Publisher © Czech Geological Survey, ISSN: 2336-5757 (online), 0514-8057 (print)
Minerals of selected critical elements (REE, Zr, Nb) detected in the Roztoky Intrusive Complex, České středohoří Mts.
Published online: 7 December 2020Abstract
Strongly carbonatized alkaline rocks and a carbonatite s. s. (xenolith) were discovered in the R-2 drillcore in the Roztoky Intrusive Complex (Figs 1, 2, 7; Rapprich et al. 2017). Both carbonate-rich rock types contain minerals of critical elements reported for the first time in the alkaline volcanic rocks of the České středohoří Mts.
These include (Ca)-REE fluorocarbonates, minerals of Zr (zircon, baddeleyite; Fig. 3) and columbite. Chemical composition of these minerals was determined using SEM and EMP and the crystal-chemical formulae were calculated. Columbite was classified as columbite-(Mn) (Tab. 3).
(Ca)-REE fluorocarbonates were detected in two different samples and rock types. Ca-poor bastnäsite-(Ce) represents the only REE mineral present in carbonatized trachyte at the depth of 156.5 m. Synchysite-(Ce), bastnäsite-(Ce) and parisite-(Ce) were found in dolomite carbonatite at 286.5 m. These minerals form epitactic intergrowths up to 30 μm in size (Fig. 5, 8), which is, according to Armbrustmacher (1979), typical for primary (magmatic) REE minerals. (Ca)-REE fluorocarbonates are Ce-dominant (0.5 apfu), containing significant amounts of La (0.3 apfu) and Nd (0.2 apfu) (Tab. 1). The REE minerals show a typical Y3+-REE3+ substitution, which is especially significant in Ce-poor phases represented by analyses e3 and e4, where Ce is substituted by Y (0.1 apfu) and minor Sm, Eu, Gd and Th (Tab. 1).
