Ishikawaite and samarskite-(Y) are indicators of an NYF signature in the mildly to moderately fractionated parts of the hybrid NYF + LCT Julianna pegmatitic system at Piława Górna, the Góry Sowie Block, SW Poland. The minerals were metasomatically altered by a Ca-bearing and Ta-enriched highly fluxed residual silicate melt. The intensity of the metasomatism and the chemical composition of its products strongly depended on the efficiency of the transport of ions to and from the replacement front. Restricted access of the fluid promoted the formation of calciosamarskite and oxycalciopyrochlore. Where the fluid could migrate more easily, with decreasing pH and temperature the alteration products became deficient in A-site cations (mainly Ca2+ and Mn2+). Consequently, earlier-crystallized pyrochlores transformed to zero-valent-dominant pyrochlores and microlites enriched in U+Th, Y+REE, or Ca. Low-temperature hydrothermal fluids were responsible for the final enrichment in Pb and Bi.

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