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Thomaston Dam railroad cut, Thomaston Dam, Thomaston, Litchfield Co., Connecticut, USA

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Latitude & Longitude (WGS84): 41° 41' 50'' North , 73° 4' 5'' West
Latitude & Longitude (decimal): 41.69750,-73.06806
Köppen climate type:Dfb : Warm-summer humid continental climate

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 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 Mountian 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, harmotome). Other minerals are found in the metamorphic rocks, the pegmatite, or are secondary in origin.

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

Hydrothermal mineralization appears to be in four separate stages. Deposition of quartz [and] sphalerite...are the first stage. Quartz cemented the shattered country rock and is the most abundant mineral,...and sphalerite [is] rooted against the quartz. Next, fluorite and calcite, along with galena, [sphalerite], and chalcopyrite, crystallized in a second set of fractures as well as within the quartz-lined cavities. Calcic zeolites, stilbite, heulandite and laumontite, crystallized with calcite in the third stage. They are intergrown and perched on quartz and fluorite druze [sic]. The less siliceous zeolites, [harmotome] and chabazite, are rare and usually found alone on fluorite. The final stage is pyrite in minute crystals that are sprinkled randomly on other minerals.

Collecting is no longer allowed there per US ACOE rules.

There has long been controversy regarding the identification of the brown, generally 6-sided pagoda-like micro-crystals of ZnS found here and generally referred to as wurtzite. This ID was first made by Myer (1962). However, Pete Dunn analyzed crystals via XRD in 1972 (see Yedlin 1973a and b) and obtained "perfect sphalerite patterns". 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 prismatic 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.

Mineral List

61 valid minerals. 5 erroneous literature entries.

Regional Geology

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

Ordovician - Neoproterozoic
443.8 - 1000 Ma

ID: 3190671
Precambrian-Phanerozoic sedimentary rocks

Age: Neoproterozoic to Ordovician (443.8 - 1000 Ma)

Lithology: Mudstone-carbonate-sandstone-conglomerate

Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. [154]

Early Ordovician
470 - 485.4 Ma

ID: 3020282
Ratlum Mountain Schist

Age: Early Ordovician (470 - 485.4 Ma)

Stratigraphic Name: Ratlum Mountain Schist

Description: Gray, medium-grained, interlayered schist and granofels, composed of quartz, oligoclase, muscovite (in the schist), biotite, and garnet, also staurolite and kyanite in the schist. Numerous layers and lenses of amphibolite; also some of quartz-spessartine (coticule) and calc-silicate rock.

Comments: Part of Central Lowlands; Iapetus (Oceanic) Terrane - Connecticut Valley Synclinorium; Hartland Belt. Equivalents of Savoy Schist of Massachusetts (Emerson, 1898, 1917) (CT009, CT010) and of Moretown Formation of Vermont and Massachusetts (Bedrock Geologic Map of Massachusetts, Zen and others, 1983) (CT011) (includes Golden Hill and Ratlum Mountain Schists) (Lower? Ordovician).

Lithology: Major:{schist,granofels}, Minor:{amphibolite}, Incidental:{calc silicate rock}

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. [133]

Data and map coding provided by Macrostrat.org, used under Creative Commons Attribution 4.0 License

This page contains all mineral locality references listed on mindat.org. This does not claim to be a complete list. If you know of more minerals from this site, please register so you can add to our database. This locality information is for reference purposes only. You should never attempt to visit any sites listed in mindat.org without first ensuring that you have the permission of the land and/or mineral rights holders for access and that you are aware of all safety precautions necessary.


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Zodac, Peter. (1959), Minerals at Thomaston Dam, Connecticut; Rocks & Minerals: 34: 3.
Schooner, Richard. (1961), The Mineralogy of Connecticut. Fluorescent House, Branford, Connecticut.
Myer, George H. (1962), Hydrothermal Wurtzite at Thomaston Dam, Connecticut. American Mineralogist: 47(7-8): 977-979.
Hiller, John. (1968), Collecting at Thomaston, Conn. Rocks & Minerals: 43(10): 746-747.
Ryerson, Kathleen. (1972), Rock Hound's Guide to Connecticut. Pequot Press: 36-37.
Yedlin, Neal. (1973a), Yedlin on Micromounting. Mineralogical Record: 4(2).
Yedlin, Neal. (1973b), Yedlin on Micromounting. Mineralogical Record: 4(6).
Januzzi, Ronald E. and Seaman, David. (1976), Mineral Localities of Connecticut and Southeastern New York State and Pegmatite Minerals of the World. The Mineralogical Press, Danbury, Connecticut.
Molon, Joseph. (1976), Minerals from Thomaston Dam, Thomaston, Connecticut; Rocks & Minerals: 51: 449.
Henderson, William A. (1979), Microminerals. Thomaston Dam. Mineralogical Record: 10: 239-241.
Shelton, Bill and Webster, Bud. (1979), Mineral Collector's Field Guide: The Northeast. Mineralogy, Wallingford, Connecticut: 46-49.
Segeler, Curt and Molon, Joseph. (1985), The Thomaston Dam Site, Thomaston, Connecticut; Rocks & Minerals: 60(3): 119-124.
Vogt, Wolfgang. (1991), Rediscovering Thomaston Dam. Lapidary Journal: April.
Januzzi, Ronald. E. (1994), Mineral Data Book - Western Connecticut and Environs. Mineralogical Press, Danbury, Connecticut.
Henderson, William A. (1995), Microminerals of Connecticut; Rocks & Minerals: 70(6): 420-425.
Weber, Marcelle H. and Earle C. Sullivan. (1995), Connecticut Mineral Locality Index. Rocks & Minerals (Connecticut Issue): 70(6): 399.

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