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The Niederschlag fluorite-barite vein deposit in the Western Erzgebirge, Germany, has been actively mined since 2013. We present the results of a first comprehensive study of the mineralogy, petrography, fluid inclusions, and trace element geochemistry of fluorite related to the Niederschlag deposit. Two different stages of fluorite mineralization are recognized. Stage I fluorite is older, fine-grained, associated with quartz, and forms complex breccia and replacement textures. Conversely, the younger Stage II fluorite is accompanied by barite and often occurs as banded and coarse crystalline open-space infill. Fluid inclusion and REY systematics are distinctly different for these two fluorite stages. Fluid inclusions in fluorite I reveal the presence of a low to medium saline (7–20% eq. w (NaCl+CaCl2)) fluid with homogenization temperatures of 140–180 °C, whereas fluorite II inclusions yield distinctly lower (80–120 °C) homogenization temperatures with at least two high salinity fluids involved (18–27% eq. w (NaCl+CaCl2)). In the absence of geochronological data, the genesis of the earlier generation of fluorite-quartz mineralization remains enigmatic but is tentatively related to Permian magmatism in the Erzgebirge. The younger fluorite-barite mineralization, on the other hand, has similarities to many fluorite-barite-Pb-Zn-Cu vein deposits in Europe that are widely accepted to be related to the Mesozoic opening of the northern Atlantic Ocean. [ABSTRACT FROM AUTHOR]

Abstract: A world-class fluorite mining district was located NNE of Regensburg at Nabburg-Wölsendorf, Germany. Together with some smaller mining sites near Bach a.d. Donau, Germany, it provided the main part of the domestic fluorite supply until the late 1980s when the last mines were shut down in NE Bavaria. Both fluorite mining districts are separated from each other by the Bodenwöhr Halbgraben and their mineral assemblages are controlled by deep-seated fault zones. The NW–SE-running “Pfahl”, also called the “Great Bavarian Quartz Lode” (=quartz dyke along a deep-seated shear zone), is of structural control for the vein-type U-, Pb-, Zn-, Cu-, Bi, -Co-, Ni-, As-, and Se mineralization in the Nabburg-Wölsendorf District. Another prominent fault structure, the NNW-trending boundary fault of the NE Bavarian Basement, controls vein-type fluorite deposits with minor Cu-, Zn- and Pb sulfides in the Donaustauf District. Concluding from the REE distribution, primary fluorite mineralization was sourced from granitic and gneissic aquifers or intrastratal solutions, giving rise in both fluorite mining districts to similar mineral assemblages. Two fluorite mineralizations can be distinguished from each other in both mining districts. The primary fluorite mineralization evolved in the aftermaths of the granite intrusion, during the Late Carboniferous and the early Permian, whereas the secondary mineralization or remobilization of fluorite took place until post-Middle Jurassic times. Remobilization, related to the paleogeography of each mineral district, was more intensive in the northern district, because of a moderate erosion. In the southern district, erosion cut deeply into the fluorite mineralization and left behind only relics of minerals indicative of fluorite remobilization. The physical–chemical regime of the fluorite mineralization was interpreted, making use of the common phase diagrams which show Eh and pH along the x- and y axes. The temperature of formation was as high as 200°C in both districts. The fluid regime became more acidic and more oxidizing from the north toward the south. This physical–chemical change well accords with the paleogeographic situation when the fluorite mineralization took place during the Permo-Mesozoic and with the supergene alteration during the Miocene and Pliocene when the southern part of the basement was undergoing stronger uplift than the northern part. Gangue minerals, e.g., ferroan carbonate and sulfate minerals are proximity indicators marking the distance of the vein-mineralization relative to the horizontal reference plain and implying a depth zonation (unconformity), whereas minerals and elements originating from the hypogene alteration are proximity indicators relative to the vertical reference plains (veins related to deep-seated lineamentary fault zones). [Copyright &y& Elsevier]

Abstract: Fluorite vein-type deposits occur along the western edge of the Bohemian Massif in SE Germany, from Bayerischer Wald to Frankenwald. They are emplaced in various host rock lithologies, as old as Late Variscan granites and low-grade metasediments of late Paleozoic age. The Nabburg-Wölsendorf mining district in NE Bavaria, Germany, used to be the largest in Germany and one of the most important in the world. A minor proportion of fluorite is also concentrated in the Permo-Mesozoic siliciclastic sediments resting unconformably upon the basement rocks near Pingarten. Fluorite is investigated in the present study for its REE variation, the REE minerals associated with it and its Sm/Nd isotope ratios. The primary blue-black fluorites precipitated from aqueous hydrothermal solutions during early stages without noticeable interaction with wall rocks and were poor in REE and Y. The secondary green, white and yellow fluorites formed from remobilization of older fluorite and through the interaction of more viscous fluids with the wall rocks and they are abundant in REE plus Y. Monazite–(Ce) is the major source for the REE re-deposited into fluorite. In a double cross plot, designed during this study and using the Ce-, Eu anomalies and the skewness (fractionation) of the REE pattern, the geological and hydrological evolution of fluorite vein mineralizations are related to the granite and the unconformity, which is the hydraulic reference plane for the fluorite mineralization. First, blue fluorite formed at considerable depth below the unconformity. Fetid fluorite is exclusive to the deep-seated vein mineralization with abundant uranium minerals. These uranium minerals caused some radiation damage to the lattice of the blue fluorite and turned into black. Based upon the variation of REE in fluorite and the Sm/Nd dating (fluorite age: ca. 270Ma) the Permian paleogeography can be reconstructed. It is suggested that in the Nabburg-Wölsendorf District a NW–SE oriented swell stood out from a vast peneplain. Second, fluorite vein mineralization composed of green, white and yellow fluorite along the western edge of the Bohemian Massif resulted from remobilization associated with an increase in REE and Y relative to the blue and black fluorites and a strong fractionation of LREE vs. HREE. The REE signature of the latter fluorites got increasingly more “unconformity-related” (Eh>0), notwithstanding the fluid source which the mineralizing fluids originated from. The REE patterns, Sr- and Sm/Nd isotopes suggest that mineralizing fluids mainly derived from a granitic source rock with little impact from metasedimentary rocks. Cerite–(Ce) formed during the initial stages of fluorite mineralization when little REE were accommodated in the lattice of blue fluorite. During precipitation of green, white and yellow fluorite, REE minerals failed to continue developing on their own, because REE were increasingly accommodated in the structure of fluorite changing its color from blue via green into white. REE minerals were again formed during supergene alteration of fluorite mineral assemblages, terminating the entire F-REE cycle. Rhabdophane is of supergene origin and indicative of meteoric conditions that are neutral to slightly alkaline. Subsequently, florencite evolved under a more acidic regime of chemical weathering. The distinct data arrays in the cross plots of the Sm-, Nd- and Sr isotopes, likewise, indicate this subdivision into fluorite originating from fluids that interacted with granitic and gneissic rocks at depth and those being formed proximal and distal to the unconformity. [Copyright &y& Elsevier]

Abstract: The Nabburg–Wölsendorf vein-type fluorites in SE Germany which are characterized by a pronounced timebound variation in mineral color (blue→black→green→white/colorless→yellow→brown) were investigated for their accessory minerals included in or associated with these different fluorite types. Nabburg–Wölsendorf is not a unidirectional fluorite mineralization but a polyphase mineralizing process with several repetitions of the various color types during the vein mineralization event. Accessory minerals of the fluorite mineralization include silicates, sulfates, oxide/hydroxides, phosphates, sulfides and carbonates. These minerals are intimately intergrown with fluorite in the vein-type mineralization and are efficient tools to broaden knowledge on the depositional environment of fluorite that is otherwise poor in marker minerals for redox conditions, temperature, alkalinity/acidity and fluid viscosity. Pyrite, galena and cinnabar, are indicative of hypogene processes, while uranyl phosphates and APS minerals furnish evidence for supergene mineralizing processes. Some minor constituents may be used as marker minerals of redox conditions (hematite, galena, and goethite) or held to be characteristic for the changing pH regime (kaolinite and smectite). Oxidizing conditions prevailed during fluorite mineralization, excluding some phases when fetid and white fluorite precipitation ran through a maximum. Yellow-brown honey spar marks the oxidizing physical–chemical conditions by its brown mineral color at the end of fluorite mineralization. Oxidizing conditions prevailed also during supergene mineralization. The pH of the mineralizing fluids of the hypogene stages was greater than pH 7. During the supergene stage, Eh fluctuated, but was mainly >0; the pH of fluids ranged between 7 and 4. The black color of fetid fluorite and its pungent odor resulted from the U present primarily in its tetravalent state. Incomplete haloes recognized in some fetid fluorites reflect “frozen” uranium disequilibria and an early phase of supergene alteration. In this respect, the fetid fluorite (“Stinkspat”) plays an important part and held to be a marker for the radioactive “hot spot” as well as “ghost” uranium mineralization. Fetid fluorite is defined as to its mineral color, morphology and characteristic spectral features so as to allow for a precise distinction from other dark blue or green fluorite varieties by its outward appearance and easy-to-handle methods. The accessory minerals under study are indicative of the depocentre of the fluids which lies close to an unconformity but does not, however, deliver information on the source of the mineralizing fluids. The nearness to the potential source rock of the fluids can only concluded from the presence of cerite-(Ce) which formed prior to fluorite. This REE silicate is cast as a marker of the root zone of the fluorite veins which fade out into late Variscan granites. Cerium is assumed to have been derived from monazite an accessory mineral largely found outside the fluorite veins, proper, in the late Variscan granites. In the waning stages of fluorite mineralization different types of REE-bearing phosphates, belonging to the rhabdophane solid solution series, and some APS minerals (aluminum–phosphate–sulfate solid solution series) evolved. [Copyright &y& Elsevier]

Abstract: REE patterns of fluorite from 63 post-Variscan hydrothermal veins of the Schwarzwald area, Germany, can be divided into two major groups. Type A fluorite shows roof-shaped PAAS normalized patterns with enriched MREE, the turning point of the curvature is at Ho or Er position. All patterns of type A show a distinct positive Eu anomaly. Type B fluorite shows roof-shaped patterns with enriched MREE and strongly depleted HREE and a characteristic, steep slope between Nd and Sm. The turning point of the curvature is at Gd to Dy, all patterns of type B show negative or no Eu anomalies. Laser ablation-ICP-MS traverses through single grains combined with cathodoluminescence imagery revealed either (i) identical REE patterns and total REE concentrations from core to rim, (ii) identical REE patterns, but concentration gradients or (iii) variable REE patterns and concentration gradients from core to rim. These features are explained by different degrees of supersaturation or different fluid/fluorite ratios during crystallization and by complexly controlled precipitation processes under closed system conditions. Petrographically late fluorites from several deposits show the characteristics of new fluid batches rather than the expected remobilization features. LREE enrichment close to the crystal faces in some grains may result from the copreciptiation of a submicrometer-sized LREE-enriched phase or from LREE clustering in late, cool fluids. By comparison with literature data on natural waters from gneissic and granitic aquifers and of leachates of granites and gneisses, we show that the hydrothermal vein fluorite preserves the characteristic, source-related REE feature of the hydrothermal fluid that migrated through either gneissic (type A) or granitic (type B) host rocks. The immediate host rocks of the veins, however, are not the source rocks of the REE, which indicates large-scale fluid convection in the basement. Gneiss-related type A fluorite shows Tb/Ca–Tb/La trends of primary hydrothermal crystallization whereas granite-related type B fluorite indicates remobilization of possibly pegmatitic or Variscan fluorite. [Copyright &y& Elsevier]

The main ore stage of three similar unconformity-related vein systems in the Schwarzwald, SW Germany spanning a period of activity of ~150 Ma, was investigated to understand the details of fluid penetration from overlying sediments/marine environment into the basement, their evolution as well as the processes involved in vein formation. To investigate temporal and spatial variations of the hydrothermal fluids responsible for mineralization, over 1650 fluid inclusions were analyzed by microthermometry. Of these, a total of 108 fluid inclusions (mainly in fluorite) were successfully analyzed by LA-ICP-MS. The fluid inclusions reveal a binary mixing trend between a CaCl 2 - and a NaCl-rich endmember. Independent of major element composition, the fluids are metal-bearing (e.g., up to ~100 mg/kg Ba, Pb, Zn, Ni and up to 10 mg/kg Ag), show high As (up to 1000 mg/kg) and low S (below the detection limit in most analyses). Over time, the mixed fluid shows a gradual decrease in CaCl 2 and increase in NaCl with slightly decreasing total salinity. Based on earlier studies and geochemical arguments, the veins formed by anisothermal binary fluid mixing of two fluids, which both were originally derived from seawater and chemically modified through interaction with the basement and sedimentary rocks in different ways. This produced a gradual stratified basement fluid reservoir comprising a modified bittern/halite dissolution brine. The fluids involved in the vein formation are sourced from different depths of this modified bittern/halite dissolution basement brine reservoir: fluid A, a CaCl 2 -dominated, KCl-poor, deeper seated brine with a salinity of ~25 wt% CaCl 2 + NaCl, and fluid B, an NaCl-dominated and KCl-richer brine situated at shallower depths in the crystalline basement with salinities of ~22 wt% NaCl+CaCl 2. Based on the Na-Ca-K and Na K thermometers and on Rb/Cs systematics, fluid A records alteration of the Na-, K- and Ca-bearing feldspars of the host rocks; progressive alteration led to consumption of mainly Ca-rich plagioclase in contact with these basement brines. Accordingly, fluid B that subsequently entered the basement was only in equilibrium with alkali feldspars and clay minerals. This scenario produced a gradual change of fluid composition with depth that was pushed to greater depth over time. The source temperatures are estimated to ~250 °C while vein formation occurred at 100–170 °C, based on fluid inclusion homogenization temperatures. Thus, significant fluid cooling without abundant fluid mixing (and without major mineralization) must have occurred during fluid ascent (3–7 km, depending on the assumed geothermal gradient). Fluid mixing then resulted in the formation of the major gangue minerals fluorite, quartz, barite and calcite, while the locally confined sulfides must have formed due to an influx of sulfide into the binary mixed fluid. The process of fluid mixing is a rapid and turbulent process. This is recorded by a great diversity of fluid compositions (including ones close to the mixing endmembers) trapped within the same crystal. Compositional variations are even visible within individual fluid inclusion trails. [Display omitted] • Major and trace element analysis by LA-ICP-MS on fluid inclusions in fluorite. • ~150 Ma spanning evolution of a basement brine reservoir from CaCl 2 -rich to NaCl-dominated. • Basement fluid stratification of shallow NaCl-dominated and deeper CaCl 2 -rich fluids. • Spatial and temporal fluid evolution linked to aquifer, Ca-rich plagioclase, alteration. • Near endmember fluid composition constrained by variability in fluid mixing ratio. [ABSTRACT FROM AUTHOR]