Sr-Ca-REE (sulfát-)fosfáty ze svrchnokarbonských sedimentů ze Semil (podkrkonošská pánev, Česká republika)

Abstract

Sporadic grains of the Sr-Ca-REE-rich members of the plumbogummite and beudantite groups were identified in a heavy mineral concentrate separated from Upper Carboniferous freshwater sandstone/arkose of the Semily Fm. at the Semily site (Krkonoše Piedmont Basin, Bohemian Massif, Czech Republic). These minerals display very fine-grained nature, increased porosity and contain abundant inclusions of both detrital (quartz, illite-muscovite) and authigenic (Fe-oxides, anatase) phases. The electron microprobe study revealed four phases including predominant crandallite, less frequent goyazite, and rare woodhouseite and florencite-(Ce), which, however, display great similarities on both cationic (Sr, REE, Ca, Fe contents) and anionic (As and S contents) sites of the formula. Cerium is always the dominating cation among REEs. The studied phases show low degree of fractionation of REEs (La/SmCN = 2.0 - 8.1), absence of CeCN anomaly and mostly slightly positive EuCN anomaly. They likely originated during diagenetic processes operating in the host sediments. When compared with other occurrences of these minerals in the area of Czech Republic, a very low level of REE fractionation is obvious, which can be explained either due to significant contribution of basic volcanites in the host rock environment and/or due to low activity of strong REE-complexing ligands in the parent fluids. However, the character of Ce and Eu anomalies is similar to other occurrences suggesting for similarities of temperature and redox conditions during crystallization.