Andover Iron Mine, Andover Township, Sussex Co., New Jersey, USA
|Latitude & Longitude (WGS84):||41° 0' 16'' North , 74° 44' 6'' West|
|Latitude & Longitude (decimal):||41.00444,-74.73500|
An iron mine in hematite and magnetite ore. Located 1¾ miles NNE of Andover Borough in Andover Township. Workings were an open pit 70 feet deep and 1,000 feet long. This mine was operational by 1762. Worked 1847-1863, again from 1879 to 1880. One of three mines near Andover.
The Andover Iron Mine is fundamentally different than all but a very few of the very many iron deposits in northern New Jersey. The mine is a long, narrow pit, now thickly forested. The configuration of the pit’s floor can only be roughly estimated because back fill and debris, including some very large fragments, has obscured much of it. The mine predates the Revolutionary War but its principal period of activity began circa 1847 after it was acquired by the Trenton Iron Company, owned by Abram S. Hewitt and Peter Cooper. The renovation and operation of the mine was directed by Phillip R. George, my Uncle Ray’s grandfather, who was descended from a long line of Cornish mine captains and who had recently come to America from Redruth, Cornwall. Mr. George remained Superintendent at Andover until 1854 when he was transferred to the Ringwood Mines.
The Andover Mine ore body is interpreted as a stratiform body that occupies a small, local graben (Volkert and Puffer, 2001). Within this graben the ore is underlain by approximately 85 feet of Neoprotorozoic greenschist-facies clastic metasediments. These are assigned to the Chestnut Hill Formation, a group of sedimentary and volcanic rocks best exposed near the Delaware River just north of Easton, Pa that was named by Drake (1984). The sediments in the graben resulted from very local erosion as they contain detrital goldmanite, the vanadian garnet abundant in the skarns at the nearby Sulphur Hill Mine.
Volkert and Puffer (2001) describe the ore at Andover as mostly hematite and lesser magnetite with calcite and jasper. Puffer (2001) likens the ore to banded iron formation.
Along the middle portion of the pit the southeast wall is very straight for at least 250 feet. In this area the limit of both the ore and the pit was marked by a fault. This fracture contained a narrow vein filled mostly by quartz and calcite. The calcite ranged from off white to pale salmon colored. The vein ranged in thickness from nil, just sheared hematite-rich gneiss, to several inches. In at least 2 local areas the vein assemblage included a very dark gray sphalerite which contained tiny, exsolution inclusions of a copper sulphide. Such exsolution inclusions are common in sphalerite and are usually chalcopyrite but in this case freshly broken surfaces had the distinct purplish bronze color of bornite. Both of these sulphide-bearing areas had been oxidized.
The northern example was commonly referred to as the “Wllemite Pocket”. It was apparently discovered by Al Lord, a local collector, circa 1968 and by the early 1970’s was mostly worked out. The remaining void was 6 to 8 inches high and deep and approximately 3 to 4 feet long. The vein material was very siliceous. The cavities in this material were drusy quartz which was thickly encrusted with small, stubby crystals of willemite. Radiating aggregates of bladed hemimorphite crystals were also common. The willemite fluoresced in short wave ultraviolet light a bright orange-yellow.
The southern example, approximately 150 feet to the south of the “willemite pocket”, was more copper-rich. Dioptase occurred as micro size crystals, mostly elongated prisms in individuals and radiating or spherical aggregates. A very few crystals, including the best one seen by this author, were equant, miniature versions of the well known examples from Tseumeb.
Aurichalcite, with the typical rich, light blue color, graded into similar species (?), one white, possibly hydrozincite, and another distinctly green, perhaps rosasite.
Malachite, in groups of radiating crystals, was locally present locally. The entire copper-rich area of the vein was approximately 2 to 3 feet square and less than a foot deep.
The vein mineralization, that includes the sulphides, is later than the iron ore. It is probably Paleozoic age (Permian ?). It may be related to the widespread hydrothermal overprint of the region that produced numerous minor Mississippi Valley Type lead-zinc and alpine cleft occurrences (Cummings, 1997).
The willemite from the Andover Iron Mine is not the same material that was reported from this area in the 19th century. That willemite occurrence was at the Sulphur Hill Mine, less than 1000 feet northeast of the northern end of the Andover Mine pit. Because of the proximity of these two mines and a shared access road there has been some confusion in the assignment of some mineral species even though the deposits are very distinct in their age, character and origin. There has been a tendency to lump both mines under the heading “Andover” or “Andover Iron Mine”. Some of the species listed in Mindat for the Andover Iron Mine, including carbonate-apatite, garnet group and leadhillite, are suspect, should probably be assigned to the Sulphur Hill Mine and should be rigorously reviewed.
12 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
1000 - 1600 Ma
Age: Mesoproterozoic (1000 - 1600 Ma)
Description: Gray- to grayish-black, medium-grained amphibolite composed of hornblende and andesine. Some phases contain biotite and (or) clinopyroxene. Ubiquitous and associated with almost all other Middle Proterozoic units. Some amphibolite is clearly metavolcanic in origin, some is metasedimentary, and some appears to be metagabbro.
Comments: Rocks of Uncertain Origin (Middle Proterozoic)
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. 
New Jersey State Geologist Annual Report (1854): 38-41.
New Jersey State Geologist Annual Report (1855): 149-163.
New Jersey State Geologist Annual Report (1868): 640-657.
New Jersey State Geologist Annual Report (1873): 88.
Spencer, et al (1908).
Bayley (1910), The Iron Mines of New Jersey: 81-83.
Drake, A.A. (1984), The Reading Prong of New Jersey and eastern Pennsylvania: An appraisal of rock relations and chemistry of a major Protorozoic terrane in the Appalachains, Geological Society of America Special Paper 194, p. 75-109.
Dunn, P.J. (1995), Part 1: 89.
Cummings, Warren L. (1997), A sketch of the Limecrest Quarry, the Franklin Marble and some interesting minerals, in Benimoff, A.I. and Puffer, J.H., eds, The economic geology of northern New Jersey: Field guide and proceedings of the fourteenth annual meeting of the Geological Association of New Jersey, p. 61-70.
Cook, D.K. (1998), "Willemite from the Andover Iron Mine, Andover, New Jersey." Mineralogical Record: 3(2): 63-64.
Puffer, J.H. (2001), Origin of five types of Protorozoic magnetite deposits in the New Jersey highlands, in Slack, J.F, ed., Part 1. Protorozoic iron and zinc deposits of the Adirondack Mountains of New York and the New Jersey highlands, Society of Economic Geologists Guidebook Series, Vol. 35: 103-110.
Volkert, R.A., and Puffer, J.H. (2001), Field trip day five. Road log for the Sulphur Hill and Andover Iron Mines, New Jersey, in Slack, J.F, ed., Part 1. Protorozoic iron and zinc deposits of the Adirondack Mountains of New York and the New Jersey highlands, Soc. of Econ. Geologists Guidebook Series, Vol. 35: 99-101.