Hello,
I deal with detrital heavy minerals. I found quite numerous topaz grains in several samples. I wonder if this is possible to distinguish between different source rocks (e.g., various pegmatites and skarns from the Bohemian Massif, Central Europe) using chemical composition of the topaz grains/crystals (major and minor elements using electron microprobe, and/or trace elements using laser ablation). This subject is completely new for me, I will be grateful for any tips.
Monika
I think it is possible to distinguish various source rocks using chemistry of topaz grains from heavy minerals in sands. But here is the caveat: You have to first establish a correlation between the chemistry of a specific source rock and its topaz content, and then apply that knowledge to use topaz grains as source indicators. Lot of home work there!
In absence of core data, Gamma Ray log, Bulk density log, Sonic logs are helpful in generating the mineral identification cross plots. The data density of the field data (logs) falls on respective minerals position/levels which are standard. The graphical results are present discrimination among constituting minerals.
Hmm, thank you, however I'm looking for specific information, i.e., if I’m able to link a detrital topaz to/with a specific source rock using its (topaz) chemical compositions (concentration of elements, ratios between elements, etc.). Papers dealing with detrital topaz are quite scarce.
Dear Dr. Kowal-Linka,
I know topaz from HM suites around granites and pegmatites. As the hydroxyl group (OH)- can substitute for fluorine F- up to 25 % there may be a clue to discriminate different types of topaze. I would suggest to contact gemologists who can analyze even small grains with non-destructive methods and try and find out if their methods are suitable to detect such substitutions or lattice defects caused by radiation damage as a function of the host rock. Maybe the mineral color direct your thoughts to one or the other effects. Chromophores in whatever way they show up in the lattice also fall into the field of experience of gemologist.
I wish you much success
H.G.Dill
I think that the easiest way to do it is by analyzing topazs from the suspected source rocks and correlating their microprobe and laser ablation data with those from the detrital ones. Every other calculation, if you don't know precisely what you are searching for, may be misinterpreted and wrongly attributed.
As Prof Dill said, the OH-F substitution in topaz has been related to its environment of formation. You may like to look into some of the following, rather oldish, publications:
Dear, Howie and Zussman, Rock-forming minerals, Vol. 1A.
Chaudhry & Howie, 1970. Mineralogical Magazine, 37, 717-720.
Jan, 1979. Mineralogical Magazine, 43, 175-176.
Rosenberg, 1967. American Mineralogist, 52, 189-195.
Rosenberg, 1969. Geol Soc America Abs, 7, 192-193.
best wishes
Do you have other heavy minerals as well, such as zircon? You might do some geochronology using detrital zircons to help you estimate provenance. Just a thought!
Considering the limited variability of topaz in terms of major elements, studying trace elements, both of your heavy minerals and suspected source areas, might be the most fruitful approach. Perhaps cathodoluminescence imaging may also give some information (e.g. Agangi et al., 2016. Relation between cathodoluminescence and trace-element distribution of magmatic topaz from the Ary-Bulak massif, Russia. Mineralogical Magazine, 80(5): 881-899.
In the western United states, there are topaz bearing rhyolites. Flourite and yitrroflourite are also reported in some of the HREE-enriched rhyolite laccoliths of similar age at the eastern shoulder of Rio Grande rift. These HREE-enriched rhyolites are full of Yitrroflourite and devoid of topaz, I assume topaz was not stable possibly due to increased F-content sufficient enough to dissolve almost all the zircons in the rocks ( I tried separating zircons from the HREE-rhyolites, despite several efforts not even a single zircon was found. There are some hydrothermal zircon reported from the contact of the rhyolite where flourite is abundant indicating low flourine activity to regrow some of the zircons.There is stability issue of these three minerals depending on the fluorine concentration and other geochemistry. the rocks I mentioned have A-type geochemistry. Look for fluorine-enriched rocks in the surrounding region
Do you find only topaz or flourite and Yittroflourite too?