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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]

Fluid mixing is an important process in the formation of many hydrothermal vein-type deposits. Here, we present evidence from hydrothermal fluorite-barite-quartz veins with Pb-Zn-Cu-(Ag)-sulfides and associated mineralization, indicating that mineral precipitation was initiated by mixing of fluids derived from multiple sources, including mixing between more than two end-member fluid compositions. Based on our observations, we relate the diversity of the hydrothermal veins of the Schwarzwald mining district in terms of mineral assemblage and fluid inclusion chemistry to the disturbed and transient geological environment during ongoing rifting. Literature data on the regional geology, current groundwater reservoirs, formation processes and hydraulic features are augmented by new fluid inclusion analyses from post-Cretaceous, hydrothermal vein minerals including microthermometry, crush leach, Microraman and LA-ICP-MS analyses of individual fluid inclusions. Petrography and microthermometry of fluid inclusions show complex sequences of alternating fluid signatures within different growth zones of one crystal. High (20–26 wt% NaCl + CaCl 2 ), moderate (5–20 wt% NaCl + CaCl 2 ) and low salinity (< 5 wt% NaCl + CaCl 2 ), sulfate- and/or CO 2 -bearing primary fluids were trapped during crystal growth. Such variations are commonly observed in minerals from different localities. Bulk crush leach analyses show significant variations in major element composition of the trapped fluids, within the overall Na-Ca-Cl-SO 4 -HCO 3 -system. These variations are caused by mixing of fluids from different aquifers and in various proportions. Ancient fluids show chemical similarities to modern groundwater aquifers that are available for direct sampling, such as granitic basement, Lower Triassic sandstones or Middle Triassic limestones and evaporites. Analyses of individual fluid inclusions by LA-ICP-MS support this interpretation and document the multi-component fluid mixing processes at individual localities recorded on the scale of single crystal growth zones. The latter data are used in a diffusion model to obtain the duration of mineral growth (before the fluid is homogenized), which implies very short-lived fluid events on the order of seconds to hours. By defining end member fluids and their proportions, we show that nearly all fluid mixtures are saturated with respect to barite. By contrast, fluorite-saturated fluids can only be modelled by mixing of a basement brine with fluids from Triassic sandstones. All fluid mixtures are strongly undersaturated with respect to galena, chalcopyrite and sphalerite, the most commonly observed ore minerals in the hydrothermal veins. As the calculated fluid mixtures are typically relatively oxidized and contain high sulfate/sulfide ratios, precipitation of sulfides was probably related to short-lived reduction events caused by an influx of hydrocarbons, by reactions with graphitic wall rocks in fractures by sulfidation related to fluid-rock reaction with the surrounding host rocks or an external influx of hydrocarbon-bearing fluids. The multi-aquifer fluid mixing processes involving aquifers of different chemical and physical constitution were triggered by brittle deformation related to rifting of the Rhine graben. This appears to be essential for the formation of a large number of mineralogically diverse hydrothermal ore deposits. [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]

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]

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]

The significance of magnesium isotope (δ 26 Mg) fractionation in the continental hydrothermal domain is poorly explored. Here, a detailed Mg isotope dataset from various aquifer host rocks and corresponding hydrothermal carbonate precipitates from the Schwarzwald mining area in SW Germany is documented and discussed. This study is motivated by the very considerable existing data set on hydrothermal mineralizations in the study area and the excellent regional control of sampling points. Based on structural, mineralogical and microthermometrical arguments, we here subdivide three clusters of veins: (i) Permian, (ii) Jurassic–Cretaceous and (iii) post-Cretaceous (Cenozoic). The focus is on clusters ii and iii and their corresponding, texturally older hydrothermal fluid inclusion-rich, coarse-grained, low-Mg calcite and dolomite–ankerite solid solutions and younger, oscillatory zoned low-Mg calcite and dolomite–ankerite solid solution crystals precipitated from fluids with temperatures between 50 and 350 °C. In terms of their δ 26 Mg ratios, three characteristic groups of hydrothermal carbonates can be distinguished: (i) Jurassic–Cretaceous veins (δ 26 Mg = − 3.38 to − 0.82‰) in the central Schwarzwald precipitated from fluids containing a significant proportion of sedimentary cover-derived waters. They yield 26 Mg-depleted Mg isotope signatures typical of sedimentary carbonate lithologies. (ii) Jurassic–Cretaceous vein carbonates in the southern Schwarzwald precipitated from mainly basement-derived fluids with 26 Mg-enriched signatures (δ 26 Mg = − 1.22 to + 0.05‰). Supporting evidence for the origin of these fluids comes from Sr–C–Pb isotope systematics and trace element compositional variations of fahlore and sphalerite. The Mg isotope variations are controlled by differences in fluid source characteristics dominate over variations exerted by mineralogical differences (where low-Mg calcites are generally, but not in all cases, more depleted in 26 Mg compared to dolomite–ankerite solid solutions). (iii) Post-Cretaceous (Cenozoic), Rhinegraben-related veins represent a complex tectonic juxtaposition of different aquifer lithologies. As expected, this phase spans the full range from silicate to carbonate Mg isotope signatures (+ 0.45 to − 3.4‰). Magnesium isotope data are supported by textural analyses (optical and cathodoluminescence microscopy), electron microprobe, microthermometry and published radiogenic 87 Sr/ 86 Sr isotope systematics. Our data are not in agreement with a significant temperature-controlled Δ 26 Mg fluid-calcite and no obvious relation between fluid salinity and hydrothermal carbonate δ 26 Mg is found. The results of this study suggest that hydrothermal carbonate magnesium isotope ratios have a significant potential as tracer of hydrothermal fluid sources and corresponding aquifer lithologies. [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]

Hydrothermal five-element veins (Ag-Co-Ni-Bi-As) are mineral successions of native metals, encapsulated by Fe-Co-Ni arsenides and carbonates. Recent studies focused on the evolution from ordinary base-metal systems (sulfide-rich) to five-element veins (sulfide-poor) and revealed the importance of hydrocarbon-dominated fluids as essential redox agent in these systems. Although mineral successions reveal the natural subdivision into native As-, native Ag/Bi- or arsenide-dominated vein types and suggested processes explain the mineralogical variation to a certain degree, the Ni-Co-Fe-variations among the arsenides are not well understood yet. This is the first case study explaining compositional, mineralogical, and textural features of five-element veins by changing metal contents, arsenic/sulfur activities, pH and temperatures, applied to a multi-stage vein mineralization in the Penninic Alps, Switzerland. Textural relationships, mineral chemistry, fluid inclusion compositions (microthermometry and Raman spectroscopy), stable S-isotopes and in-situ U-Pb age dating of carbonates, magnetite and multi-mineral isochrons were investigated. U-Pb ages of paragenetic mineral fractions constrain a primary formation of löllingite-skutterudite-dolomite-dominated ores at 233 ± 10 Ma and niccolite-gersdorffite-skutterudite-ankerite-dominated ores at 188 ± 32 Ma, which links their formation to crustal thinning caused by the breakup of the Meliata ocean and Alpine Tethys. As secondary processes during the Alpine Orogeny, a in-situ remobilization of the ores occurred as mostly ternary Fe-Co-Ni sulfarsenides at ∼73–24 Ma due to continent-continent collision and neoformations of safflorite-cobaltite-skutterudite-dominated ores at ∼29–16 Ma due to transtensional strike-slip tectonics. Ore textures indicate that the dissolution of primary siderite, oxidation of ferrous iron and its precipitation as magnetite was the redox couple to precipitate native Bi, arsenides and sulfarsenides (Bi 0 , As 3− and As −1 ) from their oxidized aqueous species Bi 3+ Cl 4 − , and As 3+ (OH) 3 at temperatures between 200 and 300 °C. Ni and Co signatures of the arsenides/sulfarsenides show increasing mobilization from the host rocks during the successive evolution of the hydrothermal system in the Triassic, Jurassic and during the Alpine Orogeny. Stable S-isotopes and As/S signatures of the sulfarsenides indicate an increasing mobilization of host rock sulfides (i.e. fahlbands) from the primary to the secondary ores. Fluid inclusions and stable S-isotopes suggest involvement of fluid modification by water-rock interactions (i.e. cover rocks: carbonate, sulfate and halite dissolution; basement rocks: albitisation of plagioclase) during fluid descent. [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]

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]