Geochemistry and petrology of late K-and Rb-feldspars in the Rubellite pegmatite, Lilypad Lakes, NW Ontario

Three types of late, subsolidus (K-Rb)-feldspars occur in the spodumene-sub type, complex-type, pollucite-bearing Rubellite rare-element pegmatite at L ilypad Lakes, near Fort Hope in northwestern Ontario: (1) Exsolution follow ed by hydrothermal reconstitution generated diffuse bands, patches and vein lets of Rb-enriched (< 43 mol.% Rbf) and Rb-depleted triclinic K-feldspar i n black blocky microcline; (2) metasomatism of pollucite produced aggregate s of adularia which range from 14 to 73 mol.% Rbf, rimmed by pure 100 mol.% Kfs feldspar, and are most likely highly disordered; (3) blocky K-feldspar was replaced by salmon-coloured, fine-grained, Na-poor and Rb-free maximum microcline with a composition close to 100 mol.% Kfs, associated with calc ite and fluorite. After initial solid-state unmixing in blocky microcline, the reconstituted and metasomatic (K-Rb)-feldspars of the first two types c ould have formed simultaneously, under the influence of the same aqueous fl uid. The exsolved and reconstituted Rb-rich feldspar inherited the (Al,Si)- ordered framework of the blocky precursor, whereas metasomatic neocrystalli zation generated metastable disordered (K-Rb)-adularia. In contrast, a diff erent, F- and CO3-rich fluid produced the metasomatic salmon-coloured K-fel dspar, and promoted its perfect (Al,Si)-order.