Geochemical constraints on the distribution of trace elements and volatiles in fluorapatite from the Panasqueira tin-tungsten deposit (Portugal) |
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| Institution: | 1. Institute of Geological Sciences, Polish Academy of Sciences (ING PAN), Research Centre in Kraków, Senacka 1, PL–31002 Kraków, Poland;2. AGH University of Science and Technology, Faculty of Geology, Geophysics and Environmental Protection, Al. Mickiewicza 30, PL–30059 Kraków, Poland;3. The Czech Academy of Sciences, Institute of Geology, Rozvojová 269, Prague 6 16500 Czech Republic;4. Institute of Geological Sciences, Jagiellonian University, Gronostajowa 3a, PL–30087 Kraków, Poland |
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| Abstract: | A large, euhedral crystal of fluorapatite (ca. 19.5 × 20.0 mm) from the Panasqueira tin-tungsten deposit (Portugal) was investigated in terms of the distribution of trace elements by using several microanalytical techniques. The studied material represents almost pure fluorapatite with minor amounts of other cations (mainly Sr, Mn, REE and Fe), OH and Cl. Particular interest was given to the distribution of rare earth elements with respect to the crystallographic orientation. A broad range of analytical techniques were used, including optical microscopy coupled with cathodoluminescence imaging, electron probe microanalysis (EPMA), laser ablation – inductively coupled plasma mass spectrometry (LA-ICPMS), Raman microspectroscopy, and simultaneous thermal analysis coupled with mass spectrometry. The investigated crystal consists of the main crystal with a distinct core and rim (Ap2core and Ap2rim, respectively), which grew on a previous, euhedral crystal (Ap1). The fluorapatite demonstrates various types of zoning: regular oscillatory, irregular, and internal sectoring, which is also reflected in trace elements concentrations. The rim Ap2rim has lower concentrations of Mn, Sr and Fe, and significantly higher concentrations of REE compared to the core Ap2core and older crystal Ap1. Furthermore, the rim Ap2rim is strongly depleted in Th, U and Pb. The entire crystal shows elevated Eu contents, expressed as a strong positive anomaly in chondrite-normalized REE patterns. With regards to the volatiles, F concentrations are constant in Ap1, Ap2core and Ap2rim, whereas Cl is below the EPMA detection limit. The Ap2rim was the only part of the investigated material containing OH and CO3, which were observed in the Raman spectra. Furthermore, part of the crystal Ap2core is extensively altered, likely due to fluid-induced metasomatic processes. LA-ICPMS U-Pb dating yielded highly discordant dates due to common Pb content. A lower intercept age of 297 ± 13 Ma (MSWD = 0.13) indicates the age of the fluorapatite crystallization. The overall analytical data constrain growth and post-growth processes, including crystallization of Ap1 and Ap2core, which both have typical hydrothermal Sn-W deposit characteristics, whereas Ap2rim is related to a carbonate stage of the mineralization in the Panasqueira deposit. |
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| Keywords: | Fluorapatite Phosphates LA-ICPMS Rare earth elements Crystal zoning Substitution mechanisms Dissolution-reprecipitation reactions Metasomatic alterations |
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