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XRF Analysis of Topaz-Bearing Hydrothermal Veins in Trumbull, Connecticut

Last Updated: 9th May 2009

Background:
Long popular among mineral collectors in the New England region, exposures of rock as well as mine dumps within and surrounding Old Mine Park provide the diligent collector with excellent hand specimens of a wide variety of minerals. Despite the fact that these sites have been very extensively collected, their geology has not been studied in depth.
Perhaps one of the more interesting geologic features of the region are the multitude of hydrothermal veins that can be found within and outside the park. Previous to performing a closer examination of these veins, the author considered them to be greisens due to their mineralogy. This classification later proved to be incorrect because no obvious protolith could be identified, thus making it quite difficult to define what these interesting rocks are.

Although many hydrothermal veins can be found within the area of interest (there could be as many as 20 or 30), they are fairly homogenous in terms of their mineralogy- they are composed of quartz + topaz + micas ± feldspar ± fluorite, but the proportions of these minerals can vary quite a great deal with respect to each other.
Typically, the most abundant minerals found within these hydrothermal veins are quartz, topaz, and micas. An analysis capable of identifying the species present has not been performed, but it has been suggested to the author through literature and conversation with local mineral collectors that margarite and muscovite are present. It appears that the margarite, reported as slightly pink, micaceous crystals (from conversation with Anthony J. Albini) may be an alteration product of topaz, as they occur most often in veins that look to be subsequentially altered by secondary hydrothermal fluids after their initial formation. In some veins, particularly those found within Old Mine Park, topaz crystals are either partially altered to a poorly defined mixture of fine-grained micas- typically gray-green and slightly porcelaneous in appearance (known as margarodite). It has been suggested that this mixture is comprised of muscovite to some extent, but its composition has not been analyzed to an extent that is conclusive enough to be reliable.

Descriptive Geology:
Old Mine Park and outcrops within the vicinity of the park are located within three rock units- the Collinsville Formation (a gray, iron rich, and readily weathering coarse to fine-grained schist, dark colored, fine to medium grained amphibolite, and hornblende gneiss), a basal member of the Straits Schist (gray schist, amphibolite, quartzite, and marble), and the Straits Schist. Long Hill itself, which is where the old mine, which Old Mine Park itself is named for contains noticeable outcrops of all three formations. The large variety of rock types within such a small area makes for an extremely complex and fascinating geology and mineralogy, with approximately 80 known minerals in the area and likely more to be discovered if one were to study the area more thoroughly than has been in the past.
Numerous contacts between the different rock types make for abundant metasomitization- creating skarns that concentrated the Tungsten (in the form of scheelite and occasional wolframite) in economical concentrations. These skarns are still frequented by mineral collectors for specimens of beautiful fluorescent scheelite, sometimes found in excellent crystals nearing two inches in diameter, and occasional excellent specimens of clinozoisite. In addition to the rocks mentioned above, small pegmatites comprised of white feldspar, quartz, and rich in biotite can be seen occasionally. Stringers of quartz assumedly related to these pegmatites contain small light green and transparent beryl crystals which some describe as Emerald. In Spring 2009, the author made a find of a calcite vein also contained in the pegmatite which yielded fine specimens of feldspar crystals, complimented by massive metallic blebs of pyrrhotite, and small amounts of what appears to be tremolite. These specimens were hosted in a calcite vein and were treated with hydrochloric acid to expose the crystallized minerals. A local collector, Michael Otto, recently made a wonderful find within the pegmatites, producing excellent specimens of deep purple fluorite cubes associated with small calcite crystals. These specimens are some of the finest fluorites the author has seen from a Connecticut locality, and they could be considered exceptional for the species within New England.



Local Mining History:
The occurrence of tungsten minerals in Trumbull, Connecticut was discovered in 1818, but this resource could not be appreciated until later on when tungsten was found to be the material of choice in the manufacture of lightbulb filaments. Mining commenced in 1897, and wasn’t especially long lasting- impurities in the ore made it very difficult to concentrate, forcing the mine to close in 1916.

Experimental Objective:
The peculiar hydrothermal veins within and surrounding Old Mine Park have not received significant attention in terms of their descriptive geology. It is noted that some of the veins appear to be composed of strictly quartz and sub to euhedral topaz crystals with minimal mica and very few accessory minerals, while others have been apparently altered. In the altered veins, the topaz crystals are almost always typically or fully pseudomorphed by micas. This experiment serves to determine whether the alteration of the topaz crystals in the hydrothermal veins happened during their initial formation, or is due to alteration by a subsequent fluid. The use of XRF spectroscopy will be employed to determine if any trace elements are present in the altered material, in order to assist in the verification that some of the material has been chemically altered by hydrothermal fluid flow after the initial formation of the rocks.

Typical assemblage of minerals found in supposedly altered hydrothermal veins:

Sampling and Analysis:
Samples of hydrothermal vein rocks were taken from two locations- one was a roadcut where veins primarily composed of topaz and quartz crosscutted a schist of the Collinsville formation, and the other being an area of mine dump within the park overturned by mineral collectors searching for topaz pseudomorphs.

Typical margarodite ps. topaz with some margarite. Sliced and polished to reveal interior.

After the samples were collected, they were prepared for analysis by slicing off pieces of the sample with a diamond-bladed lapidary saw. The samples were then polished with a moderately coarse aluminum oxide grit to ensure a smooth surface for XRF analysis.
Once the samples were polished and trimmed to the appropriate size, they were loaded into an XRF Spectrometer and a qualitiative, spacially resolved heavy element analysis using a 0.5mm wide x-ray beam was performed. The samples that were selected were chosen for their overall representivity of the mineral assemblages observed at the sampling locations. The sample from the roadcut was referred to as tpzqtz, and contained only three minerals (identified visually)- topaz, a small amount of mica, and quartz. The second sample (labeled JZ001) from the mine dumps was very mineralogically complex, containing several minerals that cannot be identified visually with much certainty. With a fair amount of certainty, it can be said that this sample contained a significant amount of mica, particularly margarite- CaAl2[(OH)2|Al2Si2O10], occurring as light pinkish-grey plates.
The purpose of using a spacially resolved analysis was to identify mineral grains that contained detectable amounts of rare heavy elements exclusive to the sample collected within Old Mine Park. These heavy elements could be concentrated by the fluids I suggest may be responsible for the altered nature of the material collected in the overturned mine dumps.

Results and Their Interpretation:
Many analyses of the same samples had to be performed, because some of the elements detected- Ni, Cr, and Co were thought to be contaminants from the polishing process. However, after preparing a sample of white bull quartz in the same manner as the other samples (diamond saw and aluminum oxide grit), it was determined that Ni, Cr, or Co are not contaminants, but naturally occurring trace elements.
In terms of trace element composition, both samples used in the analysis were very similar. However, it is especially interesting to note that the sample collected from the mine dumps contained more detectable strontium than the sample from the roadcut. In fact, only two of the nine points (M007 and M001) analyzed on the sample from the roadcut produced strontium peaks, while all nine points analyzed on the sample from the mine dump contained this element in detectable quantities.

A possible reason for the higher amounts of strontium in the sample from the mine dumps is due to the substitution of this element for some of the calcium in the large amount of margarite found in the sample. Since the sample from the roadcut didn’t contain any visually distinguishable margarite, it may have been more difficult for this trace element to be mineralized. The mica contained in the sample from the roadcut is likelty muscovite, and it would therefore have much more difficulty exchanging one of it’s constituents for an alkali earth metal, since it does not contain one itself. Supporting this is the presence of rubidium in both samples. Both contained micas- more likely than not, quite a bit of muscovite. Since muscovite contains an appreciable amount of potassium as a cation and rubidium is very chemically similar, rubidium substitution for some of the potassium in the muscovite had likely taken place. Margarite does not contain an alkali metal (such as Li, Na, or K), but contains an alkali earth metal (Ca), so substitution of this cation for something very chemically similar (Sr) is likely. Alkaline earth metal substitution, as that hypothesized as occurring in the margarite is not at all likely to occur in the muscovite.


Notably, the heavy element composition of both samples proved to be quite peculiar. Especially interesting was the appearance of the elements germanium, vanadium and molybdenum in detectable amounts on most of the plots for both samples. On many of the plots, gallium also appeared, as well as other rare unexpected elements such as niobium and barium. As the differences in mineralogy between both locations revealed themselves upon close inspection of the samples collected, looking at these elements as indicators to prove secondary alteration of the mica-rich rocks collected within the park would be inconclusive. They are however, very interesting and worth noting.

Conclusion:
While the enrichment and depletion of strontium in hydrothermal veins at this locality is significant in identifying a notable chemical difference within the rocks of interest, it does not signify that any of the veins were altered after their deposition. Further analyses, perhaps those looking at δO ratios in the rocks would be more conclusive. I theorize that the mica which pseudomorphs the topaz crystals found within Old Mine Park has been formed as a result of low-temperature hydrous fluids altering the mineralogy of certain hydrothermal veins. Looking at the δO ratios within unaltered topaz found within this vein and comparing it with the ratio obtained from analyzing the mica directly coating the topaz may somehow reveal a bit more of the story behind the formation of these extremely unusual rocks.

Sources:
Pawloski, John A. Connecticut Mining. Charleston, SC: Arcadia, 2006.
"Bedrock Geological Map of Connecticut." Talcott Mountain Science Center. 22 Apr. 2009 <." rel="nofollow">http://www.tmsc.org/geology/bedrock/>.
Mineralogy Database - Mineral Collecting, Localities, Mineral Photos and Data. 28 Apr. 2009 <." rel="nofollow">http://www.mindat.org>.






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Comments

I haven't yet received my analyses of the samples back, but when I do, I plan on scanning and uploading them to this article. I will also update this article in the future if I continue my work on this project

Jeremy Zolan
9th May 2009 6:30am
Jeremy,
Thanks for the great article.
I just recieved a few of those beautiful fluorite cubes on calcite from Mike Otto.
Almost hard to believe they are from NE because they are so nice.
Hopefully Old Mine Park will be fully understood before it is fully developed.
AK


Adam Kelly
17th May 2009 12:00am
Hello Jeremy,
I enjoyed the great article as well as learning a great deal. I still stop by the construction site some Sundays, but my collecting has been reduced to looking through the larger (6inch + or -) piles of rock still there. I occassionally find something of interest there but the new building is up and there isn't much digging being done anymore. Too bad it's going to be covered with asphalt this year, but on the other hand if they hadn't developed it, we wouldn't have had all that material exposed for us. Glad you did all the documentation and we had a chance to collect there. Mike

Michael Otto
19th Jul 2010 11:39am

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