Enrichment of critical metals in mafic rocks: the case of the Maronia Pluton, Northern Greece

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Marco E. Ciriotti
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Enrichment of critical metals in mafic rocks: the case of the Maronia Pluton, Northern Greece

Messaggio da Marco E. Ciriotti » gio 15 set, 2022 10:34

Referenza:
▪ Papoutsa, A., Skentzou, D., Voudouris, P., Stouraiti, C., Vasilatos, C. (2022): Enrichment of critical metals in mafic rocks: the case of the Maronia Pluton, Northern Greece. XXII International Congress of the CBGA, Plovdiv, Bulgaria, 7–11 September 2022, Abstracts.

Abstract:
The geochemical behavior of critical metals in magmatic and hydrothermal systems is of paramount importance in economic geology. Specific metals namely the REE, U and Th are predominately associated with alkaline felsic igneous rocks. Therefore, the presence of REE-U-Th minerals in mafic rocks is of particular interest, regarding the physicochemical conditions of the enrichment.
The Maronia pluton is an Oligocene intrusion, emplaced within the Rhodope Core Complex, as a result of arc-magmatism during subduction of continental lithosphere after the closure of the Pindos Ocean (Schaarschmidt et al., 2021). The pluton comprises three distinct lithologies: a) the basic group of gabbro; b) the intermediate group of monzonite, quartz monzonite, monzogabbro and quartz monzogabbro; and c) the felsic group, comprising granite cut by aplitic dykes and porphyry-type mineralized porphyritic microgranite (Melfos et al., 2020; Schaarschmidt et al., 2021). Felsic magmas, including the porphyritic microgranite, were generated by fractional crystallization of mafic magmas (Schaarschmidt et al., 2021). The shoshonitic affinity of the Maronia pluton is considered to reflect hydrous melting of a phlogopite-bearing mantle that was enriched by 3–5% of slab-derived sediment melts (Schaarschmidt et al., 2021).The REE are presented in remarkably constant concentrations throughout the compositional range of the pluton, which is in contrast with the variable compatibility of these metals in minerals of mafic and felsic rocks. Therefore, a geochemical and mineralogical study was conducted in the mafic rocks to determine a) the host minerals of these metals and b) the geological processes responsible for their formation. Wholerock composition of the studied samples was determined through XRF and ICP-MS, while the identified REE-U-Th minerals were investigated by SEM-EDS.
Geochemically, the studied samples are classified as metaluminous monzogabbros/diorites, and are composed of plagioclase, K-feldspar, pyroxene, amphibole, minor quartz, and iron oxides (ilmenite, magnetite). Accessory minerals include "apatite", epidote and zircon. In addition, REE-U-Th minerals such as allanite-(Ce), monazite-(Ce), perrierite-(Ce), uraninite-thorianite, thorite, and zirkelite, were identified. The studied samples show a weak hydrothermal alteration, including biotite, sericite, actinolite and chlorite. A preliminary petrographic investigation suggests that perrierite-(Ce) and monazite-(Ce) are tentative magmatic phases, whereas the U-Th minerals and allanite-(Ce) could be of both magmatic and late magmatichydrothermal origin. It is possible that late magmatic-hydrothermal fluids, potentially derived also from the surrounding granites and associated porphyry-type mineralization, may have contributed to the relative enrichment of REE, U, and Th in the monzogabbros. Furthermore, a possible dissolution of main magmatic REE-U-Th phases, could have, locally, remobilized U and Th (Papoutsa and Pe-Piper, 2013), and resulted in the formation of uraninite-thorianite, thorite and zirkelite.
Marco E. Ciriotti

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