Great Hill cobalt mines, Cobalt, East Hampton, Middlesex Co., Connecticut, USA

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This locality at the southern foot of Great Hill consists of two parallel veins, one containing cobalt and nickel, the other gold (Chomiak 1989, Gray 2005). Both are located partly on private residential properties and no longer accessible (please respect their wishes and do not trespass) and partly in the Meshomasic State Forest, where collecting is not allowed without a permit. No significant dumps remain in any case, having been hauled away by the state years ago for use as fill.

Legend had it that John Winthrop the Younger, the first Governor of Connecticut, supposedly discovered gold here in 1641 and mined it in the 1660s. Until the University of Connecticut Summer field school identified native gold at the same site in 1985 (Frahm, 1986) there was no record of anyone having mined or even seen gold at Great Hill since Winthrop's day.

Instead, in 1762 German prospector John Stephauney found a small deposit of cobalt ore. In 1770 he organized a serious mining effort, opened a major adit and actually shipped some ore. The enterprise was abandoned within a few years.

The mines were dormant until 1818 when Seth Hunt sunk a quarter-mile-long trench along the main vein and shipped some supposed cobalt ore to England. He abandoned the enterprise after assays showed mostly nickel.

Charles U. Shepard examined and described these workings (Shepard, 1837). An assay of the dressed ore indicated 28% Co + Ni. He purchased a 99-year lease of Hunt’s properties. In 1844 Shepard himself opened an inclined adit on the vein (mostly west of Gadpouch Road and called “Shepard’s Lode”) and although he abandoned all work within a year he retained the mineral rights.

In the mid 1800s interest shifted to nickel as the US Mint was looking to that metal for coinage. In 1850 Brown sunk a 38-foot-deep shaft just east of Shepard's and opened a new adit in the ravine of Mine Brook 700 feet to the east. The eastern part of the Co-Ni vein was called “Robert’s Lode” and it trends parallel to and just south of Gadpouch Road on state property. The two access points were meant to connect, but funds ran out and the workings were acquired by "The Chatham Cobalt Mining Company" organized by Eugene Francfort. Between 1853 and 1859 they sank two major shafts, one 120 feet deep, drove at least 100 meters of adits and drifts along the main ore zone, and built a stamping mill and assay lab. The Brown and Francfort workings are the primary ones still visible today. Some dressed ore containing about 10% nickel and 10% cobalt was shipped but investor interest lapsed and work ceased by 1859.

What was not documented until Chomiak (1989) is that there are two different but parallel mineralized veins lying stratigraphically within a few tens of meters of each other. Cobalt-nickel arsenides extend for 800 meters along a 1-cm-thick garnet gneiss bed just below (south of) the Clough Quartzite and Collins Hill Formation contact. Two shorter gold-bearing arsenopyrite-quartz veins are situated just above (north of) this contact. The gold bearing veins are now called Winthrop’s Lode on the west (on private property and no longer accessible - please respect their wishes and do not trespass – there’s nothing to see there now anyway) and Champion Lode on the east in the state forest. This latter arsenopyrite-quartz vein is visible where it crosses Mine Brook just downstream of where the brook flows under Gadpouch Road. The locality coordinates are for the position of this crossing and are near the major mining shafts and ruins on state land.

According to Chomiak (1989) and Gray (2005), the two Co-Ni lodes are similar, the main difference being the dominance of hornblende to the west and biotite to the east. The rock at Shepard’s Lode is essentially a banded amphibolite. Hornblende, manganous garnet, sphene and plagioclase make up the bulk of the rock. 5 to 10% sulphides and arsenides are intergrown with the silicates. Pyrrhotite and nickel-cobalt rich loellingite (not chloanthite (aka “chathamite”) as erroneously reported through the 7th edition of Dana's Manual of Mineralogy, and by Shannon (1921) and Foye (1922) and others) constitute the bulk of the ore minerals although chalcopyrite, sphalerite, galena, nickeline, and gersdorffite are reasonably common. Tailings from Robert’s Lode are gneisses containing biotite rather than hornblende, plus manganous garnet, feldspar, quartz and staurolite. Loellingite is apparently the only ore mineral there and it is present in small amounts ranging from 1 to 5%. The flashy loellingite and various brightly colored secondary minerals helped the miners follow these very thin veins.

Gray (2005) described the gold-bearing veins:

“Gold occurs in decimeter wide quartz-arsenopyrite veins in the Silurian Clough Quartzite just above its contact with the Ordovician Collins Hill formation. Although the Clough appears to be concordant with the layering in the Robert's and Shepard's lodes the evidence suggests that the contact is tectonic and not an unconformity. The arsenopyrite in these quartz veins occurs as centimeter sized massive concentrations. Pyrrhotite, locally altered to pyrite, is the only other sulfide present in abundance. Native gold, generally as micron sized grains, is found, along with pyrite and chalcopyrite, in a network of thin fractures and veins cutting the arsenopyrite. Although much of the gold is very fine grained and is difficult to see, even with a strong hand lens, grains up to a mm are present and are quite noticeable on bright sunny days.”

Concerning the gold, the following text from Gray (2005) says it best:

“The biggest mystery of the Cobalt area is why all the miners, except Winthrop, failed to recognize and exploit the high-grade gold ores on their property. Some of the arsenopyrite-quartz veins, which contain no cobalt or nickel but run up to 6 oz of gold per ton were clearly explored by means of adits, shafts and trenches but there is no written or even verbal record of any gold having been extracted. Were the miners hiding the presence of gold from the owners, and all the inquisitive locals, or were they just totally incompetent?”

Mineral List

Actinolite Albite Almandine Annabergite Arsenolite ? Arsenopyrite var: Danaite 'Biotite' Breithauptite ? Chalcopyrite 'Chlorite Group' Cobaltite ? 'Copiapite Group' Cordierite ?

Diopside ? Erythrite Fluorapatite ? Gahnite Galena 'Garnet' Gersdorffite Goethite Gold Grossular 'Hornblende' Jarosite ? Kaolinite 'Limonite'

Löllingite Melanterite Microcline Muscovite var: Sericite Nickeline ? Nickelskutterudite var: Chloanthite Opal var: Opal-AN Orthoclase Pickeringite 'Pitticite' ? Pyrite Pyrolusite

Pyrrhotite Quartz Rammelsbergite ? Safflorite ? Scorodite Siderite ? Sillimanite ? Skutterudite var: Smaltite Sphalerite Staurolite 'Tourmaline' 'Voltzite' ?

55 entries listed. 38 valid minerals. 5 erroneous literature entries.

References