescu, M., Bailey, D, Hawkins, M. 2010. Mineralogy of the Benson mines Proterozoic iron deposit, Star Lake, St. Lawrence County, NY. In: Proceedings of the 37th Rochester Mineralogical Symposium, 14-15.

Miller, W. J. 1921. Geology of the Luzerne quadrangle. New York State Museum Bulletin, No 245-246, p. 35-36.

Nason, F. L. 1922. The sedimentary phases of the Adirondack magnetic iron ores. Economic Geology 17: 633-54.

Newland, D. H. and J. F. Kemp. 1908.Geology of the Adirondack magnetic iron ores with a report on the Mineville-Port Henry group. New York State Museum bulletin 119.

Palmer, D. F. 1970. Geology and ore deposits near Benson Mines, New York. Economic Geology, 65. 31-39.

Smock, J. C. 1889. First report on the iron mines and iron-ore districts in the State of New York. New York State Museum bulletin 7.

 


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