Blue, complexly zoned, (Na,Mg,Fe,Li)-rich beryl from quartz-calcite veins in low-grade metamorphosed Fe-deposit Sk��ly near Ryma?ov, Czech Republic

Syn-tectonic quartz-calcite veins containing blue beryl are enclosed in hematite > magnetite-rich portions of the low-grade metamorphosed Fe-deposit Skály near Ryma?ov, Czech Republic. Aggregates of pale to deep blue beryl, up to 2?cm in diameter, are associated with euclase, clinochlore, hematite, albite and dravite. Complexly zoned beryl crystals consist of skeletal aggregates of beryl I randomly distributed within volumetrically dominant beryl II with narrow rims of beryl III. All types of beryl have similar contents of Na (0.32?C0.49 apfu) and Mg (0.31?C0.41 apfu) but variable contents of Fe_tot (0.05?C0.34 apfu) and Al (1.20?C1.62 apfu). The LA-ICP-MS study yielded elevated contents of Li, up 1,314?ppm (0.28?wt.% Li₂O) in beryl I. The quartz-calcite veins represent an unusual type of low-T metamorphic-hydrothermal vein related to Fe-ore deposit characterized by single-stage fracturing and mobilization in a closed system at T~200?C300°C and CO ₃ ^2- as a major complexing agent for the mobility of Be.     

Origin of the Zálesí U–Ni–Co–As–Ag/Bi deposit,Bohemian Massif,Czech Republic: fluid inclusion and stable isotope constraints

The Zálesí vein-type deposit is hosted by Early Paleozoic high-grade metamorphic rocks on the northern margin of the Bohemian Massif. The mineralization is composed of three main stages: uraninite, arsenide, and sulfide. The mineral assemblages formed at low temperatures (~80 to 130°C, locally even lower) and low pressures (<100 bars). The salinity of the aqueous hydrothermal fluids (0 to 27 wt.% salts) and their chemical composition vary significantly. Early fluids of the oldest uraninite stage contain a small admixture of a clathrate-forming gas, possibly CO₂. Salinity correlates with oxygen isotope signature of the fluid and suggests mixing of brines [δ ¹⁸O around +2‰ relative to standard mean ocean water (SMOW)] with meteoric waters (δ ¹⁸O around −4‰ SMOW). The fluid is characterized by highly variable halogen ratios (molar Br/Cl = 0.8 × 10⁻³ to 5.3 × 10⁻³; molar I/Cl = 5.7 × 10⁻⁶ to 891 × 10⁻⁶) indicating a dominantly external origin for the brines, i.e., from evaporated seawater, which mixed with iodine-enriched halite dissolution brine. The cationic composition of these fluids indicates extensive interaction of the initial brines with their country rocks, likely associated with leaching of sulfur, carbon, and metals. The brines possibly originated from Permian–Triassic evaporites in the neighboring Polish Basin, infiltrated into the basement during post-Variscan extension and were finally expelled along faults giving rise to the vein-type mineralization. Cenozoic reactivation by low-salinity, low-δ ¹⁸O (around −10‰ SMOW) fluids of mainly meteoric origin resulted in partial replacement of primary uraninite by coffinite-like mineral aggregates.     

The Malé Vrbno magnetite occurrence of the Velké-Vrbno Unit, Czech Republic: petrology, mineralogy, geochemistry and genesis

The mineralization investigated is that of a stretched magnetite-rich body consisting of three subparallel layers. These layers are each 200 m long, have in total a thickness of close to 2 m and are asymmetrically enveloped by amphibolite which is the host rock. Both, host rock and ore body belong to the Velké-Vrbno Unit which is a constituent of the Prevariscian Orogen and lie at the NW margin of the Bohemian Massif about 500 m SW of the village Malé Vrbno close to the town of Staré-Mešto (Czech Republic).The host rocks are composed of amphiboles (up to 90 vol%) with Mg# of 0.54–0.62 (pargasite, tschermakite and magnesiohornblende), Ab₆₄An₃₅K_0.1 feldspar (up to 50 vol%), mica (eastonite) and Mg-dominated chlorite. The only oxide mineral is manganese-rich ilmenite. The ore body is composed of homogeneous, nearly pure magnetite (up to 50 vol%), amphibole with Mg# of 0.14–0.26 (ferrotschermakite, ferrogedrite and ferroanthophyllite), garnet (almandine-dominated with varying proportions of grossularite, andradite and pyrope), mica (annite), local Ab₆₅₋₇₂An₂₈₋₃₄K_0.5−0.8 feldspar, apatite, secondary Fe_tot-dominated chlorite and quartz.Deduced from REE distribution patterns, the distribution of the HFS elements and the SPIDER diagram, host rocks and ore body originated from continental tholeiitic magmas. The magmatic origin is also supported by the occurrence of chromite and the abundance of apatite in the ore body. The rocks derived from two subvolcanic emplaced partial melts that originated from a deep-seated intrusion. The first melt, identical to the protolith of the host rocks, emplaced in an early stage of differentiation in which the Mg/Fe ratio was relatively high. In an advanced stage of differentiation a second melt was released from the deep-seated intrusion leading to the emplacement of the ore-body protolith. In this stage, the Mg/Fe ratio was remarkably lower and the oxygen fugacity higher causing not only the formation of Fe-rich minerals, but also the crystallization of magnetite. After their emplacement and consolidation, both rock types suffered the same geological development during which they were overprinted by various tectono-metamorphic events (which were not separately investigated).