Search

The highly fractured Jurassic carbonates at Jbel Tirremi in northeastern Morocco host fluorite-baryte deposit. The mineralization occurs both as stratabound in marl-limestone contact and as fault-hosted veins (N-S- and NNW-SSE). The mineral paragenesis consists of two fluorite and baryte generations and calcite with subordinate amounts of quartz, dolomite, traces of sulfides (chalcopyrite, pyrite, galena), and oxidized minerals. Fluid inclusion data reveal that hot moderately saline fluids derived from the Paleozoic basement mixed with Triassic brines and cooler, meteoric waters. The REY inventory, C-O-S-Pb-Sr-Nd isotope, and crush-leach data point to the Paleozoic basement as the primary source of metals with a contribution from the Triassic red beds. The refined ore genetic model developed in this study from our new geological and geochemical data includes the downward movement of Triassic-Jurassic evaporated seawaters along normal faults, followed by the leaching of metals from Paleozoic and Triassic rocks, and the subsequent upward flow of these metalliferous fluids. During the Late Cretaceous-Paleocene basin inversion, the deep-seated ore-forming fluids migrated upward, which eventually mixed with Triassic brines and cooler meteoric waters. This fluid mixing caused the precipitation of multiple generations of fluorite and baryte. • The Triassic-Jurassic evaporated seawater infiltrated into the Paleozoic basement. • During Cretaceous-Paleocene time, the emergence of land allowed the ascension of the ore-bearing fluids. • The metalliferous fluids mixed with shallow fluids, causing fluorite-baryte precipitation. • Barium and fluorine originated mainly from the Paleozoic basement. [ABSTRACT FROM AUTHOR]

Unusual hydrothermal native As-sulfide ± native Ag ± arsenide ± antimonide ± sulfosalt ore shoots and their co-genetic sulfide-fluorite-barite-quartz host veins, which are common in the region and in whole Central Europe, were investigated at three localities in the Schwarzwald, SW Germany, to understand the physico-chemical processes governing the change from a normal (= common) hydrothermal to an exceptional ore shoot regime. Based on fluid inclusions, the formation of the gangue minerals is the result of binary mixing between a NaCl-rich brine and a CaCl2-rich brine (both ~ 20 wt% NaCl aq.). This mixing correlation, major and minor fluid composition, formation temperature (~ 150 °C), and δ34S signature are identical (within error) in ore shoots and host veins. Thermodynamic modeling indicates that ore shoot formation must have resulted from a change in redox conditions by a local influx of a volumetrically minor reducing agent, probably hydrocarbons. The elemental content and the mineralogy of each ore shoot locality (Ag-As-rich: Münstertal; Ag–Ni-As-rich: Urberg; Ag–Ni-As-Sb-rich: Wieden) reflect the metal content of the binary mixed fluid, while mineral textures, successions, and assemblages are thermodynamically and, regarding sulfur, kinetically controlled. The formation of vein and ore shoot sulfides requires an addition of sulfide, most probably from the sulfide-bearing host rocks, because thermodynamic and kinetic reasons suggest that the two major vein-forming and metal-bearing fluids are not the source of the sulfur. The final ore shoot textures are influenced by later hydrothermal remobilization processes of As and Ag. This results in a number of sulfosalts, mostly proustite-pyrargyrite. Interestingly, the greater thermodynamic stability of Sb-endmember sulfosalts enables them to form even in As-dominated fluid systems. [ABSTRACT FROM AUTHOR]

This work evaluates an exceptionally complex natural laboratory, the Middle Triassic Latemar isolated platform in the northern Italian Dolomite Mountains and explores spatial and temporal gradients in processes and products related to contact metamorphism, dolomitization and dedolomitization of marine limestones. The relation between petrographic change and re‐equilibration of geochemical proxy data is evaluated from the perspective of carbonate‐archive research. Hydrothermal dolomitization of the limestone units is triggered by dykes and associated hydrothermal fluids radiating from the nearby Predazzo Intrusion. Detailed petrography, fluid inclusion analysis, δ13C and δ18O data and 87Sr/86Sr isotope ratios shed light on the extreme textural and geochemical complexity. Metamorphic and diagenetic patterns include: (i) peak‐metamorphic and retrograde‐metamorphic phases including three dolomite marbles, two dedolomite marbles, brucite, magnesium silicates and late‐stage meteoric/vadose cement at the contact aureole; (ii) four spatially defined episodes of dolomitization, authigenic quartz, low magnesium calcite and late‐stage meteoric cement at the Latemar isolated platform; and (iii) kilometre‐scale gradients in δ13C values from the contact aureole towards the platform interior. Results shown here are relevant for two reasons: first, the spatial analysis of alteration products ranging from high‐grade metamorphic overprint of marbles at temperatures of 700°C in the contact aureole to moderately altered limestones in the platform interior at temperatures <100°C, allows the observation of processes that commonly occur along vertical (prograde) gradients from shallow burial to metamorphism at depths >20 km. Second, under rock‐buffered conditions, and irrespective of metamorphic to diagenetic fluid−rock interactions, both marbles, and low‐temperature hydrothermal dolomites have conservative marine δ13C and δ18O values. The fact that metamorphism and hydrothermal dolomitization of precursor limestones and early diagenetic dolostones did not per se reset environmental proxy data is of interest for those concerned with carbonate archive research in Earth's deep time. [ABSTRACT FROM AUTHOR]

The crush-leach technique is a frequently used method to determine the bulk composition of fluid inclusions trapped in a range of geological samples. We present a modified crush-leach technique combining ion chromatography (IC) and total reflection X-ray fluorescence spectroscopy (TXRF) which allows to determine a range of major, minor and trace elements out of one leachate. To date, trace element detection by means of TXRF is barely used in geosciences, although it combines the advantages of low to very low detection limits (µg/L to ng/L range), small sample amount needed (µL-range) and a fast and inexpensive analytical procedure. Previously described problems of adsorption of polyvalent cations at sample surfaces have been overcome by using acidified water as a leachate. Instead, it has been demonstrated that, for example, the syringe filter type used for IC measurements influences contamination and/or adsorption for a number of elements. The proposed method combination was evaluated for accuracy, reproducibility and system blanks and subsequently applied to quartz samples from hydrothermal vein deposits of the Schwarzwald ore district, SW Germany. [ABSTRACT FROM AUTHOR]

• Fluid inclusion data from carbonatite. • A new approach for quantification. • Exsolution of (Na,K)Cl + (Na,K) 2 CO 3 + (Na,K) 2 SO 4 fluids with salinities up ∼80 wt.% • Fluid boiling as trigger for apatite crystalization. Studies of fluid inclusions in carbonatitic rocks are essential for understanding physicochemical processes involved in carbonatite-related hydrothermal ore mineralization and fenitization. However, the composition of many carbonatite-derived fluids is challenging to quantify, which hampers their detailed interpretation. Here, we present a systematic study of microthermometry of fluid inclusions found in carbonatites from the Kaiserstuhl (SW Germany), and a simple numerical model to estimate the compositions of such fluids, which are typical of numerous carbonatites worldwide. Four types of fluid inclusions have been identified in the Kaiserstuhl carbonatites: (I) vapor-poor H 2 O-NaCl fluids with <50 wt.% salinity; (II) vapor-rich H 2 O-NaCl-CO 2 fluids with <5 wt.% salinity; (III) multi-component fluids with high salinity and high CO 2 contents; and (IV) multi-component fluids with high salinity but little to no CO 2. At present, it is only possible to quantify fluid compositions for types I and II. For the complex types III and IV, we conducted predictive modeling of the liquidus surface based on the Margules equations. The results suggest that carbonatite melts predominantly exsolve Na-K-sulfate-carbonate/bicarbonate-chloride brines (types III or IV). Such fluid inclusions may represent immiscible fluids that were trapped after segregation by boiling from a parental highly saline brine (type I). Fluid boiling, in turn, was probably triggered by a rapid pressure release during melt ascent. The present model enables quantification of fluid compositions associated with carbonatitic magmatism. [ABSTRACT FROM AUTHOR]

Early marine diagenetic dolomite is a rather thermodynamically‐stable carbonate phase and has potential to act as an archive of marine porewater properties. However, the variety of early to late diagenetic dolomite phases that can coexist within a single sample can result in extensive complexity. Here, the archive potential of early marine dolomites exposed to extreme post‐depositional processes is tested using various types of analyses, including: petrography, fluid inclusion data, stable δ13C and δ18O isotopes, 87Sr/86Sr ratios, and U‐Pb age dating of various dolomite phases. In this example, a Triassic carbonate platform was dissected and overprinted (diagenetic temperatures of 50 to 430°C) in a strike‐slip zone in Southern Spain. Eight episodes of dolomitization, a dolostone cataclasite and late stage meteoric/vadose cementation were recognized. The following processes were found to be diagenetically relevant: (i) protolith deposition and fabric‐preservation, and marine dolomitization of precursor aragonite and calcite during the Middle–Late Triassic; (ii) intermediate burial and formation of zebra saddle dolomite and precipitation of various dolomite cements in a Proto‐Atlantic opening stress regime (T ca 250°C) during the Early–Middle Jurassic; (iii) dolomite cement precipitation during early Alpine tectonism, rapid burial to ca 15 km, and high‐grade anchizone overprint during Alpine tectonic evolution in the Early Eocene to Early Miocene; (iv) brecciation of dolostones to cataclasite during the onset of the Carboneras Fault Zone activity during the Middle Miocene; and (v) late‐stage regression and subsequent meteoric overprint. Data shown here document that, under favourable conditions, early diagenetic marine dolomites and their archive data may resist petrographic and geochemical resetting over time intervals of 108 or more years. Evidence for this preservation includes preserved Late Triassic seawater δ13CDIC values and primary fluid inclusion data. Data also indicate that oversimplified statements based on bulk data from other petrographically‐complex dolomite archives must be considered with caution. [ABSTRACT FROM AUTHOR]

The Garub fluorite deposit in south Namibia is hosted by a Neoproterozoic tuffisite diatreme which is cut by a hydrothermal vein with base metal mineralization. Petrographic observations indicate a single hydrothermal event producing a paragenesis of fluorite-baryte with base metal sulfides, quartz and carbonates. Microthermometry data from fluid inclusions show that hydrothermal fluorite and baryte were precipitated from a high-salinity (16.9–22.7 wt% NaCl + CaCl 2), moderate-temperature brine (180–210 °C). This brine formed by mixing between a warmer, Ba- and F-rich endmember likely representing a deep-seated basement brine (chemically equilibrated with Namaqua-Natal-Metamorphic Complex basement rocks) and a cooler, SO 4 - and Ca-rich endmember that likely equilibrated with Nama Group limestone. Chlorine/Br mass ratios between 152 and 612 indicate the mixing of a fluid with a halite dissolution brine signature (endmember 1) and an equally saline brine derived from Nama Group limestone (endmember 2). Rubidium/Cesium (Rb/Cs) ratios between 2.2 and 20.5 provide evidence for significant fluid interaction with clay minerals. Since clay minerals are abundant in the tuffisite of the Garub diatreme, these Rb/Cs ratios indicate that fluids migrated along the diatreme-gneiss contact along a zone of weakness. Late-stage quartz and calcite likely reflect cooling and induced a shutdown of the hydrothermal system. The sulfide phases in this hydrothermal vein are strongly depleted in trace elements compared to other hydrothermal vein districts, which is interpreted as an inherited signature of the fluid source. There is no record of evaporites in the sedimentary rocks of the Nama Group. Based on the fluid chemistry of the mixed fluid, which clearly indicates halite dissolution and water-rock interaction with basement rocks, it is concluded that the basement brine fluid endmember is likely generated through cryogenic brine formation during the large scale Dwyka glaciation event. As a farfield consequence of Pangea breakup fluid pathways were established that acted as host structures for the Garub fluorite deposit. This could be the first evidence for unconformity-related hydrothermal fluorite deposit formation in the context of Dwyka cryogenic brine formation in southern Africa. • Ore formation in context of cryogenic brine development. • A new unconformity related hydrothermal fluorite mineral system in S-Namibia. • Fluid inclusion analyses and crush leach data. • Fluorite formation as a consequence of fluid mixing. [ABSTRACT FROM AUTHOR]

Abstract Six mineralogically exemplary barren and mineralized hydrothermal veins (with Pb and Zn ores) of Jurassic-Cretaceous and Cenozoic age in the Schwarzwald mining district, SW Germany were chosen to shed light on the origin of their mineralogical diversity. The selection of the veins was guided by the fact that they represent the largest number of veins in the region, are very well known mineralogically and geochemically, and they provide nice examples of barren and mineralized veins of similar age. Fluid inclusion data from the individual veins overlap implying their diverse mineralogy is not caused by different fluid compositions participating during fluid mixing. LA-ICPMS data of single fluid inclusions indicate no systematic variations in major elements, but significant changes in fluid mixing ratios which caused variable trace element concentrations of different fluid inclusion assemblages in one sample. We suggest that different ore-gangue-assemblages can be produced by mixing of identical fluid endmembers, but variable mixing ratios. LA-ICPMS analyses of single fluid inclusions in barren and mineralized veins record similar base metal and sulfur concentrations. Hence, sulfide mineralization in the veins appears not to be controlled by metal solubility. Thermodynamic modeling based on the fluid data indicates that the presence of a reducing phase during fluid mixing is required for sulfide mineralization to precipitate. LA-ICPMS trace element analyses of feldspars, biotites, chlorites and clay minerals in granites and paragneisses which are the source of the metal content in the ore-forming fluids demonstrate that the dominant provenance for Pb, Zn, As, Sb, Ba, Tl, Mo, Fe and Mn are probably paragneisses, while Co, Cu and Ni are probably sourced from S-type granite. A rough quantification indicates that <1 m3 paragneiss or granite has to be altered (10% alteration) to supply sufficient Zn to 1 l of hydrothermal fluid reaching a concentration of 2 g/l Zn. Hence, this study confirms that ore fluids can be produced from any lithology in the upper crust that contains at least some trace metals (μg/g level is sufficient). Highlights • LA-ICPMS trace element data on host rock minerals • LA-ICPMS trace element data on single fluid inclusions • Comparison barren and mineralized veins • Host rock redox potential as key for sulfide mineralization • Mass balance calculation for basement leaching [ABSTRACT FROM AUTHOR]

Abstract Understanding the physical basics of tectonic events, related fluid flow and ore deposition represents one of the great challenges in modern geosciences. In this contribution, Sm-Nd ages of hydrothermal fluorites and U-Pb ages of carbonates and iron oxides from unconformity-related vein type deposits in the Schwarzwald next to the Upper Rhinegraben rift in SW Germany are used to distinguish different pulses of hydrothermal fluid activity and to understand their relation to large-scale tectonics and magmatism. While the fluorites are of Early Jurassic to Oligocene age, carbonate (calcite, dolomite, siderite) and hematite dated by U-Pb small-scale isochrons records formation from Permian to Quaternary with a clear culmination in the Neogene. These new age-data in combination with microthermometry data of primary fluid inclusions from growth zones in the fluorites and carbonates are used to constrain the timing of fluid signatures. This contribution shows that the ages are correlated with changes in fluid properties and/or tectonic events in the evolving continental crust. Comparison with published thermochronological data, apparent ages of URG-related volcanic rocks and the tectono-sedimentary evolution of the Upper Rhinegraben rift show clear correlations between the intensity of hydrothermal mineralization (and, hence, the intensity of fracture-bound fluid flow) with the U-Pb carbonate ages. This method accordingly provides an excellent tool to date rift-related processes like fluid flow, ore deposition and tectonic activation or reactivation of fractures. Fluid properties changed after the deposition of Middle Triassic Muschelkalk evaporites from low salinity, high temperature (1–6 wt.% NaCl eq , 200–270 °C, cooling late-metamorphic basement fluids) to high salinity, moderate temperature (20–26 wt.% (NaCl + CaCl 2), 50–170 °C, mixture of a modified bittern brine (“basement brine”) with halite dissolution brine). This change led to large scale ore deposition (fluorite-barite-quartz with Pb-Zn-Cu, Bi-Co-Ni-Ag-U, Fe-Mn ores). During the Paleogene and Neogene, previously separated aquifers from various sedimentary units were connected by juxtaposition of the fluid source rocks during Rhinegraben rifting, which resulted in variable salinity and temperature fluids (1–26 wt. % (NaCl + CaCl 2), 50–350 °C) by a multi-component fluid mixing process. Typical mineralization related to this shows barren or Pb-Zn-Cu veins with large amounts of barite. [ABSTRACT FROM AUTHOR]

High purity fluorite veins (acid grade > 97 % CaF 2) are generally rare in a global context. The fluorite-(±quartz-calcite) veins in the Aukam Valley mining district in SW Namibia have not previously been studied but due to their excellent exposure they are an ideal natural laboratory to study such mineralization. These veins cross-cut Mesoproterozoic high-grade gneisses and granites (Namaqua-Natal Metamorphic Province) close to the unconformity with the overlying late-Neoproterozoic Nama Group sedimentary rock cover. Based on the regional context, their genesis has been assumed to be related to nearby fluorite-bearing pegmatites. However, the present contribution provides new microthermometry data from vein fluorite (with 25.0–25.6 wt% NaCl and T h of 270–300 °C) that indicate precipitation from a mixed fluid with two chemically contrasting end-members for the early paragenetic fluorite stage. However, it was not possible to identify the chemical composition of the endmembers. Based on the chemical composition of the fluid mixture and the regional geological context, we propose that these end-members are F-rich brines sourced from the Namaqua basement and Ca-rich limestone-derived formation fluids of the Nama Group. A fluorite-overgrowing quartz precipitation is likely the effect of post-mixing fluid cooling as the fluid inclusions hosted in quartz show lower homogenization temperatures (T h 230–260 °C) with similar salinity. Locally, late-stage calcite is recognized with fluid inclusion compositions (0.2–2.7 wt% NaCl and T h of 60–92 °C) different from those observed in the fluorite and quartz. With significantly lower homogenization temperatures, they likely represent precipitates formed during the shutdown and collapse of the hydrothermal system. The fluorite veins are hosted in post-Cambrian N-trending brittle structures that probably formed during the opening of the South Atlantic in late Mesozoic times. Interestingly, similar massive fluorite mineralization on both sides of the Atlantic Ocean is also likely associated with Pangea rifting and known from numerous mining districts operating on unconformity-related hydrothermal veins in Central and North America, northern Africa and Europe. • First recognition of fluorite mineralization in relation to Pangea rifting on the southern hemisphere • Fluid inclusion and crush leach data • First description of the Aukam mining district including a first map • A genetic model, which is similar to those veins recognized on the northern hemisphere. [ABSTRACT FROM AUTHOR]

Sulfate is among the most abundant ions in seawater and sulfate-bearing brines are common in sedimentary basins, among other environments. However, the properties of sulfate-bearing fluid inclusions during microthermometry are as yet poorly constrained, restricting the interpretation of fluid-inclusion compositions where sulfate is a major ion. The Schwarzwald mining district on the eastern shoulder of the Upper Rhinegraben rift is an example of a geologic system characterized by sulfate-bearing brines, and constraints on the anion abundances (chloride versus sulfate) would be desirable as a potential means to differentiate fluid sources in hydrothermal veins in these regions. Here, we use the Pitzer-type formalism to calculate equilibrium conditions along the vapor-saturated liquidus of the system H 2 O-Na-Ca-Cl-SO 4 , and construct phase diagrams displaying the predicted phase equilibria. We combine these predicted phase relations with microthermometric and crush-leach analyses of fluid inclusions from veins in the Schwarzwald and Upper Rhinegraben, to estimate the compositions of these brines in terms of bulk salinity as well as cation and anion loads (sodium versus calcium, and chloride versus sulfate). These data indicate systematic differences in fluid compositions recorded by fluid inclusions, and demonstrate the application of detailed low-temperature microthermometry to determine compositions of sulfate-bearing brines. Thus, these data provide new constraints on fluid sources and paleo-hydrology of these classic basin-hosted ore-forming systems. Moreover, the phase diagrams presented herein can be applied directly to compositional determinations in other systems. [ABSTRACT FROM AUTHOR]

The temporal evolution of fracture networks below hydrothermal veins (root zones) and their impact on fluid chemistry, vein mineralogy and ore formation is insufficiently understood for unconformity-related hydrothermal veins in regions of extension, such as the common epithermal sediment-, unconformity- and basement-hosted Pb–Zn deposits. As metals and other trace elements are presumably mobilized during water–rock interaction of highly saline brines with igneous and metamorphic basement rocks, the evolution of these fracture zones seems to play a major role for hydrothermal ore formation. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) microanalysis of individual, texturally well-characterized fluid inclusions, hosted in fluorite and quartz of the Jurassic–Cretaceous Brandenberg fluorite–barite–quartz–galena–sphalerite vein near Todtnau, Schwarzwald, SW Germany, was carried out. Fluid mobile elements (Rb, Cs, Li, W, Ba, Zn, Pb, Sr), preferentially released by the alteration of primary rock-forming minerals (process tracer) were analysed as well as the Cl/Br ratio (source tracer) of fluid inclusions in genetically early fluorite and later quartz. A distinct decrease of trace elements within the fluid inclusions with time indicates successive alteration of primary minerals at the fracture wall to clay minerals with consecutive fluid pulses. A maximum concentration of trace elements in the fluid and consequent ore precipitation is associated with the initial phase of formation of a fracture root. Later fluid pulses migrate along pre-existing fractures so that the amount of fresh reactive rock material decreases with each fluid pulse. As a consequence, multiple generations of ore minerals require the formation of new fracture branches in the root zone of hydrothermal veins. Therefore, it seems that cataclastic zones below hydrothermal veins essentially control ore formation, and their tectonically induced dynamics might be one key parameter that governs the temporal interval of ore precipitation. [ABSTRACT FROM AUTHOR]

Calcite veins hosted in the Triassic Stockton, Lockatong and Passaic formations of the Newark Basin are investigated to reconstruct the fluid evolution. To constrain the parameters of calcite precipitation, a microthermometry study was carried, which reveals precipitation of calcite from a low to moderate saline H 2 O-NaCl fluid (0.4 to 13.2 wt% NaCl equiv.) under low to moderate hydrothermal (137 °C to 232 °C) conditions. This fluid composition is interpreted to reflect mixing between a deep basement-derived heated diluted fluid and relatively low to moderate saline diagenetic formation waters hosted in the different Triassic formations. Carbon and strontium isotope analysis on the vein calcites suggests that these elements are derived from the pre-Triassic basement and the sedimentary cover through fluid-rock interactions. The aforementioned geochemical findings are supported by Rare Earth Elements and Yttrium (REY) systematics and oxygen isotope data. The Late Triassic extensional activity and gravity-driven fluid flow mechanism facilitated the infiltration of meteoric waters to deeper lithostratigraphic units (i.e., Precambrian-Paleozoic basement-Triassic Stockton Formation) where they became heated. In response to the extensional tectonics, the deep-seated hydrothermal basement-derived diluted fluids migrated upward along the tectonic-related fractures and the major faults to upper shallow crustal levels. Here, the heated, diluted meteoric waters were mixed with low, moderately saline, and relatively cooler formation waters, leading to calcite precipitation. The pH increase is suggested to be a contributing factor in the precipitation of calcite. [Display omitted] • During Late Triassic extensional activity, the meteoric waters descended to deep parts of the Newark basin. • Cross-basinal migration of these deep-seated hydrothermal fluids was ensured primarily by the topographically-driven flow mechanism • During Early Jurassic, extension tectonics facilitated the upward migration of the deep-seated fluids to shallow crustal levels • Fluid mixing occured between the basement-derived diluted fluids and shallowformational waters, leading to calcite precipitation. [ABSTRACT FROM AUTHOR]

Carbonatites are formed from volatile-rich melts, commonly associated with a characteristic hydrothermal footprint. However, studies of their fluid inclusions are relatively scarce and heterogeneous in terms of detail and completeness of the data presented. Here, we review and discuss comprehensively the results of previous studies and derive a general model for the formation and properties of fluids associated with carbonatitic magmatism. Worldwide, four types of fluid inclusion occur in carbonatites: (type I/HS) vapour-poor H 2 O-NaCl fluids with up to 50 wt% salinity; (type II/HC) vapour-rich H 2 O-NaCl-CO 2 fluids with <5 wt% salinity; (type III/DS) multi-component fluids with high salinity and without CO 2 ; and (type IV/CDS) multi-component fluids with high salinity and high CO 2. This global dataset suggests continuous fluid release from deep to shallow-level intrusions. Modelling of the liquidus surface indicates that carbonatite magmas generally exsolve a saline brine (type I/HS). This brine separates/evolves into a Na-K-sulfate-carbonate/bicarbonate-chloride brine with or without CO 2 (types III/DS and IV/CDS), trapped together with low salinity CO 2 -rich fluids produced by immiscibility. Fluid immiscibility is related to rapid pressure release during fast, forceful and discontinuous magma ascent, which we envisage as a "pneumatic jackhammer" model for carbonatite ascent and emplacement. In this model, cyclic and progressive fluid flux via pressure build-up and subsequent catastrophic pressure release results in a self-sustaining crustal ascent of the buoyant, low-viscosity magma. This process allows for rapid and efficient magma ascent, in particular above the brittle-ductile transition zone, where pressures that prevailed during apatite crystallization have been estimated in numerous complexes. Moreover, this model provides an explanation for the apparent absence of shallow carbonatite magma chambers (in a classical sense) and identifies fenitization as a phenomenon induced by both fluids released during magma ascent and residual fluids. [Display omitted] [ABSTRACT FROM AUTHOR]

Hydrothermal fluorite-hematite veins represent a relatively rare vein type, which is sometimes associated with more common, fluorite-barite-quartz‑carbonate veins containing base metal mineralization. The origin and significance of the fluorite-hematite veins, and how they relate to the more common expression of mineralization, is essentially unknown. Here, we present new data illuminating the formation conditions of these "sulfide-barren" fluorite-hematite veins in a mining district known for hydrothermal Pb + Zn + Ag ores for which formation conditions are well understood: the Schwarzwald district in SW Germany. The exemplary Ödsbach-Hesselbach vein was chosen to study the fluids involved in fluorite-hematite precipitation through the first, direct investigation of fluid inclusions hosted in hydrothermal hematite using combined infra-red light microscopy, microthermometry and LA-ICPMS. The results indicate that the hematite-precipitating fluids are nearly identical to those observed in the associated fluorite, showing consistent salinities of 24.6–25.0 wt% (NaCl+CaCl 2) and homogenization temperatures of 150 to 155 °C. Moreover, the trace element abundances determined by the LA-ICPMS analyses show a composition similar to other inclusions observed in the common mineralization in the mining district, including elevated Pb (>5000 μg/g) and Zn (>2000 μg/g) concentrations at low Cl/Br mass ratios, and sulfur contents mostly below the limits of detection. The results suggest fluid mixing between a continental basement brine and a redbed-derived fluid from the Triassic Buntsandstein. Therefore, the formation of the rare fluorite-hematite veins was probably triggered by a change of the sedimentary fluid endmember representing a shift from the Middle Triassic Muschelkalk fluid, which drove the more common Pb + Zn + Ag mineralization, towards fluids derived from the Buntsandstein. Furthermore, the absence of sulfur is interpreted to have precluded base metal sulfide precipitation, and the formation of hematite points to unusually oxidized conditions and hence towards a lack of a reducing agent, which would also have been required to generate a mineralization of base metal sulfides. • First LA-ICPMS data from hematite-hosted fluid inclusions • How a missing redox agent influence vein mineralogy. • Low S systems are needed to precipitate fluorite-hematite veins. [ABSTRACT FROM AUTHOR]

Mixing of compositionally contrasting fluids is widely accepted as the major ore-forming process in unconformity-related hydrothermal Pb-Zn-fluorite-barite veins. Although the general process is relatively well understood, the temporal evolution of such fluid systems which may result in distinct mineralogical changes of the ore and gangue minerals precipitating are insufficiently constrained. One specific example is the occurrence of late-stage siderite-chalcopyrite-gersdorffite mineralization in 28 hydrothermal veins of the Schwarzwald, SW Germany post-dating the major fluorite-barite-galena-sphalerite phase which is very common in the whole area. To investigate the underlying process for this late-stage Cu-Ni-stage, published fluid inclusion data have been complemented by detailed confocal Raman micro-spectroscopy of fluid inclusions, petrographic observations and electron microprobe analyses. Petrography as well as mineral compositions of sphalerite, gersdorffite, pyrite, arsenopyrite, fahlore and galena record a gradual, but significant influx of Cu and Ni into a prevailing Pb-Zn hydrothermal system. This influx caused a distinct shift towards lower fO 2 and Pb + Zn + Sb + Ag (+ S) in the hydrothermal system at the transition from the Pb-Zn to the Cu-Ni mineralization stage. This transition in mineralogy and the related gradual change of the hydrothermal fluid can be explained by changes in the relative proportion of involved fluid components. However, the transition to the Cu-Ni mineralization stage by fluid influx is significantly different compared to the common rapid fluid mixing from below (which is thought to be responsible for the typical Pb-Zn veins) and probably records the gradual transition from long-term stable deep fluid reservoirs with constant chemical compositions towards more shallow and perturbed fluid reservoirs during rifting and exhumation. [ABSTRACT FROM AUTHOR]

Five-element veins (Ag, Bi, Co, Ni, As) have been valuable mineral deposits since medieval times. The characteristic occurrence of large aggregates of native metals (up to several dm) surrounded by a succession of arsenides makes this vein-type attractive for mining industry, natural history museums and private mineral collectors. Nevertheless, the exact formation process of these specific vein types has not been fully understood. This is the first case study applying a new model, which includes methane as a reducing agent to two typical examples of such mineralisations from the Odenwald, SW Germany. We analysed all mineralogical varieties (Ag-, As- and Bi-dominated) of the five-element veins in the Odenwald (SW Germany) in terms of ore textures, mineral chemistry, fluid inclusion compositions (microthermometry and Raman spectroscopy), stable isotopes (C, O and S) and in-situ U-Pb age dating of calcite and prehnite. A variety of Ag-, Bi- and As-dominated native metal-arsenide-calcite veins, sulphide-calcite veins and arsenide-free Ag-Hg-barite veins occurs in the Odenwald and has been examined in this study. All arsenide veins have in common that up to dm-sized, often dendritic native metals are overgrown by a succession of arsenides, followed by carbonate and finally sulphides. The succession of arsenides shows a distinct spatial and temporal chemical trend in their composition. This trend evolves from Ni- to Co and finally Fe-dominated compositions from the core to the rim. In contrast, spatially closely related Ag-Hg-barite veins consist of almost mono-mineralic amalgam inter-grown with barite. In-situ U-Pb age dating of low-U calcite and prehnite was applied to constrain the age of hydrothermal mineralisation. The results imply that the five-element veins formed at 170–180 Ma from Na-Ca-Cl fluids at 290 °C, salinities of ~ 27 wt.% and Ca/(Ca + Na) of 0.30 to 0.35 in the presence of methane. The age data clearly relate the relevant fluid migration to extension and crustal thinning caused by the opening of the North Atlantic. Ternary mixing of a deep-seated metal-rich basement brine (fluid A), a sulphide-bearing (H 2 S and HS − ) basinal/sedimentary brine (fluid B) and methane-dominated fluid/gas (fluid C) induce ore formation. Mixing of such chemically contrasting fluids results in a strong chemical disequilibrium of the mixed fluid, which potentially leads to rapid precipitation of native metals and arsenides with these specific ore textures. In contrast, sulphide-bearing calcite veins formed under similar P-T-conditions due to mixing of fluid A and B, while fluid C was absent. Additionally, U-Pb analyses of post-ore calcite yields ages of ~ 60 Ma, which indicates that a second calcite formation event is associated with the onset of the Upper Rhine Graben rifting. Veins with amalgam and barite form as a consequence of post-ore oxidation of Ag 2 S at ~ 135 °C. Conspicuously, this secondary silver II has Hg contents of up to ~ 30 wt.%, in contrast to Hg contents below 1 wt.% of primary silver I and Ag 2 S. The formation of amalgam was most likely related to decrease of the S 2 − /SO 4 2 − ratio due to cooling of the late hydrothermal fluid resulting in the destabilisation of Hg-bisulphide complexes in this fluid. [ABSTRACT FROM AUTHOR]

This paper documents original petrographic and structural data on metamorphic and magmatic rocks as well as the occurrence of gold mineralization in the Dourou Tchaga area (SW Poli) in the northwestern Cameroon domain. The study area is dominated by high-grade gneisses derived from a volcanic-sedimentary series alternating with orthogneiss and magmatic rocks. Four main deformation stages were distinguish comprising the early D 1 and D 2 deformations showing ductile behavior under amphibolite to granulite facies, followed by retrogression into greenschist facies. D 3 and D 4 are characterized by ductile-brittle and late brittle structures. Gold mineralization is hosted in laminated quartz veins in steeply dipping, ENE–WSW-trending, sinistral shear zone developed during D 3. Host rocks are high-grade metamorphic paragneiss and orthogneiss. Ore minerals include native gold, galena, sphalerite, pyrite and chalcopyrite; wall rock alteration assemblage includes stable pyrite, magnetite and hematite. This assemblage together with specific quartz microstructures indicate gold mineralization at lower greenschist facies conditions. Gold is disseminated in quartz veins and wall rock alteration zones, where it occurs as free grains but also as inclusions in pyrite and sphalerite. Two types of fluid inclusions are distinguished: aqueous-carbonic CO 2 –H 2 O and aqueous (H 2 O), both with low to moderate (2.0–13.9 wt% NaCleq) salinity and variable CO 2 content of 13–23 mole% for the aqueous-carbonic fluid inclusions. Fluid inclusions data and alteration assemblage provide evidence that gold precipitation in quartz veins occurred in response to an increase in fO 2 caused by fluid pressure fluctuations in the shear zone. Gold mineralization in the northern Cameroon domain was formed during post-collisional stage and retrograde exhumation of the terrane in the greenschist facies toward the end of the Panafrican Orogeny. [ABSTRACT FROM AUTHOR]

Unconformity-related hydrothermal ore deposits typically form by mixing of hot, deep, rock-buffered basement brines and cooler fluids derived from the surface or overlying sediments. Current models invoking simultaneous downward and upward flow of the mixing fluids are inconsistent with fluid overpressure indicated by fracturing and brecciation, fast fluid flow suggested by thermal disequilibrium, and small-scale fluid composition variations indicated by fluid inclusion analyses. We propose a new model where fluids first descend, then evolve while residing in pores and later ascend. We use the hydrothermal ore deposits of the Schwarzwald district in southwest Germany as an example. Oldest fluids reach the greatest depths, where long residence times and elevated temperatures allow them to equilibrate with their host rock, to reach high salinity, and to scavenge metals. Youngest fluids can only penetrate to shallower depths and can (partially) retain their original signatures. When fluids are released from different levels of the crustal column, these fluids mix during rapid ascent in hydrofractures to form hydrothermal ore deposits. Mixing from below during ascent provides a viable hydromechanical mechanism to explain the common phenomenon of mixed shallow and deep fluids in the formation of hydrothermal ore deposits. [ABSTRACT FROM AUTHOR]

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]

• Contrast-comparison of dolostones located on flanks and crests of anticlinal hydrocarbon reservoirs. • Petrographic, isotopic and fluid inclusion data set provides evidence of coeval formation of burial dolostone. • Replacive and porosity-destructive dolomitization has deteriorated reservoir properties. • The temperature range, as based on fluid-inclusion data, points to an abnormally high-temperature background thermal regime in the Volga-Ural basin. A significant proportion of the remaining oil reserves worldwide is concentrated in dolostone rock bodies. The present paper focuses on Tournaisian and Visean limestones and geologically younger diagenetic dolomite units in two mature oil fields of the Volga-Ural Basin, Russia. In contrast to many dolostone-hosted hydrocarbon reservoirs worldwide, the Tournaisian and Visean dolostones in the Demkinskoye and the Onbiyskoye oil fields plot near the low end of the porosity–permeability range. Replacive and porosity-destructive burial dolomitization has deteriorated the reservoir properties of these units. Dolomitization is related to the ascent of hot (170–215 °C) and saline (22–24 wt%) fluids as based on fluid inclusion data. This temperature range points to an abnormally high temperature background thermal regime of the Volga-Ural basin. The onset of burial dolomitization predates oil charge as documented by the fact that oil-bearing inclusions are only present in the outermost zones of porosity-occluding dolomite cements. Isotope geochemical data, dolomite petrography and fluid inclusion data reveal important similarities between hydrothermal dolostone units in the Demkinskoye and the Onbiyskoye oil fields. Based on these data and the detailed cement stratigraphy presented here, it is argued that dolomite bodies in both oil fields formed as the result of one main, Alpine tectonic event pushing hot and saline dolomitizing brines ahead of ascending hydrocarbons. [ABSTRACT FROM AUTHOR]