With the construction of this dam by the US Army Corps of Engineers following the devastating flood along the Naugatuck River in 1955, the railroad line, which originally followed the river, was moved into a cut along the west side of the dam above the spillway level. At least two workmen died in accidents during the making of this cut (Zodac, 1959). The cut through the Ordovician Ratlum Mountain Schist and some outlier Devonian Nonewaug Granite and pegmatite also exposed numerous hydrothermally mineralized fault zones of unknown origin, but possibly related to Mesozoic rifting. The veins are typically about 0.25 to 1.5 meters thick and contain mostly fluorite, quartz, sulfides (galena, sphalerite, pyrite), calcite, and zeolites (stilbite, heulandite, laumontite, and harmotome). Other minerals are found in the metamorphic rocks, the pegmatite, small Alpine clefts, or are secondary in origin.

The railroad cut likely obliterated the long-sought (see Chipman, 1860) colonial-era "lead mine" vaguely described from this area, which probably lay on a galena-rich vein later exposed along the cut. Ed Force, in a self-published essay called "The Mysteries of Mattatuck" found out this information:



Note the important point that the mine was opposite (west) of the mouth of Lead Mine Brook. Many an expedition in the 19th and 20th centuries mistakenly journeyed far up the brook looking for the old mine. No mine can be found on detailed LiDAR topographic imagery of the area, supporting its obliteration by the cut.

The mine's location coincident with the later railroad cut is also supported by multiple specimens of fluorite, with other minerals, in the Yale Peabody Collection labeled as coming from "Plymouth". Although no such fluorite locality is known within present-day Plymouth, these are "Old Peabody" or Brush collection specimens from before 1875, when Thomaston was still part of the town of Plymouth. They show the same habits, mineral associations, and matrices as post-railroad cut specimens. It is interesting, but essentially undocumented, that the veins were exposed for at least 80 years before the famous railroad cut was made.

Myer (1962) noted the paragenesis of the hydrothermal minerals in the fault veins as follows:



There has long been controversy regarding the identification of the brown, generally 6-sided pagoda-like micro-crystals of ZnS found here and historically referred to as wurtzite. This mis-ID was first made by Myer (1962). However, Pete Dunn analyzed crystals via XRD in 1972 (data from crushed and whole crystals made no difference; see Yedlin 1973a and b) and obtained "perfect sphalerite patterns." They are NOT pseudomorphs of sphalerite after wurtzite—they are translucent at the edges and monocrystalline (demonstrated by fully penetrative cleavage and parting). Subsequent authors were apparently unaware of Dunn's analyses. Henderson (1979) showed diagrams of sphalerite crystals "epitaxial" on "wurtzite", and the other way around, with a (0001) (pinacoidal) face of "wurtzite" matching a (111) (tetrahedral) face of sphalerite. Yet (111) is the face that sphalerite twins on. The crystals are actually combined positive and negative tetrahedra of sphalerite twinned on a 6-sided (111) face. Vogt (1991) shows a nice photo of a tetrahedron with a (111) face capping a stack of polysynthetic sphalerite twins resting on multiple sphalerite tetrahedra, but still referring to it as "wurtzite" even though he notes the presence of sphalerite associated with the "wurtzite". Note the re-entrant angles that circumscribe the "prisms" of these pagoda crystals, plus the varying width of the crystals, which are indicative of twinning. Here are some examples of sphalerite tetrahedra with both simple twinning and polysynthetic twinning, some on the same crystal: http://www.mindat.org/photo-783527.html http://www.mindat.org/photo-783535.html and http://www.mindat.org/photo-783519.html The pagoda stacks also cleave along the dodecahedral form {011} and part parallel to the twin plane (111), which is across the "prism" of the stack of twins, whereas wurtzite's best cleavage is parallel to its pyramid and is not evident here.

A mindat discussion found here http://www.mindat.org/forum.php?read,9,318431,318568#msg-318568 makes an argument that wurtzite is not a separate mineral at all but merely a sphalerite polytype, while others hold that it is.

Collecting is no longer allowed there per US ACOE rules, plus careless collectors dumped rocks onto the railroad tracks.

Other Regions, Features and Areas containing this locality

References