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 Post subject: Simsbury copper mine info
PostPosted: Sun Nov 20, 2011 10:35 pm 

Joined: Mon Apr 04, 2011 6:52 pm
Posts: 5
Location: CT
I had a couple questions about the mine in this topic: viewtopic.php?f=15&t=21834

First off, can I have the link to those map pictures? They're very difficult to read, it looks like someone actually took a picture of their computer screen instead of a screen capture. lol

Second, is this the same mine as the New Gate Prison mine? I've been to New Gate and I remember all of the tunnels going off in the opposite direction. Maybe they're two different, non-connected workings of the same mine?

Third, it looks like you guys dug into a tunnel that was at ground level, yet the map labels everything as shafts. What exactly did you enter the mine through? Was that a collapsed audit or did you actually have to go down a vertical shaft? Having a ground level entrance at the top of a hill doesn't seem to make much sense.

 Post subject: Re: Simsbury copper mine info
PostPosted: Mon Nov 21, 2011 10:16 am 

Joined: Wed Jul 06, 2005 10:17 am
Posts: 755
Location: Monroe, CT
There were 3 seperate areas of copper mining in Simsbury all within a mile of each other and unconnected. The principal mine was what became New-Gate prison but there were also workings at Castine and North Hill. Castine has very little left to see, just some rock cuts and a 5' long partially filled adit as seen in the 6th and 7th photo. North Hill is where the rest of the photos were taken. According to the map there were 3 adits there which I located and found covered over by soil erosion. There were also several shafts which likely accessed the main workings dating to the early 18th century, these were also in bad shape with no possibilty of rappelling down. The adit shown in the last photos was enterable with minor digging, it was 66' long and did not reach any workings, there was a short exploratory incline off the side a few feet inside the portal. I belive this adit was driven in the 1830s with the intent of draining the main workings but it obviously never made it far enough. The proportions are tiny at about 4.5' high and 3' wide! Below is some additional info you might like to read. ... harles.htm ... nnover.htm ... =471&y=273






Department of Geology and Geophysics
Yale University
New Haven, Connecticut

Department of Geology and Geophysics
Yale University

Connecticut Geological and Natural History Survey
Eastern Section of SEPM
National Association of Geoscience Teachers
Peabody Museum of Natural History

FIELD TRIP A1 pages 9 to 18


Norman H. Gray, Department of Geology, University of Connecticut, Storrs, CT 06269

During the 1700 and 1800s Connecticut was the scene of several small entrepreneurial mining ventures. Most of these attempts were short-lived and it is difficult today from the limited tailing piles and collapsed or flooded workings to understand what these early miners were after. The underground workings of two of the more important deposits are still accessible and the nature of the mineralization that attracted the miner's interest can be seen in place (Fig. 1).
The New-gate mines of Simsbury are hosted by grey colored sandstones in the Hartford Basin Mesozoic redbeds. The main ore (grading up to 13% Cu) was a massive chalcocite replacement of the ankerite cemented zones. The mineralization at New-Gate was discovered in the early 1700s, and worked at intervals up to 1900. For a brief period in the late 1700s the underground workings served as Connecticut's State Prison!
The Cobalt mines of East Hampton were first prospected in the 1790s, opened as a cobalt source in the early 1800s and then for nickel in the 1860s. Mineralization occurs in Ordovician metasediments just below the Silurian Clough Quartzite. Local legend has it that the first Governor of Connecticut found gold in the area in the mid 1600s. Later miners make no mention of gold even though stringers of native gold are visible in arsenopyrite veins close to their main workings in the Clough. The early miners, who obviously were very resourceful, traced a single folded thin seam of Fe-Co-Ni arsenide bearing garnet amphibolite along strike for over 1200 ft.

Figure 1. Locations of the New-Gate and Cobalt mines in Connecticut. The Mesozoic Hartford Basin is shaded

Of the many small redbed type copper occurrences in the Mesozoic Hartford Basin the New-Gate deposit is the largest. Although never profitable, it was actively worked over a period of several decades The geology in the vicinity of the mine is outlined in detail by Schnabel and Eric (1964), Perrin (1976), and Gray (1982, 1989) on which figure 2 and much of the following is based.

Copper was discovered in the New-Gate prison area in 1705. Two years later the first charter mining company in North America was organized to work the deposits. Although numerous prospect pits, declines, shafts and adits testify to the scale of the effort the venture never proved especially profitable. The mines absorbed much more capital than their output ever provided in return. By 1741, more than $200,000 had been spent to recover little more than 100 tons of dressed chalcocite ore which at best averaged 13% Cu (Richardson, 1928).
Water was one of the most difficult problems faced by the early miners. Between 1721 and 1730 two neighboring mine owners combined resources and drove a 100 meter tunnel to drain the lower workings of one of the principal deposits. Because it was drained and secure the mine served as the State Prison from 1773 to 1827.
In 1831 New-Gate and number of the other workings in the area were reopened by the by the Phoenix Mining Company. Forty-two experienced miners were induced to emigrate from Europe to work the deposits. The old workings were pumped, mapped and new level at the lowest point driven 60 meters to the south. Unless purely exploratory, the purpose of these new tunnels is not clear as they were driven through entirely unmineralized strata.
Perhaps that fact alone explains the failure of the Phoenix Company three years later, in 1834. A drainage tunnel designed to intersect a shaft sunk to what must have been extensive underground workings at the North Hill Mines had been driven 65 meters before work was abandoned.
Two other attempts in 1855 and 1907 to reopen the mines also ended in failure. The focus of these ventures seem to have been speculation rather than actual mining but it is possible that the lower workings at New-Gate were dewatered at these times.

Figure 2. Generalized geology of the New-Gate area showing the locations of the New-Gate mine (1001370E, 911260N) and the North Hill workings (1001630E, 913970N). The Higley mine (1000470E, 902640N) is situated in the Talcott Basalt 2 miles south of New-Gate. (Coordinates are SPC83 in feet).

Geology and Mineralization
Copper occurrences in the vicinity of the New-Gate mine are associated with a major erosional unconformity which truncates the Talcott basalt (Fig. 2). Disseminated copper mineralization can be traced for over 2 km along strike in a series of small prospect pits just below the unconformity. Bornite and chalcopyrite, in places marginally replaced by chalcocite, fill interstitial pores between detrital grains in the grey siltstones. The mineralization is statabound and is present only in gray to black colored sediments. The interbedded red-colored sandstones and shales are everywhere barren. In the coarser grained grey sandstones the sulfides form millimeter sized nodular concretions. The detrital feldspars in these concretions are replaced by bornite and chalcopyrite while quartz grains are unaffected and even show euhedral diagenetic overgrowths. A small amount of uraninite is associated the nodular copper sulfides. Grey sandstones less than 20m below the unconformity that truncates the Talcott basalt are the most intensely mineralized. Copper averages less than 1% in most of these rocks but in the area around the North Hill mines the average grade of the 10 disseminated mineralization ranges from 2% to as much as 5%.
Figure 3. Underground workings at the New-Gate Prison mine.

The main copper ore at the New-Gate prison mine is stratigraphically lower and both texturally and mineralogically distinct from the disseminated mineralization. It is also significantly higher grade. Copper averages 2.5 to 10% and silver up to 10 ounces per ton. Chalcocite rather than bornite or chalcopyrite is the principal ore mineral. The highest grade chalcocite occurs in curious mottled zones surrounding centimeter sized poorly cemented porous patches within structureless, bioturbated sandstones. The mottled ore bed is part of a well defined sedimentary fill of a large north-south channel eroded into a thick sequence of red mudstones. Locally the base of the channel is defined by a thin (0-10 cm) intraformational breccias containing pebble sized clasts of an iron-rich dolostone. A few thin (cm) gray to black shales are the only clear evidence of stratification within the ore bed. In the area of the highest grade mineralization the ore bed is overlain by festoon bedded medium grained laminated grey sandstones. However in the now flooded southern workings a meter thick black mudstone overlies the stratigraphic equivalent of the ore bed which although unmottled contains the disseminated and millimeter nodular .concretions of bornite and chalcopyrite, typical of the regional disseminated Cu-U mineralization.
The ore bed which as befits a channel fill is discordant with the underlying redbeds, thickens to the Northeast, and wedges out to the southwest (Fig. 3). Where less than a couple of meters thick at the edge of the channel fill the ore bed was cemented by ferroan dolomite prior to compaction and mineralization. The carbonate cemented zones are cut by numerous calcite-chalcocite veins which do not extend into the surrounding non-carbonated cemented sandstones. The high grade mottled ore irregularly crosscut and replaces the unmineralized ferroan dolomite cemented zones (Fig. 4).

Figure 4. Schematic cross-section of the ore bed at New-Gate.

The disseminated copper sulfides either precipitated in pore spaces between detrital grains or formed nodular concretions early in the diagenetic history of the gray sandstones. The close relationship of the disseminated mineralization and the Talcott unconformity suggests saline copper-bearing waters percolated down into the sediments during a prolonged erosional interval. Considering the extent of the mineralized area, the amount of copper precipitated in near surface reducing environments was substantial. The ultimate source of the metals is somewhat of a mystery but one clue may be found at a small, now inaccessible Higley deposit 3 km to the south. In spite of the fact that they both lie at essentially the same stratigraphic level the copper mineralization at the Higley mine is quite unlike that of the New-Gate area. Rather than filling pore spaces in sedimentary rocks, the Higley mineralization occupies fractures and vesicles in altered Talcott Basalt. Vesicles are lined by copper sulfides, calcite and zeolites. The amygdaloidal basalt is extensively replaced by ferroan dolomite, especially where intensely mineralized. Hematite stained fractures and bornite-chalcopyrite veins up to 2 cm wide extend well below the porous vesicular zone. The flow is overlain unconformably by a coarse breccia consisting of angular clasts of chloritized basalt and bleached clay-rich hydrothermally altered basalt in a red sandstone matrix. Chalcopyrite-filled fractures crisscross the bleached basalt fragments but nowhere extend into the sandstone matrix. The bleached and mineralized basalt clasts imply extensive hydrothermal activity after the Talcott event during the erosional interval associated with the Talcott unconformity. Hot springs sited along the surface traces of the mapped northwest trending normal faults may have source of the copper bearing waters for both Higley and the disseminated mineralization in the New-Gate area.
The mottled high-grade chalcocite ore at New-Gate was the result of a major change in the chemistry groundwaters late in the diagenetic history of the sediments. Partial dissolution of ferroan dolomite cemented sandstones produced a favorable local environment for the precipitation as chalcocite of copper leached from the surrounding disseminated mineralization. The fact that the syn-sedimentary ferroan dolomite cemented zones may have shielded some of the adjacent sandstones from the full effects of compaction may also helped preserved some of their original porosity which would have further controlled access by solutions responsible for the chalcocite mottling. The cause of the dramatic change in the water chemistry is not obvious. Perhaps the cooling of the overlying Holyoke generated a thermal regime which flushed new waters through the New-Gate deposit.

 Post subject: Re: Simsbury copper mine info
PostPosted: Mon Nov 21, 2011 6:29 pm 

Joined: Mon Apr 04, 2011 6:52 pm
Posts: 5
Location: CT
Thanks, Mike. That's a lot of good info, going to take me awhile to read through.

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