The Nb-Ta-U minerals of the María Elena pegmatite, San Luis range, Argentina

Abstract

The Nb-Ta-U mineral association of the María Elena pegmatite, San Luis range, Argentina is described. The assemblage of columbite-(Mn), kenopyrochlore, plumbopyrochlore and minor plumbomicrolite and kenomicrolite occurs in a rare-element pegmatite of beryl type. Primary, magmatic columbite-(Mn) forms up to 10 cm dark gray crystals covered by thin yellow films in the core-margin association of the pegmatite. Crystals show #Ta variable from 0.12 to 0.14, with TiO₂ contents between 0.54 and 2.18 wt.% and of UO₂ from 0.01 to 0.89 wt.%. The columbite-(Mn) crystals show minor local replacement by irregular millimetric veinlets and patches of pyrochlore supergroup minerals. Kenopyrochlore, associated with plumbopyrochlore and minor amounts of U-Ca-rich pyrochlore, contains irregular domains, fine cracks and tabular crystals mainly of plumbomicrolite and kenomicrolite. Besides, a second generation of secondary minerals contain yellow uranium-rich kenopyrochlore in fine veinlets that cut across columbite-(Mn). The hydrothermal overprint that reworked the primary phase mobilizing its Nb, Ta and U contents, contained a Pb input from a possibly exogenous source.