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2 Acta Geologica Polonica

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Stability relationships of REE-bearing phosphates in an alkali-rich system (nepheline syenite from the Mariupol Massif, SE Ukraine)

Dumańska-Słowik M., Budzyń B., Heflik W., Sikorska M.

Acta Geologica Polonica

2012

Vol. 62, no. 2

247-265

Primary REE-enriched fluorapatite and fluorbritholite-(Ce) in nepheline syenite from the Mariupol Massif (SE Ukraine), contain textural and chemical evidence of late- to post-magmatic metasomatic alteration. REE mobilization and replacement of the primary phases by fluid-mediated coupled dissolution-reprecipitation strongly depended on the distance between the altered minerals in the host rock. Fluorapatite and fluorbritholite-(Ce) forming individual pristine grains were partially replaced by the same phase with a new composition, resulting in the presence of patchy zoning in altered grains. the increased REE contents in altered fluorapatite rim domains are related to REE mobilization from the altered REE-depleted rim domains of the fluorbritholite-(Ce). The REEs were transported by a fluid with high F activity. The alteration of fluorapatite and fluorbritholite-(Ce) grains in contact resulted in the partial replacement of the primary phases by the same phase with a new composition, but also in the partial replacement of the fluorapatite by secondary monazite and fluorite. The REE mobilized from the fluorbritholite-(Ce) in the presence of a F-rich fluid in an alkali-rich system promoted formation of monazite as the new phosphate REE-host. The presence of secondary parisite in the altered domains of the fluorapatite and fluorbritholite-(Ce) indicates a CO2 component in the fluid during metasomatic alteration.

Dissolved-recrystallized zircon from mariupolite in the Mariupol Massif, Priazovje (SE Ukraine)

Dumańska-Słowik M., Sikorska M., Heflik W.

Acta Geologica Polonica

2011

Vol. 61, no. 3

277-288

Zircon in mariupolite from the alkaline Mariupol Massif has a relatively simple, monotonous composition with only some variation in its REE2O3 and ThO2 content, i.e., 0.00-1.55 and 0.00-0.34 wt. %, respectively. It contains numerous inclusions such as albite, lepidomelane, aegirine, K-feldspars, pyrochlore, paristite and bastnńsite-(Ce). The crystallization of the inclusions (except for REE-bearing carbonates) was contemporaneous with the formation of the zircon crystals. These inclusions were enclosed by faster growing zircons. The SEM-CL images of the zircon are typical of crystals with signs of local thermal recrystallization, i.e., primary oscillatory zoning along the grain margins and secondary irregular patchy zoning in the interior. Its complex internal texture most probably formed as a result of a coupled dissolution-recrystallization process during the late magmatic or post-magmatic stage of the cooling of the host rocks. The abundant micropores could have originated as a result of leaching of the zircon. The recrystallization process definitely disturbed the concentric oscillatory zoning pattern characteristic of magmatic zircon.