REE distributions of some varieties of flourite minerals from sedimentary rocks-hosted hydrothermal vein display LREE enrichment relative to HREE (30-300x Chondrite). All patterns show very visible positive Eu anomalies. What gave rise to these +Eu anomalies? What could have been the origin of the mineralizing fluid?
Dear Uriah,
Positive Eu anomalies suggest feldspar destruction (Moller et al., 1976; Moller and Morteani, 1983). Feldspars typically have positive Eu anomalies and if the solutions responsible for the deposition of the fluorite are involved in the decomposition of feldspars, the Eu can be carried to the site of fluorite deposition.
If oxidizing conditions exist at the deposition site, the Eu +2 derived from the feldspar will be converted to Eu +3 and incorporated in the fluorite which will then exhibit a positive Eu anomaly (Constantopoulos, 1988).
Möller, P., and G. Morteani. "On the geochemical fractionation of rare earth elements during the formation of Ca-minerals and its application to problems of the genesis of ore deposits." The significance of trace elements in solving petrogenetic problems and controversies (1983): 747-791.
Möller, P., P. P. Parekh, and H-J. Schneider. "The application of Tb/Ca-Tb/La abundance ratios to problems of fluorspar genesis." Mineralium Deposita 11.1 (1976): 111-116.
Constantopoulos, James. "Fluid inclusions and rare earth element geochemistry of fluorite from south-central Idaho." Economic Geology 83.3 (1988): 626-636.
Kind Regards,
Masoud Ovissi
Dear Mr. Lar,
we studied fluorites from several deposits and I have added below four of our studies. What Mr. Ovissi wrote is correct but for the state of oxidation it is the Ce anomaly which counts. This is a significant parameter especially for fluorite of younger generations, precipitated at shallow depth and last-but-not least in sedimentary rocks.
I hope these studies and comments will be of help for you
H.G.Dill
Dear H.G.Dill, I appreciate your input and the literature; They will be of immense help in my further work. Suggested literature from M. Ovissi is acknowledged. Many thanks.
All I can add to Ovissi's answer is that "feldspar destruction" in this case means albitization or sericitisation of plagioclase, since Eu substitutes for Ca in plagioclase and both cations are released during those alteration processes. This could help to explain the origin of the mineralising fluids. With regards, Sebastian.
Dear Mr. Lar,
It is well known that REE are hosted in the fluorite structure due to ion-substitution processes (Möller et al., 1998). A possibility for Eu anomaly is temperature of fluids. This means that, Eu anomaly depends on environment conditions (mainly temperature) during the precipitation. Another possibility is that Eu anomaly is inherited from the reaction of fluid with source rock (Schwinn and Markl, 2005) because of alteration of feldspars (as mention by Mr. Ovissi). Have a look on papers:
Möller, P., Bau, M., Dulski, P., Lüders, V., “REE and Y fractionation in fluorite and their bearing on fluorite formation” In: Proceedings of the Ninth Quadrennial IAGOD Symposium, Schweizerbart, Stuttgart, (1998) pp. 575–592.
Schwinn, G., Markl, G., “REE systematics in hydrothermal fluorite” Chemical Geology 216,(2005) 235–248.
Mondillo, N., Boni, M., Balassone, G., Spoleto, S., Stellato, F., Marino, A., Santoro, L., Spratt, J., “Rare earth elements (REE)-Minerals in the Silius fluorite vein system (Sardinia, Italy)” Ore Geology Reviews, Vol. 74, (2015) p. 211-224.
Thank you Mr Azizi for the contribution. I will have a check on the literature you recommended.
Thanks
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