During the last 20 years, new tools, such as Cd and Zn isotope ratios, have been developed to better understand the ore-formation processes for Pb-Zn deposits. Here, we investigate Cd concentrations, Cd and Zn isotope compositions and Zn/Cd ratios in different sphalerite generations from three Ag-Pb-Zn +/- Sb +/- Cu deposits and one U deposit of the Pribram ore district, a classical vein-type Variscan ore district in the Bohemian Massif, Czech Republic. Our results suggest that Zn, Cd and S were derived from Neoproterozoic/Cambrian volcano-sedimentary rocks including a fraction of deep-seated S and that sphalerite isotope chemistry was controlled by mixing of fluids and Rayleigh fractionation.

Stromatolites are remarkable organosedimentary laminated structures that grow gradually through time due to the activity of microorganisms, mostly algae and cyanobacteria. The appearance of stromatolites in formations as old as - 3.7 Ga represents the earliest macroscopic traces of life on Earth. Yet, the biogenic origin of some presumed stromatolites has been questioned as abiotic processes (e.g., fluid precipitation, diagenesis and metamorphism) can also be responsible for the formation of stromatolite-like textures. This is especially true in case of pervasively silicified stromatolites (stromatolitic cherts) that have rarely been reported from Neoproterozoic successions. Here, we present a detailed study of the Ediacaran stromatolitic cherts from the Blovice accretionary complex (Bohemian Massif) that combines petrography, trace element and Si-O isotopic compositions and Raman spectroscopy of organic matter to decipher the complex processes leading to their formation and evaluate the biogenicity of their precursor. Textural evidence (i.e. the presence of pseudomorphs after carbonate/evaporate and micro-ooids, variable thickness and spacing of organic-rich laminae) as well as trace element/isotopic compositions argue for a multi-stage formation of stromatolitic cherts. The primary carbonaceous stromatolitic precursors first formed in shallow-water lagoons with variable salinity at seamount slopes, were then fragmented by surface processes (erosion, mass wasting), and finally silicified by low-temperature hydrothermal fluids. The primary biogenic character of stromatolitic textures is evidenced by the nonequidistant alternation of dark (organic matter-rich) and light laminae with markedly different oxygen isotopic compositions reflecting probably seasonal cycles during stromatolite deposition, enrichment of bioessential elements (e.g., Cu, Zn, Co and Mn) in dark laminae, and Raman spectra of organic matter indicative for variably matured kerogens. The presence of late-stage diagenetic modifications might be related to subsequent burial and low-grade metamorphism of stromatolitic cherts within the accretionary wedge.

The Ediacaran to early Cambrian Blovice accretionary complex, Bohemian Massif, hosts abundant chert bodies that formed on an oceanic plate and were involved in subduction beneath the northern margin of Gondwana. Field relationships of cherts to their host, their microstructure and elemental as well as isotopic compositions revealed diverse processes of chert petrogenesis reflecting depositional environment and position on the oceanic plate. The deep-water cherts formed through a hydrothermal precipitation of silica-rich gels on outer trench swell of the subducted slab with none or only minor addition of terrigenous material. On the contrary, the shallow-water cherts formed in lagoons on seamount slopes, and at least some of them represent a product of hydrothermal replacement of former carbonate and/or evaporite precursors. For both chert types, the hydrothermal fluids were of low temperature and continuous pervasive hydrothermal alteration of oceanic crust, together with an elevated Si content in Neoproterozoic seawater, served as the major source of silica. On the other hand, minor carbon enrichment in chert is mostly linked to variable incorporation of organic matter that was deposited on the seafloor. Rare earth element (REE) systematics of the cherts indicate predominantly oxygenated environment for the shallow-water cherts whereas the deep-water cherts were deposited in diverse redox conditions, depending on their distance from hydrothermal vent. Using these data, we demonstrate that the cherts once formed a part of Ocean Plate Stratigraphy (OPS) now dismembered and mixed with terrigenous siliciclastic material to form OPS mélanges. Combining our data with those from the existing literature, we show that cherts can serve as significant markers of OPS since the Archean, recording a complex interplay between seafloor-related volcanic (production of MORB- and OIB-like magmas) and sedimentary processes, hydrothermal activity at mid-ocean ridges and seamount chains as well as at outer slopes of subducting slabs. However, the cherts also exhibit a secular change in composition and petrogenesis most profoundly affected by an overturn in seawater silica cycle across the Precambrian–Phanerozoic boundary.

A small and isolated hydrothermal vein-type Sb deposit is hosted by a lamprophyre dike which penetrated Neoproterozoic to early Cambrian volcanosedimentary successions near Chric in the Tepla-Barrandian Unit, Czech Republic. Hydrothermal minerals were formed in several consecutive mineralization stages, separated by long pauses in hydrothermal fluid flow. Early Stage I is characterized by the formation of carbonates of dolomite-ankerite series (Carb I) and arsenopyrite with an imprecise Re-Os age of similar to 570 to 510 Ma. Stage I was followed by tectonic movements and brecciation. The second mineralization stage (Stage II) is characterized by the precipitation of quartz (Qtz I) and Sb minerals (predominantly stibnite) while the third stage (Stage III) is marked by the presence of carbonates of dolomite-ankerite series (Carb II) and quartz (Qtz II) along with Cu, Zn, Pb, Ag and Fe sulfides. Lead isotope data of galena indicate a Variscan age of Stages II-III. These stages were followed by precipitation of another generation of carbonate of dolomite-ankerite series (Carb III, Stage IV), quartz (Qtz III, Stage V) and formation of Sb supergene minerals. The earlier reported occurrence of gold was not confirmed in the studied rock samples but three types of gold particles of variable fineness, with one of them indicating a local origin, were found in the heavy mineral concentrate from a local creek. Elevated Au concentrations were found in arsenopyrite and pyrite. The ore-forming hydrothermal fluid of the mineralization stage III was of H2O-CO2 type with minor CH4 admixture, medium salinity, and temperatures in the range of 258-350 degrees C with an observed temperature decrease from Stage II to Stage V. Oxygen isotope data of Carb II correspond to high delta O-18(fluid) values (similar to+7.5 parts per thousand VSMOW) with a decrease in the fluid delta O-18 to later stages. An important part of the carbonate carbon was derived from decomposition of organic matter or its high-temperature reaction with hydrothermal fluid. Considering the wide variation in delta S-34 values of sulfides and elevated Sb contents in the TBU black shales, we anticipate that the origin of the Chric mineralization is closely associated with a metal and carbon-sulfur remobilization from the TBU volcanosedimentary successions in response to Cadomian and Variscan tectonothermal events.

The Ediacaran to early Cambrian limestones preserved within the Blovice accretionary wedge, Bohemian Massif, are part of Ocean Plate Stratigraphy (OPS) and provide an intriguing example of how paleooceanographic and palaeoenvironmental conditions at active plate margins may be recorded by isotopic composition. Field relations, petrography and geochemical data elucidate the complex interplay of oceanic, diagenetic and volcanic processes. Rare calcite-dominated primary oolitic/pisolitic limestones were deposited on the slopes of seamount volcanoes in shallow-water lagoons and their formation was simultaneous with volcanic activity. In contrast, volumetrically more abundant re-deposited limestones occurring as layers within siliciclastic, graywacke-dominated successions consist of clastic fragments derived from previously formed carbonate ramps, which were disrupted during subduction and subsequently incorporated in deep-water turbidity currents. In both limestone types, the major/trace element as well as C, O and Sr isotope signatures are well preserved and provide a fingerprint of various limestone depositional conditions that can be directly linked with paleooceanic and paleogeographic interpretations. The observed spatial distribution of carbonate-bearing silicate rocks with elemental and isotopic compositions that differ from primary limestones suggest their common origin at active margins and indicate that some of these rocks have been formed by hydrothermal activity. Finally, we demonstrate that the carbonate member of OPS may provide important insights into the polarity and dynamics of subduction zones.

Potassium elemental and isotope systematics were investigated for a suite of central European tektites from three strewn sub-fields in Czech Republic and possible parent sedimentary materials from the vicinity of the Ries impact structure in SE Germany, supplemented by data for several other impact-related materials (bediasites, Ivory Coast tektites, Libyan Desert Glass). This is paralleled by computation of potential K loss and attendant isotope fractionation for physico-chemical conditions typical for formation of tektite precursor melts. These theoretical calculations indicate a <0.1% loss of K from tektite precursor melts up to 2,500 K and <0.002% change in the K-41/K-39 ratio even for a small sphere of 0.002 m at 2,500 K, precluding any significant K loss and isotope fractionation. Numerical modelling also indicates that differential velocities between surrounding gas and liquid are not sufficient to remove the gaseous boundary layer, such that the partial pressure of potassium developed around the molten moldavite beads impedes further evaporation and also contributes to back-condensation of the already evaporated potassium. Central European tektites (moldavites) are enriched in K compared to the assumed sedimentary sources from the wider Ries area whereby the latter materials do not exceed 2.9 wt.% K2O compared to 2.5-4.1 wt.% K2O in moldavites. The apparent K enrichment in moldavites may be explained by a yet unaccounted process during formation of tektite precursor melts and/or unidentified source, such as volcanoclastic deposits that were produced by large Mid-Miocene volcanic centers in the Pannonian Basin. The K isotope compositions of tektites are more variable than those of sediments from the wider Ries area but they largely overlap (delta K-41 from -0.78 +/- 0.03 parts per thousand to -0.13 +/- 0.03 parts per thousand versus -0.72 +/- 0.03 parts per thousand to -0.28 +/- 0.02 parts per thousand, respectively). These ranges mimic K-41/K-39 variations reported for igneous and sedimentary portions of the upper difference among the three investigated strewn sub-fields, depending on their respective distance from the impact. In detail, moldavites from the closest strewn sub-field in the Cheb Basin show predominantly heavy K isotope compositions and those from the farthest strewn sub-field in Western Moravia are uniformly isotopically light. The origin of this difference may reflect lithological heterogeneity of the target area. Potassium contents in bediasites and Ivory Coast tektites range between 1.3 and 1.8 wt.% K2O and their corresponding delta K-41 values vary from -0.57 +/- 0.02 parts per thousand to -0.41 +/- 0.03 parts per thousand. Both ranges are significantly narrower than those observed for moldavites. When compared to data for possible sedimentary precursors in the Chesapeake Bay and Bosumtwi impact structure, respectively, it is apparent that these tektites were neither depleted nor enriched in potassium. The extent of their K isotope fractionation relative to plausible sources remains unconstrained. The Libyan Desert Glass displays invariant delta K-41 of similar to-0.57 +/- 0.06%0 at <= 0.01 wt.% K2O. Given the silica-rich nature of LDG , the lack of possible parent materials, no further constraints can be placed at present to further resolve the source material or reveal details of LDG formation process. (C) 2021 Elsevier Ltd. All rights reserved.

The late Neoproterozoic Lecice black shales and an overlying siliciclastic succession, Bohemian Massif, were deposited in a marine environment on a volcanic arc and provide insights into palaeoenvironmental conditions at the former active margin of northern Gondwana. Field relationships integrated with major/trace element and Mo-Cr-S isotope systematics in two different sections (Stechovice and Brezany) indicate complex depositional settings intimately connected with the waning activity of the underlying volcanic arc. The Stechovice black shales represent a deeper part of the basin and were first deposited in weakly anoxic conditions (lower strati-graphic level) with limited terrigenous supply, as reflected by their higher total organic carbon (TOC) and U contents and positive delta Cr-53, but negative delta S-34(pyrite) values accompanied by late-stage silicification by arc-related fluids. An abrupt change to oxygenated conditions and increased terrigenous flux, characterized by lower TOC, U and Mo and coupled, negative delta Cr-53 and delta Mo-98, is documented in the upper level of the same section. By contrast, the Brezany black shales were deposited in a shallower part of the basin and their composition (e.g., low metal contents, negative delta Cr-53 and delta Mo-98) suggests oxygenated conditions. Furthermore, the extensive syn-to post-depositional silicification by low-temperature hydrothermal fluids was associated with Si and Ba enrichment and elevated bacterial productivity leading to higher organic matter input. The black shale deposition was terminated by arc uplift, which supplied vast amounts of terrigenous material to the basin and produced a thick flysch sequence deposited at oxygenated conditions. The estimated age of the Lecice black shales (similar to 580-560 Ma) suggests that their deposition may be linked to interactions between global eustatic sea level changes in response to the late Neoproterozoic glaciations (Gaskiers, Farquar) and dynamic arc topography.

This contribution introduces a profile of the Czech geologist and Quaternary malacologist doc. RNDr. Jiří Kovanda, CSc., who passed away 12th August 2020. A bibliography of his abundant work (yrs 1951–2019) is also included.

This contribution introduces a profile of the Czech geologist and Quaternary malacologist doc. RNDr. Jiří Kovanda, CSc., who passed away 12th August 2020. A bibliography of his abundant work (yrs 1951–2019) is also included.

The Australasian tektite (AAT) strewn field is the largest strewn field on the Earth with about similar to 10-30% coverage, both land and ocean, but a clearly identified source impact crater is absent despite the young age of AAT of ca. 790 ka. A genetic link between the Australasian tektites and their unequivocal parental materials is therefore largely impossible to establish. Nevertheless, the nature of the parental materials and the extent of volatilization can be constrained using the splash form tektites, carrying the chemical signatures of high-temperature processes, and the layered (so-called Muong Nong-type) tektites, which are less chemically homogenized and exceptionally abundant in the AAT field. New high-precision Sr, Nd and Pb isotopic measurements were obtained for a chemically and petrographically well-characterized suite of AAT, which included the Muong Nong-type (MN-AAT) with precisely known field locations in Laos and splash forms (SF-AAT) from different parts of the strewn field. In addition, optically dark and light zones of the MN-AAT were also separately analyzed. Homogeneous epsilon(Nd) values from -11.8 to -11.2, combined with a narrow range of two-stage Nd model ages from 1.67 to 1.72 Ga for the entire AAT suite, point to a well-mixed source, in terms of REE, of the crustal segment from which the sedimentary material for tektites was ultimately derived. The Sr isotopic data largely overlap for SF-AAT and MN-AAT (Sr-87/Sr-86 = 0.71636-0.72021) and indicate Paleozoic to Mesozoic sedimentary parentage. However, late Neogene to early Quaternary re-deposition and formation of a thick silt-sized sedimentary section with vertical stratification is required to comply with Be-10 data. Lead isotope systematics documents at least three different components which can perhaps be represented by different mineral phases, such as feldspar, zircon, organic matter adsorbed on young sediments etc., sorted during fluvial transport and final deposition. In addition, the SF-AAT have systematically lower Pb contents than the MN-AAT, and generally show isotopically heavier Pb isotopic ratios. This is theoretically consistent with a preferential volatilization of lighter Pb isotopes during evaporation and considerably larger Pb loss from SF-AAT when compared to MN-AAT. Nevertheless, further experimental work would be necessary to unambiguously distinguish kinetic fractionation from source mixing. (C) 2020 Elsevier Ltd. All rights reserved.

12 Australasian Muong Nong-type tektites from the locality centered at 16.46150° N, 106.48917° E 13 in Laos contain sporadic spherical heterogeneous sulfide inclusions less than 10 μm in diameter, 14 which have been identified by electron probe microanalyzer and electron back scatter diffraction 15 to represent a mixture of rare mineral shenzhuangite with a pyrrhotite polytype (possibly troilite). 16 Contrary to type shenzhuangite found in the shocked L6 chondrite Suizhou the mineral 17 embedded in the tektite glass is nearly free of copper providing the composition close to its 18 expected end-member not yet found in the nature; the empirical formula closest to ideal end19 member composition based on 4 atoms per formula unit is Ni1.007Fe0.998Cu0.016Co0.058S1.922. The 20 described occurrence also represents the first find of shenzhuangite in terrestrial material. 21

Little is known about water mixing in deep underwater cave shafts of hypogene karst. The Hranice Abyss (HA) in Czechia is currently the deepest underwater cave in the world. It shares a thermal and CO2-rich water source with an adjacent spa. Based on chemical and isotope composition, water in the HA is a mixture of shallow and thermal groundwaters. The shallow local groundwater is distinctly different from the adjacent Bečva River water in its elemental chemistry and sulfate δ34S values. The thermal water is mixed with 5–10% of modern water, based on tritium content and chlorofluorocarbons. Vertical profiling and deep sampling in the HA showed distinct changes with depth in temperature and TDS. Density-driven flow controls the mixing. In winter, the shallow water of the open HA lake is efficiently cooled; the denser surface water sinks to greater depths, which mixes the water column in the HA. During the summer the shallow water stagnates at the depth of 0–15 m. Periods of stagnation and of accelerated water flow and mixing in the HA perfectly fit with the periodic occurrence of CO2 evasion in the lake and the overall characteristics of the microbial communities, which showed the absence of any functional stratification. Ferric oxyhydroxide precipitation is the major cause for turbidity in the HA. Elevation-specific hydraulic responses of the HA groundwater, caused by the adjacent river’s level pulses, enabled a determination of the points along the river course at which the river is connected to groundwater by karst conduits.

Major and trace element analyses and triple oxygen isotope measurements were performed on 11 individual specimens of Australasian tektites ( AAT ) with exactly known field positions from Laos. The sample set was dominated by Muong Nong‐type tektites ( MNAAT ), including separated layers of glass of different appearance and chemistry from four samples. This first larger set of oxygen isotope data of MNAAT revealed the δ 18 O range 8.7 ≤ δ 18 O ≤ 11.6‰ on VSMOW 2 scale (12 analyses), only slightly wider than the previously reported range for splash‐form AAT . The Δ’ 17 O values of MNAAT (−0.098 ≤ Δ’ 17 O ≤ −0.069‰; 12 analyses) and splash‐form AAT (−0.080 ≤ Δ’ 17 O ≤ −0.068‰; three analyses) are all in the range of data typical for terrestrial crustal rocks, with no mass‐independent oxygen isotope fractionation (from impactor or from exchange with atmospheric O 2 ) being observed.

The Kašperské Hory deposit represents one of the economically most important hydrothermal gold deposits in the Bohemian Massif (Czech Republic). We present new Re–Os age determinations for sulfides complemented by Ar–Ar determinations on silicate minerals in order to provide temporal constraints on the deposit formation. Arsenopyrite and molybdenite formed during gold-bearing stages I–III yielded similar Re–Os ages between ~ 341 and 333 Ma, overlapping with the Ar–Ar cooling age of the metamorphic host rocks below ~ 500 °C. Continuous cooling of the ore system below 270 °C during stage III is reflected by Ar–Ar muscovite ages spanning from ~ 332 to 325 Ma representing the long interval of formation of gold-productive stage III. Thus, the Kašperské Hory gold deposit is younger than the gold deposits spatially associated with the Central Bohemian Plutonic Complex (~ 349–339 Ma) belonging to the Central Bohemian Metallogenetic Zone (e.g., Mokrsko-West, Jílové, Petráčkova Hora). This may indicate a rather close relationship to the nearby Moldanubian Batholith while still suggesting an intimate link between mineralization and Variscan orogenesis and associated magmatism.

Australasian Muong Nong-type tektites from the locality centered at 16.46150 degrees N, 106.48917 degrees E in Laos contain sporadic spherical heterogeneous sulfide inclusions less than 10 mu m in diameter, which have been identified by electron probe microanalyzer and electron backscatter diffraction to represent a mixture of rare mineral shenzhuangite with a pyrrhotite polytype (possibly troilite). Contrary to type shenzhuangite found in the shocked L6 chondrite Suizhou, the mineral embedded in the tektite glass is nearly free of copper, making the composition close to its expected end-member that has not yet been found in nature; the empirical formula closest to ideal end-member composition based on four atoms per formula unit is Ni1.007Fe0.998Cu0.016Co0.058S1.922. The described occurrence also represents the first find of shenzhuangite in terrestrial material. Presented is the Raman spectrum for shenzhuangite with the tentative assignment of spectral bands based on the analogy with synthetic chalcopyrite-structured phases. The chemical composition of shenzhuangite close to NiFeS2 is not consistent with any stable phase in the Fe-Ni-S system up to melting temperature. Available data so far on phase relations in this system do not allow unambiguous interpretation of conditions under which the sulfide association within inclusions had formed.

Extremely low and variable concentrations of osmium (Os) and other highly siderophile elements (HSE) in most tektites make it challenging to establish direct links between these impact-related materials and their possible extraterrestrial contribution. New Os concentrations (2-43 ppt) and Os-187/Os-188 ratios (0.131-0.68) in a suite of fifteen well-characterized Australasian tektites from Laos (Muong Nong and splash-form types) with variable Ni enrichment indicate a maximum of similar to 0.005% addition of a chondritic impactor. This is similar to some Australasian tektites from Vietnam with similarly low siderophile contents, but significantly lower than found in previous studies of more Ni-rich Australasian splash-form tektites and microtektites from different parts of the Australasian strewn field (e.g., Indonesia, South China Sea). The contents of HSE and Re-Os isotopic compositions of layered Muong Nong-type Australasian tektites are highly variable, suggesting mingling of crustal-derived (siderophile element-poor) and extraterrestrial (siderophile element-rich) materials. The absence of a direct correlation between HSE and Ni contents is interpreted to result from a fractionation process related to their different vaporization/condensation temperatures. The low Os abundance in most of the analyzed Australasian tektites, combined with non-radiogenic Os-187/Os-188 far below average upper continental crust, may provide a direct test to distinguish continental versus seawater impact scenario. In the absence of any specific low-Os target, a particular process of Os loss following impact is required. We envisage a scenario where evaporative loss of >>90% Os in the form of Os oxides from the overheated tektite melt is aided by volatile species derived from dissociated seawater and/or saline pore water embedded in sediments off-shore Indochina, consistent with elevated contents of halogens in Australasian tektites. This water-assisted Os loss could also play significant role for Central European tektites, while the continental surface with limited amount of water would prevent from more efficient HSE loss as could be the case for Ivory Coast tektites. (C) 2019 Elsevier Ltd. All rights reserved.