Výskyt hydroxylsynchyzitu-(Ce) na hydrotermální žíle ve vápencích z lomu cementárny Hranice

A new occurrence of a mineral of the synchysite group was identified in a hydrothermal vein in the limestone quarry of the cement plant at the Hranice town (Moravo-Silesian Palaeozoic, Bohemian Massif, Czech Republic). The vein is formed by two chemically distinct generations of calcite (Cal97-100Rdc0-2Sid0-1), Fe-rich dolomite (Dol65-75Ank21-31Ktn2-6), quartz, pyrite and chalcopyrite. The synchysite-group mineral forms microscopic inclusions hosted by Fe,Mn-enriched calcite associated with pyrite and chalcopyrite. Since Ce predominates within the sum of REE+Y and the contents of F are low (0.14–0.29 apfu), the mineral can be classified as hydroxylsynchysite-(Ce). Chondrite-normalized REE distribution patterns show a decrease from La to Er and a distinct positive Eu anomaly. The LaCN/SmCN ratios of the mineral (1.51–2.49) are different from those of the host limestones (2.81–3.14). Similar discrepancy is observed also in case of two other synchysite occurrences from area of the Moravo-Silesian Palaeozoic hosted by Culmian siliciclastic sediments. Therefore, the exact source of REE cannot be identified on the basis of determined elemental ratios only. The fractionation of REE during synchysite crystallization was mostly negligible at all three above-mentioned sites, which suggests a low content of strong REE-complexing ligands (i.e., F-, OH-, or CO32-) in the fluids and a dominant role of weak ligands (i.e., Cl-, SO42-,HCO3-). The positive Eu anomaly indicates an increase of redox potential of fluids, but not up to strongly oxidizing conditions (as Ce anomaly is absent). The fluoride ions could act as a depositional anion, allowing the precipitation of REE into a mineral phase. The given parameters (chloride solutions, geochemically open system, mixing of brines with other types of fluids) reflect well the features of hydrothermal system of post-Variscan mineralizations, which is also consistent with mineral composition of veins at all discussed sites.