The Iron Ores of Connecticut

The Iron Ores
by John Carter

One must remember that there are many minerals that contain iron, but only those containing enough iron to be profitable can be classified as an ore. In Connecticut there are four iron minerals containing enough of the metal to be profitably mined. These are: bog ore, limonite, magnetite and siderite all of these ores were used at one time or another to produce iron. Of these ores limonite was the most abundant and created a thriving industry in northwestern Connecticut that lasted for almost two centuries.

Bog ore is a form of limonite that precipitates into the bottom of a bog from the action of iron fixing bacteria that are present in the water. It was the earliest iron ore mined in Connecticut and was the ore that was smelted in the furnace that was erected in West Haven by John Winthrop Jr. It also found use in the many forges of Connecticut until the discovery of the Salisbury ore beds in 1734.

Limonite is actually a hydrous iron oxide with the generic formula of FeO(OH)·nH2O. this formula is not especially accurate because at times limonite is apt to contain more oxide than hydroxide. Limonite was discovered in the town of Salisbury, Connecticut in 1734. It has been estimated that the single deposit of ore that was mined on Ore Hill produced over 7,000,000 tons of iron ore from its initial discovery until 1923 when the mine was finally closed. There were other mines producing the same ore found throughout the Tri-state area and as far north as Manchester, Vermont. This particular ore became known as “Salisbury Ore.”

The Salisbury ore is intimately associated with the Walloomsac formation that extends from West Central Vermont under the Taconic range of mountains as far south as the tip of Manhattan. The Walloomsac formation is an iron rich schist interbedded with Dolostone. The ore itself is found at the contact with the Walloomsac formation and the Stockbridge formation that is marble. It is found throughout the length of Manhattan Island intimately associated with the Inwood Marble and underlies Wall Street. Because of this formation you could say that Wall Street and the New York Stock Exchange has a foundation of iron.

Magnetite occurs in two different forms. One form is when it occurs in the sands of rivers and beaches. Its general formula is: Fe2O3. One of the early smith’s in Connecticut mined 85 pounds of the ore on one of the Long Island Sound beaches from which he produced fifty pounds of iron. Another use for this magnetic sand was as a blotting material for wet ink. Because it is naturally porous it was the best material available for this purpose. In use it was sprinkled onto a document while the ink on the paper was still wet.

The other type of magnetite is a solid that is intimately associated with the Precambrian rocks of the Berkshire Highlands complex and other Precambrian rocks in Western Connecticut. Although iron was produced from the ore to a limited extent it was never a real factor in the iron industry as it was too difficult to smelt. There were several mines mentioned producing the ore, but the largest was Tilly Foster mine in Brewster, New York. The only mine in Connecticut that had a name was the Tuttle mine in Winchester. This mine produced about 50 tons of ore that proved to be so difficult to smelt the mine ceased operations and was abandoned. All that remains of this mine today is an open pit in the sight of the Grantville Road that is full of water. Directly under the Gilbert high school at the end of Williams Avenue the aeromagnetic map of the Winchester quadrangle shows an extremely high magnetic anomaly that is probably a large deposit of magnetite. It is interesting to note that in Nova Scotia gold deposits are often found associated with magnetite. The geological terrain of Winsted is very similar to the geological terrain found in Nova Scotia.

Siderite is the last of the four ores to be found in Connecticut. It is found in the town of Roxbury in Mine Mountain. Unlike the other iron ores siderite is composed of iron carbonate, FeCO3. This is also termed “spathic iron ore.” The deposit of this mineral in Roxbury is one of the largest in the world. When it is first dug out of the ground siderite is a cream color but very quickly gathers a crust of limonite when exposed to the air. The siderite of Roxbury gave rise to another company organized to smelt the special ore. This company was named the silver steel company of Bridgeport. The mine in Roxbury was originally opened for silver. There may have been some there but it is doubtful. Many tons of iron ore were mined here, but the company formed to smelt the ore proved to be unprofitable. After the mine was closed it was bought by Columbia University who operated it for years as a field laboratory for their school of mining engineering. You can visit the mine today as a park in the town of Roxbury.