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Abstract: The hydrothermal Mn–Fe–As vein mineralization at Sailauf (Germany) hosts a complex sequence of oxide and carbonate minerals that record a protracted fluid history. The mineralization is related to a major unconformity that separates Permian (Zechstein) sedimentary rocks from underlying Variscan crystalline basement. The hydrothermal veins contain two principal mineralization stages, which are Mn oxides associated with calcite and hematite associated with Mn-calcite. The fluid evolution of the hydrothermal system has been reconstructed from fluid inclusion petrography, microthermometry, and LA-ICPMS microanalysis, coupled with stable isotope geochemistry of carbonate and oxide minerals. The fluid inclusions are high salinity sodic–calcic brines and the bulk fluid properties show no major differences between the Mn oxide and the hematite stage. LA-ICPMS analysis of major and trace elements demonstrates that the mineralization formed from chemically distinct fluid pulses characterized by variations in their K, Li, B, Pb and Zn concentrations. The fluid that precipitated the Mn oxide stage has anomalous Pb/Zn and Li/B ratios, which closely resemble fluids found in fracture fillings in red beds of the Permian Rotliegend basin. By contrast, the fluids associated with the hematite stage have Pb/Zn and Li/B ratios typical of crustal fluids that were derived from interaction with crystalline basement. Both fluids possess characteristic element ratios including Cl/Br, but variable absolute concentrations of most metals. This suggests that both fluids were modified by mixing with a common metal-depleted brine that had a similar Cl/Br ratio, most likely a formation water from the overlying Zechstein sedimentary rocks. The Sailauf mineralization provides insight into the protracted post-Variscan fluid evolution at the basement–cover interface. The compositionally anomalous fluid that precipitated the Mn oxides is comparable to brines derived from interaction with red beds and likely represents the ore fluid of Kupferschiefer-type sediment-hosted Cu deposits. Conversely, the fluid that deposited the hematite mineralization resembles fluids that typically form basement- and sediment-hosted Pb–Zn deposits. [Copyright &y& Elsevier]

Abstract: The Sailauf Mnph name="sbnd" />As vein mineralization, located in the Spessart district (central Germany), is characterized by complex hydrothermal carbonate and oxide assemblages. The mineralization is hosted by a Permian rhyolite body and is structurally related to the Variscan unconformity that separates Permian sedimentary rocks from the underlying Variscan crystalline basement. The hydrothermal vein system has been studied by optical microscopy, electron-microprobe and LA-ICPMS analysis of major and trace elements (including the REE). Four distinct mineralization stages that are characterized by diagnostic carbonate-oxide assemblages are identified, which are (1) pre-ore stage, (2) ore stage 1, (3) ore stage 2, and (4) the replacement stage. Hydrothermal carbonates show complex compositional trends in Caph name="sbnd" /> (Fe+Mg) space, and comprise calcite, Mn-calcite, Fee, Ca-rhodochrosite, and Ca-kutnahorite. Oxide assemblages are dominated by braunite and hematite, with minor amounts of manganite and hausmannite. The mineralization is enriched in a distinct suite of trace elements, including As, W, Pb, Zn and Cu. Analysis of the paragenetic evolution, in conjunction with the major and trace element data, allows to reconstruct the fluid evolution of the hydrothermal system. The first order change in mineralogy between the two main ore stages (Mn oxides and calcite evolving into hematite and Mn-rich calcite) records a pronounced shift in fluid pH and silica activity of the system. This interpretation is also supported by variations in the behavior of Ce in different carbonate generations. The late stage replacement carbonates relate to destabilization of the primary ore assemblages. The distinct geological setting immediately below the Permian unconformity, in conjunction with the mineralogical and chemical data, suggests that dynamic fluid mixing processes involving basement-derived brines and more shallow groundwaters were important in the formation of the Sailauf Mnph name="sbnd" />As mineralization. The significant enrichment in the Ash name="sbnd" />Pbph name="sbnd" />Cu element suite resembles that of other Mnts, and points to felsic lithologies as the main metal source of the mineralization. [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]