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This mineralogical and fluid inclusion study was conducted on an Au-bearing quartz–sulfide vein encountered in the deep parts of the Bytíz deposit in the Příbram uranium and base-metal district, Bohemian Massif, Czech Republic. The samples were taken where the Au-bearing vein is crosscut by the common base-metal Zn-Pb ore vein Bt23C. The early mineralization of the Au-bearing vein is composed mainly of quartz (Q-1 to Q-3), illite–muscovite, Fe-Mg chlorite, arsenopyrite, and Au-Ag alloys, showing a wide range of compositions (4–69 at. % Ag) and a decrease in Au/(Au + Ag) ratios during vein evolution. Younger hydrothermal processes led to the crystallization of nests and veinlets composed of late quartz (Q-4), carbonates (siderite, dolomite–ankerite and calcite), base-metal sulfides (galena, sphalerite, chalcopyrite, and tetrahedrite), a suite of Ag and Bi-tellurides, and acanthite. The input of Sb is manifested by the partial to complete replacement of some gold grains by aurostibite and an unnamed (Ag,Au)-Sb oxide with a composition close to AuSbO3. The fluid inclusion study, combined with chlorite thermometry and arsenopyrite thermometry, showed that the early mineralization crystallized from progressively cooled (from 300 to 400 °C down to ca. 180 °C), diluted (1.2–7.0 wt. % NaCl eq.) aqueous solutions. The late portion of the mineralization formed from aqueous fluids with highly variable salinity (0.2–23.4 wt. % NaCl eq.) and homogenization temperatures decreasing from ca. 250 °C to < 50 °C, which compare well with the base-metal mineralization of the vein Bt23C and other base-metal veins of the Příbram ore area. Our study illustrates the nature and intensity of the processes of the reworking of the early gold mineralization mediated by the younger Ag,Sb-rich base-metal fluids, giving rise to Příbram's typical late-Variscan vein Zn-Pb mineralization. [ABSTRACT FROM AUTHOR]

Palaeostress analysis based on multiple inversion of fault-slip data in 9-dimensional space in the area of the SE margin of the Elbe Fault Zone (E part of the Bohemian Cretaceous Basin, the Czech Republic) reflects five palaeostress phases. Formation of syntectonic calcite veins mostly took place at 500-1000 m depth. Homogenisation temperature of fluid inclusions (~80 °C) and only subtle variations in fluid salinity point to syntectonic calcite vein formation from a single fluid source and one-step crystallisation. Fluid trapping occurred at 82-84 °C and 4.84-9.67 MPa. Both oxygen isotope composition of the fluid (δ18Ofluid = 2 ‰ V-SMOW) and fluid salinity (~3 wt. % eq. NaCl) reflect formation of calcite veins from formation waters expelled from the Cretaceous strata. Compressional phase D1 (σ1 trending ~N-S) is documented in a regime of reverse faults and strike-slip faults which indicate Europe-Africa coupling and N-directed thrusting of the Alps in the Palaeogene-Early Oligocene. Extensional phase D2 (σ3 generally trending N-S to NE-SW) is indicated by the formation of oblique to normal faults and accompanied by volcanic intrusions in the Middle Oligocene-Early Miocene. Palaeostress phase D3 (σ1 trending ~NE-SW) reflects Early Miocene compression in the Eastern Alps. Palaeostress phase D4 (σ3 in the direction WNW-ESE) is a result of crustal extension in the Eastern Alps and the Western Carpathians. Compressional palaeostress phase D5 (σ1 generally trending NW-SE to WNW-ESE) is evidenced by reverse faults and strike-slip faults from the Pliocene. [ABSTRACT FROM AUTHOR]

Coal petrology data and vitrinite reflectance modelling were combined with micro-geochemical and fluid inclusion analyses of coal mineralization and apatite fission track analysis (AFTA) of coal-bearing sediments to constrain the thermal history of the Blanice Graben, a narrow, nearly 150-km long, late-Variscan strike-slip basin filled with Permo-Carboniferous continental coal-bearing deposits. The vitrinite reflectance modelling and the ranking of coals, which increased along the meridional axis of the graben from bituminous coals (R r ∼ 0.70%) to high-rank anthracites (R r = 2.85–4.77%), provide evidence of maximum palaeo-temperatures ranging from about 120–145 °C to 310–390 °C. This corresponds to the diagenetic realm in the north of the graben that graded to high-grade anchizonal to low-epizonal metamorphic conditions in the south. The present level of coalification resulted from the Permian burial, 2500–5000 m thick, which aggregated under a high geothermal gradient (∼ 90 °C/km). In central and southern relics of the graben, however, specific microscopic features of coals and fluid inclusion and chlorite thermometry suggest that the anthracite rank was attained by the combined effect of a stratigraphic burial, a regional discharge of ∼300 °C hot, mineralized fluids that ascended along faults into buried coal-bearing strata and an intense Permian faulting and shearing of coal seams, which collectively contributed to the anthracitization process. The heat source, promoting high geothermal gradient and circulation of hot fluids through the coal-bearing sediments, was probably associated with a mantle magma chamber hidden deeply below the graben and manifested at current errosional level by ∼303 and ∼ 270 Ma old microdiorite dykes. Similar dramatic geothermal settings, characterized by elevated geothermal gradients, localized fault-controlled incursions of hot hydrothermal solutions, and rapid coalification of peat layers at relatively shallow depths of burial, dominated in many late Variscan coal-bearing basins during a period of post-collisional extension at ∼315–280 Ma. The AFTA method, applied to sedimentary apatites, provides evidence that following the climax of the Variscan orogeny, the Blanice Graben coal-bearing deposits were rapidly uplifted and, throughout the Mesozoic and Cenozoic times, they experienced only prolonged slow cooling (0.46–0.54 °C/My) and moderate burial temperatures of 50–70 °C. Its final ascent towards the present-day erosional surface occurred during a period of accelerated uplift that has affected the Bohemian Massif since the Oligocene. [ABSTRACT FROM AUTHOR]

The stratiform deposit of mercury at Horní Luby near Cheb (Czech Republic) is hosted by Ordovician phyllites of Vogtland-Saxony Paleozoic Unit in the Saxothuringian Zone of the Bohemian Massif. Ore is represented by cinnabar disseminated within the phyllite and by lenticular bodies rich in massive cinnabar (lenses with a thickness of up to 1 m and length along the strike varying from less than 1 m up to 20 m). Cinnabar is accompanied by pyrite and locally also siderite. The P-T history of the mineralization was deciphered by the study of fluid inclusions. Early fluids are represented by a homogenous H2O-CO2, fluid trapped in secretion quartz. This fluid underwent several heterogenization events starting from approximately 300 °C and continued down to 200-150 °C. Metamorphic quartz of secretion origin crystallized at -300 °C, while hydrothermal pyrite crystallized at 220-210°C and cinnabar at 195-160 °C. The formation of the richest ore was associated with the replacement of metamorphic quartz in phyllites by the cinnabar in weakly alkaline solutions. The cinnabar is pure phase, free of admixtures (Bi, Sb, Zn, Fe, and Cu). It is accompanied by minute blebs of Hg-bearing sphalerite (11-12 wt. % Hg) that might indicate earlier presence of the zincian metacinnabar and more complex metamorphic history of the ore. The primary source of mercury is thought to be Lower Paleozoic submarine volcanism. The formation of ore bodies is, however, associated with metamorphic mobilization of mercury during the late stages of the Variscan orogeny. Despite being relatively small, the mercury deposit at Horní Luby competed with the mines in Idrija (Slovenia) and Almadén (Spain) in the 16th century. The production of mercury at Horní Luby is estimated to have corresponded to c. 10-30 % of the mercury production in the mines of Idrija and Almadén at that time. In addition to Venice, the mercury from Horní Luby was also supplied to Niirnberg, Antwerp and Lyon. In 1520-1540, the production of mercury from the Horní Luby mines was 6 to 15 tons per annum. The mines were abandoned in 1597. Attempts to reinstitute mining activity in the 17th, 18th and 19th centuries were not very extensive and always failed. The total production of the Horní Luby mines during the whole mining history is estimated to have been at least 200 tons of mercury. Much of this amount was extracted between 1520 and 1540, with a minor portion between 1560-1570. [ABSTRACT FROM AUTHOR]

Gold-bearing quartz veins of the Bělčice ore district are hosted by I-type Variscan Blatná granodiorite of the Central Bohemian Plutonic Complex. The quartz gangue precipitated from aqueous-carbonic fluids (3-7 wt. % NaCl eq.) with significant methane admixture (up to 15 mol. % of CH4) at c. 350-400°C and 200-400 MPa. The ore stage including gold precipitation occurred at lower temperatures (300 to 250 °C) and pressures (∼ 100 MPa) from aqueous fluids. The isotope composition of the quartz and carbonate gangue indicates progressively heavier oxygen of the parental fluid (δ18Ofluid from ∼9-10 ‰ to 10-14 ‰ SMOW). This suggests ingress of metamorphic fluids equilibrated with metasedimentary country rocks into an environment dominated by I-type granitoids. Invasion of external metamorphic fluids is also supported by nearly isothermal mixing trends in aqueous fluids as documented by salinity variations (from 5.5 to 0 wt. % NaCl eq.). [ABSTRACT FROM AUTHOR]

Gothic arch calcite, a specific crystallographic variety of calcite known from some hot springs and tufa streams, has been newly recognized in the Koněprusy Caves. The gothic-arch calcite occurs on the exteriors of exotic coralloid speleothems where it coexists with scalenohedral (dogtooth) spar crystals. The crystals exhibit microscopic ultrastructural features including deeply eroded topography, etch pits, and spiky and ribbon calcite crystallites, pointing to its extensive natural etching. Many gothic-arch calcites originated as late-stage, secondary overgrowths on older, etched dogtooth calcite crystals. Its characteristic outward curvature resulted from the recrystallization of etching-liberated fine carbonate grains and newly formed needle-fiber calcite laths, which were accumulated and bound on the faces and at the bases of corroded crystals. These intimately coexisting destructive and constructive processes of carbonate crystal corrosion and growth were probably mediated by bacteria, fungi, or other microorganisms. Fluid inclusions embedded in calcite crystals point to a vadose setting and temperatures below ~50 °C. This, combined with the wider geological context, indicates that the gothic arch calcite crystals originated only during the late Pleistocene to Holocene epochs, when the cave, initially eroded by hypogene fluids in the deeper subsurface, was uplifted to the subaerial setting and exposed to the meteoric waters seeping from the topographic surface. The radiocarbon analysis shows that gothic-arch calcite crystals are generally older than ~55,000 years, but the surface layers of some crystals still reveal a weak 14C activity, suggesting that microbiologically mediated alterations of the speleothems may have been occurring locally until now. [ABSTRACT FROM AUTHOR]

Primary fluid inclusion in quartz crystals from barren subvertical quartz veins that crosscut the Upper Carboniferous (Upper Asturian) coarse-grained clastics at Butov near Stíbro were found to have almost the same microthermometric characteristics (salinity, temperature of homogenization and variability of filling) as fluid inclusions from mineralized veins of the Stíbro Ag-Pb-Zn ore district. The mineralization at Stříbro should be therefore Upper Asturian in age, or younger. It is notably, that fluid salinities and temperatures of veins from Butov, and also of the Stříbro district, are very similar to that of Triasic to Jurasic fluorite-barite mineralization from the Erzgebirge Mts. (Moldava, Vrchoslav deposits). [ABSTRACT FROM AUTHOR]

Abstract: Unusual paleofluid composition is reported for the Libčice orogenic-type gold deposit located in the Central Bohemian Plutonic Complex (CBPC), Czech Republic. Unexpected bicarbonate-rich fluids and their complex chemistry variations characterize primary fluid inclusions from the main gold-bearing quartz vein. A detailed microthermometry, Laser Raman Micro Spectroscopy (LRMS) and SEM cathodoluminescence study was used to understand the complex chemistry and P–T–X history. The results indicate the presence of H2O and H2O–CO2–CH4 (±N2; H2S) fluids, the latter displaying variations of the CO2/CH4 ratio in the gaseous phase from 6.8 to 0.06. Variation of the CH4 content across single grains and between different levels of the mine was recorded. The presence of nahcolite, H2 (up to 6mol%) and ethane (0–0.2mol%) in the fluids were also discovered by LRMS. Potential models for the formation of different types of fluids present in the deposit are discussed, including the genesis of HCO3 − rich fluids as well as H2 and C2H6 presence in the primary fluid inclusions. The potential influence of organic matter-bearing sediments, as well as the impact of the intrusion of CBPC, re-equilibration and/or re-speciation of fluid inclusions during the post-entrapment history is considered to have the main impact on the complex paleofluid chemistry. Based on the thermodynamic modeling, H2 diffusion into the fluid inclusions was shown to be the main reason for the CH4 variation on the scale of a single grain, as well as across the whole vein. Although the exact processes of production/formation of HCO3 − and H2 at the Libčice deposit remain open to discussion, reactions in the C–O–H system are considered to be a possible formation mechanism. This work also contributes to our understanding of the importance of post-entrapment modifications and reactions in the C–O–H system on interpretation/deciphering the processes in orogenic-type deposits. [Copyright &y& Elsevier]